/* Copyright 2016 Software Freedom Conservancy Inc.
 *
 * This software is licensed under the GNU LGPL (version 2.1 or later).
 * See the COPYING file in this distribution.
 */

public class InjectionGroup {
    public class Element {
        public enum ItemType {
            MENUITEM,
            MENU,
            SEPARATOR
        }
        public string name;
        public string action;
        public string? accellerator;
        public ItemType kind;

        public Element(string name, string? action, string? accellerator, ItemType kind) {
            this.name = name;
            this.action = action != null ? action : name;
            this.accellerator = accellerator;
            this.kind = kind;
        }
    }

    private string path;
    private Gee.ArrayList<Element?> elements = new Gee.ArrayList<Element?>();
    private int separator_id = 0;

    public InjectionGroup(string path) {
        this.path = path;
    }

    public string get_path() {
        return path;
    }

    public Gee.List<Element?> get_elements() {
        return elements;
    }

    public void add_menu_item(string name, string? action = null, string? accellerator = null) {
        elements.add(new Element(name, action, accellerator, Element.ItemType.MENUITEM));
    }

    public void add_menu(string name, string? action = null) {
        elements.add(new Element(name, action, null, Element.ItemType.MENU));
    }

    public void add_separator() {
        elements.add(new Element("%d-separator".printf(separator_id++), null,
                    null,
                    Element.ItemType.SEPARATOR));
    }
}

public abstract class Page : Gtk.ScrolledWindow {
    private const int CONSIDER_CONFIGURE_HALTED_MSEC = 400;
    
    protected Gtk.Builder builder = new Gtk.Builder ();
    protected Gtk.Toolbar toolbar;
    protected bool in_view = false;
    
    private string page_name;
    private ViewCollection view = null;
    private Gtk.Window container = null;
    private string toolbar_path;
    private Gdk.Rectangle last_position = Gdk.Rectangle();
    private Gtk.Widget event_source = null;
    private bool dnd_enabled = false;
    private ulong last_configure_ms = 0;
    private bool report_move_finished = false;
    private bool report_resize_finished = false;
    private Gdk.Point last_down = Gdk.Point();
    private bool is_destroyed = false;
    private bool ctrl_pressed = false;
    private bool alt_pressed = false;
    private bool shift_pressed = false;
    private bool super_pressed = false;
    private Gdk.CursorType last_cursor = Gdk.CursorType.LEFT_PTR;
    private bool cursor_hidden = false;
    private int cursor_hide_msec = 0;
    private uint last_timeout_id = 0;
    private int cursor_hide_time_cached = 0;
    private bool are_actions_attached = false;
    private OneShotScheduler? update_actions_scheduler = null;
    
    protected Page(string page_name) {
        this.page_name = page_name;
        
        view = new ViewCollection("ViewCollection for Page %s".printf(page_name));
        
        last_down = { -1, -1 };
        
        set_can_focus(true);

        popup_menu.connect(on_context_keypress);
        
        realize.connect(attach_view_signals);
    }
    
    ~Page() {
#if TRACE_DTORS
        debug("DTOR: Page %s", page_name);
#endif
    }
    
    // This is called by the page controller when it has removed this page ... pages should override
    // this (or the signal) to clean up
    public override void destroy() {
        if (is_destroyed)
            return;
        
        // untie signals
        detach_event_source();
        detach_view_signals();
        view.close();
        
        // remove refs to external objects which may be pointing to the Page
        clear_container();
        
        if (toolbar != null)
            toolbar.destroy();
        
        // halt any pending callbacks
        if (update_actions_scheduler != null)
            update_actions_scheduler.cancel();
        
        is_destroyed = true;
        
        base.destroy();
        
        debug("Page %s Destroyed", get_page_name());
    }
    
    public string get_page_name() {
        return page_name;
    }
    
    public virtual void set_page_name(string page_name) {
        this.page_name = page_name;
    }
    
    public string to_string() {
        return page_name;
    }
    
    public ViewCollection get_view() {
        return view;
    }
    
    public Gtk.Window? get_container() {
        return container;
    }
    
    public virtual void set_container(Gtk.Window container) {
        assert(this.container == null);
        
        this.container = container;
    }
    
    public virtual void clear_container() {
        container = null;
    }
    
    public void set_event_source(Gtk.Widget event_source) {
        assert(this.event_source == null);

        this.event_source = event_source;
        event_source.set_can_focus(true);

        // interested in mouse button and motion events on the event source
        event_source.add_events(Gdk.EventMask.BUTTON_PRESS_MASK | Gdk.EventMask.BUTTON_RELEASE_MASK
            | Gdk.EventMask.POINTER_MOTION_MASK | Gdk.EventMask.POINTER_MOTION_HINT_MASK
            | Gdk.EventMask.BUTTON_MOTION_MASK | Gdk.EventMask.LEAVE_NOTIFY_MASK
            | Gdk.EventMask.SCROLL_MASK | Gdk.EventMask.SMOOTH_SCROLL_MASK);
        event_source.button_press_event.connect(on_button_pressed_internal);
        event_source.button_release_event.connect(on_button_released_internal);
        event_source.motion_notify_event.connect(on_motion_internal);
        event_source.leave_notify_event.connect(on_leave_notify_event);
        event_source.scroll_event.connect(on_mousewheel_internal);
        event_source.realize.connect(on_event_source_realize);
    }
    
    private void detach_event_source() {
        if (event_source == null)
            return;
        
        event_source.button_press_event.disconnect(on_button_pressed_internal);
        event_source.button_release_event.disconnect(on_button_released_internal);
        event_source.motion_notify_event.disconnect(on_motion_internal);
        event_source.leave_notify_event.disconnect(on_leave_notify_event);
        event_source.scroll_event.disconnect(on_mousewheel_internal);
        
        disable_drag_source();
        
        event_source = null;
    }
    
    public Gtk.Widget? get_event_source() {
        return event_source;
    }

    private bool menubar_injected = false;
    public GLib.MenuModel get_menubar() {
        var model = builder.get_object ("MenuBar") as GLib.Menu;
        if (model == null) {
            return new GLib.Menu();
        }

        if (!menubar_injected) {
            // Collect injected UI elements and add them to the UI manager
            InjectionGroup[] injection_groups = init_collect_injection_groups();
            foreach (InjectionGroup group in injection_groups) {
                var items = model.get_n_items ();
                for (int i = 0; i < items; i++) {
                    var submenu = model.get_item_link (i, GLib.Menu.LINK_SUBMENU);

                    var section = this.find_extension_point (submenu,
                                                             group.get_path ());

                    if (section == null) {
                        continue;
                    }

                    foreach (var element in group.get_elements ()) {
                        var menu = section as GLib.Menu;
                        switch (element.kind) {
                            case InjectionGroup.Element.ItemType.MENUITEM:
                                var item = new GLib.MenuItem (element.name,
                                                              "win." + element.action);
                                if (element.accellerator != null) {
                                    item.set_attribute ("accel",
                                                        "s",
                                                        element.accellerator);
                                }

                                menu.append_item (item);
                                break;
                            default:
                                break;
                        }
                    }
                }
            }

            this.menubar_injected = true;
        }

        return model;
    }

    public virtual Gtk.Toolbar get_toolbar() {
        if (toolbar == null) {
            toolbar = toolbar_path == null ? new Gtk.Toolbar() :
                                             builder.get_object (toolbar_path)
                                             as Gtk.Toolbar;
            toolbar.get_style_context().add_class("bottom-toolbar");  // for elementary theme
            toolbar.set_icon_size(Gtk.IconSize.SMALL_TOOLBAR);
        }
        return toolbar;
    }
    
    public virtual Gtk.Menu? get_page_context_menu() {
        return null;
    }
    
    public virtual void switching_from() {
        in_view = false;
        //remove_actions(AppWindow.get_instance());
        var map = get_container() as GLib.ActionMap;
        if (map != null) {
            remove_actions(map);
        }
        if (toolbar_path != null)
            toolbar = null;
    }
    
    public virtual void switched_to() {
        in_view = true;
        add_ui();
        var map = get_container() as GLib.ActionMap;
        if (map != null) {
            add_actions(map);
        }
        int selected_count = get_view().get_selected_count();
        int count = get_view().get_count();
        init_actions(selected_count, count);
        update_actions(selected_count, count);
        update_modifiers();
    }
    
    public virtual void ready() {
    }
    
    public bool is_in_view() {
        return in_view;
    }
    
    public virtual void switching_to_fullscreen(FullscreenWindow fsw) {
        add_actions(fsw);
    }
    
    public virtual void returning_from_fullscreen(FullscreenWindow fsw) {
        remove_actions(fsw);
        switched_to();
    }

    public GLib.Action? get_action (string name) {
        GLib.ActionMap? map = null;
        if (container is FullscreenWindow) {
            map = container as GLib.ActionMap;
        } else {
            map = AppWindow.get_instance () as GLib.ActionMap;
        }

        if (map != null) {
            return map.lookup_action(name);
        }

        return null;
    }
    
    public void set_action_sensitive(string name, bool sensitive) {
        GLib.SimpleAction? action = get_action(name) as GLib.SimpleAction;
        if (action != null)
            action.set_enabled (sensitive);
    }
    
    public void set_action_details(string name, string? label, string? tooltip, bool sensitive) {
        GLib.SimpleAction? action = get_action(name) as GLib.SimpleAction;

        if (action == null)
            return;

        if (label != null)
            this.update_menu_item_label (name, label);

        action.set_enabled (sensitive);
    }
    
    public void activate_action(string name) {
        var action = get_action(name);

        if (action != null)
            action.activate (null);
    }
    
    public GLib.Action? get_common_action(string name, bool log_warning = true) {
        var action = get_action (name);

        if (action != null)
            return action;
        
        if (log_warning)
            warning("Page %s: Unable to locate common action %s", get_page_name(), name);
        
        return null;
    }
    
    public void set_common_action_sensitive(string name, bool sensitive) {
        var action = get_common_action(name) as GLib.SimpleAction;
        if (action != null)
            action.set_enabled (sensitive);
    }

    public void set_common_action_label(string name, string label) {
        debug ("Trying to set common action label for %s", name);
    }
    
    public void set_common_action_important(string name, bool important) {
        debug ("Setting action to important: %s", name);
    }
    
    public void activate_common_action(string name) {
        var action = get_common_action(name) as GLib.SimpleAction;
        if (action != null)
            action.activate(null);
    }
    
    public bool get_ctrl_pressed() {
        return ctrl_pressed;
    }
    
    public bool get_alt_pressed() {
        return alt_pressed;
    }
    
    public bool get_shift_pressed() {
        return shift_pressed;
    }
    
    public bool get_super_pressed() {
        return super_pressed;
    }

     protected void set_action_active (string name, bool active) {
        var action = get_action (name) as GLib.SimpleAction;
        if (action != null) {
            action.set_state (active);
        }
    }

    private bool get_modifiers(out bool ctrl, out bool alt, out bool shift, out bool super) {
        if (AppWindow.get_instance().get_window() == null) {
            ctrl = false;
            alt = false;
            shift = false;
            super = false;
            
            return false;
        }
        
        int x, y;
        Gdk.ModifierType mask;
        var seat = Gdk.Display.get_default().get_default_seat();
        AppWindow.get_instance().get_window().get_device_position(seat.get_pointer(), out x, out y, out mask);

        ctrl = (mask & Gdk.ModifierType.CONTROL_MASK) != 0;
        alt = (mask & Gdk.ModifierType.MOD1_MASK) != 0;
        shift = (mask & Gdk.ModifierType.SHIFT_MASK) != 0;
        super = (mask & Gdk.ModifierType.MOD4_MASK) != 0; // not SUPER_MASK
        
        return true;
    }

    private void update_modifiers() {
        bool ctrl_currently_pressed, alt_currently_pressed, shift_currently_pressed,
            super_currently_pressed;
        if (!get_modifiers(out ctrl_currently_pressed, out alt_currently_pressed,
            out shift_currently_pressed, out super_currently_pressed)) {
            return;
        }
        
        if (ctrl_pressed && !ctrl_currently_pressed)
            on_ctrl_released(null);
        else if (!ctrl_pressed && ctrl_currently_pressed)
            on_ctrl_pressed(null);

        if (alt_pressed && !alt_currently_pressed)
            on_alt_released(null);
        else if (!alt_pressed && alt_currently_pressed)
            on_alt_pressed(null);

        if (shift_pressed && !shift_currently_pressed)
            on_shift_released(null);
        else if (!shift_pressed && shift_currently_pressed)
            on_shift_pressed(null);

        if(super_pressed && !super_currently_pressed)
            on_super_released(null);
        else if (!super_pressed && super_currently_pressed)
            on_super_pressed(null);
        
        ctrl_pressed = ctrl_currently_pressed;
        alt_pressed = alt_currently_pressed;
        shift_pressed = shift_currently_pressed;
        super_pressed = super_currently_pressed;
    }
    
    public PageWindow? get_page_window() {
        Gtk.Widget p = parent;
        while (p != null) {
            if (p is PageWindow)
                return (PageWindow) p;
            
            p = p.parent;
        }
        
        return null;
    }
    
    public CommandManager get_command_manager() {
        return AppWindow.get_command_manager();
    }

    protected virtual void add_actions (GLib.ActionMap map) { }
    protected virtual void remove_actions (GLib.ActionMap map) { }

    protected void on_action_toggle (GLib.Action action, Variant? value) {
        Variant new_state = ! (bool) action.get_state ();
        action.change_state (new_state);
    }

    protected void on_action_radio (GLib.Action action, Variant? value) {
        action.change_state (value);
    }

    private void add_ui() {
        // Collect all UI filenames and load them into the UI manager
        Gee.List<string> ui_filenames = new Gee.ArrayList<string>();
        init_collect_ui_filenames(ui_filenames);
        if (ui_filenames.size == 0)
            message("No UI file specified for %s", get_page_name());
        
        foreach (string ui_filename in ui_filenames)
            init_load_ui(ui_filename);

        //ui.insert_action_group(action_group, 0);
    }

    public void init_toolbar(string path) {
        toolbar_path = path;
    }
   
    // Called from "realize"
    private void attach_view_signals() {
        if (are_actions_attached)
            return;
        
        // initialize the Gtk.Actions according to current state
        int selected_count = get_view().get_selected_count();
        int count = get_view().get_count();
        init_actions(selected_count, count);
        update_actions(selected_count, count);
        
        // monitor state changes to update actions
        get_view().items_state_changed.connect(on_update_actions);
        get_view().selection_group_altered.connect(on_update_actions);
        get_view().items_visibility_changed.connect(on_update_actions);
        get_view().contents_altered.connect(on_update_actions);
        
        are_actions_attached = true;
    }
    
    // Called from destroy()
    private void detach_view_signals() {
        if (!are_actions_attached)
            return;
        
        get_view().items_state_changed.disconnect(on_update_actions);
        get_view().selection_group_altered.disconnect(on_update_actions);
        get_view().items_visibility_changed.disconnect(on_update_actions);
        get_view().contents_altered.disconnect(on_update_actions);
        
        are_actions_attached = false;
    }
    
    private void on_update_actions() {
        if (update_actions_scheduler == null) {
            update_actions_scheduler = new OneShotScheduler(
                "Update actions scheduler for %s".printf(get_page_name()),
                on_update_actions_on_idle);
        }
        
        update_actions_scheduler.at_priority_idle(Priority.LOW);
    }
    
    private void on_update_actions_on_idle() {
        if (is_destroyed)
            return;

        if (!this.in_view)
            return;
        
        update_actions(get_view().get_selected_count(), get_view().get_count());
    }
    
    private void init_load_ui(string ui_filename) {
        var ui_resource = Resources.get_ui(ui_filename);
        try {
            builder.add_from_resource(ui_resource);
            this.menubar_injected = false;
        } catch (Error err) {
            AppWindow.error_message("Error loading UI resource %s: %s".printf(
                ui_resource, err.message));
            Application.get_instance().panic();
        }
    }
    
    // This is called during add_ui() to collect all the UI files to be loaded into the UI
    // manager.  Because order is important here, call the base method *first*, then add the
    // classes' filename.
    protected virtual void init_collect_ui_filenames(Gee.List<string> ui_filenames) {
    }

    // This is called during add_ui() to collect all Page.InjectedUIElements for the page.  They
    // should be added to the MultiSet using the injection path as the key.
    protected virtual InjectionGroup[] init_collect_injection_groups() {
        return new InjectionGroup[0];
    }
    
    // This is called during "map" allowing for Gtk.Actions to be updated at
    // initialization time.
    protected virtual void init_actions(int selected_count, int count) {
    }
    
    // This is called during "map" and during ViewCollection selection, visibility,
    // and collection content altered events.  This can be used to both initialize Gtk.Actions and
    // update them when selection or visibility has been altered.
    protected virtual void update_actions(int selected_count, int count) {
    }
    
    // This method enables drag-and-drop on the event source and routes its events through this
    // object
    public void enable_drag_source(Gdk.DragAction actions, Gtk.TargetEntry[] source_target_entries) {
        if (dnd_enabled)
            return;
            
        assert(event_source != null);
        
        Gtk.drag_source_set(event_source, Gdk.ModifierType.BUTTON1_MASK, source_target_entries, actions);
        
        // hook up handlers which route the event_source's DnD signals to the Page's (necessary
        // because Page is a NO_WINDOW widget and cannot support DnD on its own).
        event_source.drag_begin.connect(on_drag_begin);
        event_source.drag_data_get.connect(on_drag_data_get);
        event_source.drag_data_delete.connect(on_drag_data_delete);
        event_source.drag_end.connect(on_drag_end);
        event_source.drag_failed.connect(on_drag_failed);
        
        dnd_enabled = true;
    }
    
    public void disable_drag_source() {
        if (!dnd_enabled)
            return;

        assert(event_source != null);
        
        event_source.drag_begin.disconnect(on_drag_begin);
        event_source.drag_data_get.disconnect(on_drag_data_get);
        event_source.drag_data_delete.disconnect(on_drag_data_delete);
        event_source.drag_end.disconnect(on_drag_end);
        event_source.drag_failed.disconnect(on_drag_failed);
        Gtk.drag_source_unset(event_source);
        
        dnd_enabled = false;
    }
    
    public bool is_dnd_enabled() {
        return dnd_enabled;
    }
    
    private void on_drag_begin(Gdk.DragContext context) {
        drag_begin(context);
    }
    
    private void on_drag_data_get(Gdk.DragContext context, Gtk.SelectionData selection_data,
        uint info, uint time) {
        drag_data_get(context, selection_data, info, time);
    }
    
    private void on_drag_data_delete(Gdk.DragContext context) {
        drag_data_delete(context);
    }
    
    private void on_drag_end(Gdk.DragContext context) {
        drag_end(context);
    }
    
    // wierdly, Gtk 2.16.1 doesn't supply a drag_failed virtual method in the GtkWidget impl ...
    // Vala binds to it, but it's not available in gtkwidget.h, and so gcc complains.  Have to
    // makeshift one for now.
    // https://bugzilla.gnome.org/show_bug.cgi?id=584247
    public virtual bool source_drag_failed(Gdk.DragContext context, Gtk.DragResult drag_result) {
        return false;
    }
    
    private bool on_drag_failed(Gdk.DragContext context, Gtk.DragResult drag_result) {
        return source_drag_failed(context, drag_result);
    }
    
    // Use this function rather than GDK or GTK's get_pointer, especially if called during a 
    // button-down mouse drag (i.e. a window grab).
    //
    // For more information, see: https://bugzilla.gnome.org/show_bug.cgi?id=599937
    public bool get_event_source_pointer(out int x, out int y, out Gdk.ModifierType mask) {
        if (event_source == null) {
            x = 0;
            y = 0;
            mask = 0;
            
            return false;
        }
        
        var seat = Gdk.Display.get_default().get_default_seat();
        event_source.get_window().get_device_position(seat.get_pointer(), out x, out y, out mask);
        
        if (last_down.x < 0 || last_down.y < 0)
            return true;
            
        // check for bogus values inside a drag which goes outside the window
        // caused by (most likely) X windows signed 16-bit int overflow and fixup
        // (https://bugzilla.gnome.org/show_bug.cgi?id=599937)
        
        if ((x - last_down.x).abs() >= 0x7FFF)
            x += 0xFFFF;
        
        if ((y - last_down.y).abs() >= 0x7FFF)
            y += 0xFFFF;
        
        return true;
    }
    
    protected virtual bool on_left_click(Gdk.EventButton event) {
        return false;
    }
    
    protected virtual bool on_middle_click(Gdk.EventButton event) {
        return false;
    }
    
    protected virtual bool on_right_click(Gdk.EventButton event) {
        return false;
    }
    
    protected virtual bool on_left_released(Gdk.EventButton event) {
        return false;
    }
    
    protected virtual bool on_middle_released(Gdk.EventButton event) {
        return false;
    }
    
    protected virtual bool on_right_released(Gdk.EventButton event) {
        return false;
    }
    
    private bool on_button_pressed_internal(Gdk.EventButton event) {
        switch (event.button) {
            case 1:
                if (event_source != null)
                    event_source.grab_focus();
                
                // stash location of mouse down for drag fixups
                last_down.x = (int) event.x;
                last_down.y = (int) event.y;
                
                return on_left_click(event);

            case 2:
                return on_middle_click(event);
            
            case 3:
                return on_right_click(event);
            
            default:
                return false;
        }
    }
    
    private bool on_button_released_internal(Gdk.EventButton event) {
        switch (event.button) {
            case 1:
                // clear when button released, only for drag fixups
                last_down = { -1, -1 };
                
                return on_left_released(event);
            
            case 2:
                return on_middle_released(event);
            
            case 3:
                return on_right_released(event);
            
            default:
                return false;
        }
    }

    protected virtual bool on_ctrl_pressed(Gdk.EventKey? event) {
        return false;
    }
    
    protected virtual bool on_ctrl_released(Gdk.EventKey? event) {
        return false;
    }
    
    protected virtual bool on_alt_pressed(Gdk.EventKey? event) {
        return false;
    }
    
    protected virtual bool on_alt_released(Gdk.EventKey? event) {
        return false;
    }
    
    protected virtual bool on_shift_pressed(Gdk.EventKey? event) {
        return false;
    }
    
    protected virtual bool on_shift_released(Gdk.EventKey? event) {
        return false;
    }

    protected virtual bool on_super_pressed(Gdk.EventKey? event) {
        return false;
    }
    
    protected virtual bool on_super_released(Gdk.EventKey? event) {
        return false;
    }
    
    protected virtual bool on_app_key_pressed(Gdk.EventKey event) {
        return false;
    }
    
    protected virtual bool on_app_key_released(Gdk.EventKey event) {
        return false;
    }
    
    public bool notify_app_key_pressed(Gdk.EventKey event) {
        bool ctrl_currently_pressed, alt_currently_pressed, shift_currently_pressed,
            super_currently_pressed;
        get_modifiers(out ctrl_currently_pressed, out alt_currently_pressed,
            out shift_currently_pressed, out super_currently_pressed);

        switch (Gdk.keyval_name(event.keyval)) {
            case "Control_L":
            case "Control_R":
                if (!ctrl_currently_pressed || ctrl_pressed)
                    return false;

                ctrl_pressed = true;
                
                return on_ctrl_pressed(event);

            case "Meta_L":
            case "Meta_R":
            case "Alt_L":
            case "Alt_R":
                if (!alt_currently_pressed || alt_pressed)
                    return false;

                alt_pressed = true;
                
                return on_alt_pressed(event);
            
            case "Shift_L":
            case "Shift_R":
                if (!shift_currently_pressed || shift_pressed)
                    return false;

                shift_pressed = true;
                
                return on_shift_pressed(event);
            
            case "Super_L":
            case "Super_R":
                if (!super_currently_pressed || super_pressed)
                    return false;
                
                super_pressed = true;
                
                return on_super_pressed(event);
        }
        
        return on_app_key_pressed(event);
    }
    
    public bool notify_app_key_released(Gdk.EventKey event) {
        bool ctrl_currently_pressed, alt_currently_pressed, shift_currently_pressed,
            super_currently_pressed;
        get_modifiers(out ctrl_currently_pressed, out alt_currently_pressed,
            out shift_currently_pressed, out super_currently_pressed);

        switch (Gdk.keyval_name(event.keyval)) {
            case "Control_L":
            case "Control_R":
                if (ctrl_currently_pressed || !ctrl_pressed)
                    return false;

                ctrl_pressed = false;
                
                return on_ctrl_released(event);
            
            case "Meta_L":
            case "Meta_R":
            case "Alt_L":
            case "Alt_R":
                if (alt_currently_pressed || !alt_pressed)
                    return false;

                alt_pressed = false;
                
                return on_alt_released(event);
            
            case "Shift_L":
            case "Shift_R":
                if (shift_currently_pressed || !shift_pressed)
                    return false;

                shift_pressed = false;
                
                return on_shift_released(event);

            case "Super_L":
            case "Super_R":
                if (super_currently_pressed || !super_pressed)
                    return false;

                super_pressed = false;
                
                return on_super_released(event);
        }
        
        return on_app_key_released(event);
    }
    
    public bool notify_app_focus_in(Gdk.EventFocus event) {
        update_modifiers();
        
        return false;
    }

    public bool notify_app_focus_out(Gdk.EventFocus event) {
        return false;
    }
    
    protected virtual void on_move(Gdk.Rectangle rect) {
    }
    
    protected virtual void on_move_start(Gdk.Rectangle rect) {
    }
    
    protected virtual void on_move_finished(Gdk.Rectangle rect) {
    }
    
    protected virtual void on_resize(Gdk.Rectangle rect) {
    }
    
    protected virtual void on_resize_start(Gdk.Rectangle rect) {
    }
    
    protected virtual void on_resize_finished(Gdk.Rectangle rect) {
    }
    
    protected virtual bool on_configure(Gdk.EventConfigure event, Gdk.Rectangle rect) {
        return false;
    }
    
    public bool notify_configure_event(Gdk.EventConfigure event) {
        Gdk.Rectangle rect = Gdk.Rectangle();
        rect.x = event.x;
        rect.y = event.y;
        rect.width = event.width;
        rect.height = event.height;
        
        // special case events, to report when a configure first starts (and appears to end)
        if (last_configure_ms == 0) {
            if (last_position.x != rect.x || last_position.y != rect.y) {
                on_move_start(rect);
                report_move_finished = true;
            }
            
            if (last_position.width != rect.width || last_position.height != rect.height) {
                on_resize_start(rect);
                report_resize_finished = true;
            }

            // need to check more often then the timeout, otherwise it could be up to twice the
            // wait time before it's noticed
            Timeout.add(CONSIDER_CONFIGURE_HALTED_MSEC / 8, check_configure_halted);
        }
        
        if (last_position.x != rect.x || last_position.y != rect.y)
            on_move(rect);
        
        if (last_position.width != rect.width || last_position.height != rect.height)
            on_resize(rect);
        
        last_position = rect;
        last_configure_ms = now_ms();

        return on_configure(event, rect);
    }
    
    private bool check_configure_halted() {
        if (is_destroyed)
            return false;

        if ((now_ms() - last_configure_ms) < CONSIDER_CONFIGURE_HALTED_MSEC)
            return true;
        
        Gtk.Allocation allocation;
        get_allocation(out allocation);
        
        if (report_move_finished)
            on_move_finished((Gdk.Rectangle) allocation);
        
        if (report_resize_finished)
            on_resize_finished((Gdk.Rectangle) allocation);
        
        last_configure_ms = 0;
        report_move_finished = false;
        report_resize_finished = false;
        
        return false;
    }
    
    protected virtual bool on_motion(Gdk.EventMotion event, int x, int y, Gdk.ModifierType mask) {
        check_cursor_hiding();

        return false;
    }
    
    protected virtual bool on_leave_notify_event() {
        return false;
    }
    
    private bool on_motion_internal(Gdk.EventMotion event) {
        int x, y;
        Gdk.ModifierType mask;
        if (event.is_hint == 1) {
            get_event_source_pointer(out x, out y, out mask);
        } else {
            x = (int) event.x;
            y = (int) event.y;
            mask = event.state;
        }
        
        return on_motion(event, x, y, mask);
    }

    private bool on_mousewheel_internal(Gdk.EventScroll event) {
        switch (event.direction) {
            case Gdk.ScrollDirection.UP:
                return on_mousewheel_up(event);

            case Gdk.ScrollDirection.DOWN:
                return on_mousewheel_down(event);
            
            case Gdk.ScrollDirection.LEFT:
                return on_mousewheel_left(event);

            case Gdk.ScrollDirection.RIGHT:
                return on_mousewheel_right(event);

            case Gdk.ScrollDirection.SMOOTH:
                {
                    double dx, dy;
                    event.get_scroll_deltas(out dx, out dy);

                    if (dy < 0)
                        return on_mousewheel_up(event);
                    else if (dy > 0)
                        return on_mousewheel_down(event);
                    else if (dx < 0)
                        return on_mousewheel_left(event);
                    else if (dx > 0)
                        return on_mousewheel_right(event);
                    else
                        return false;
                }
           
            default:
                return false;
        }
    }
    
    protected virtual bool on_mousewheel_up(Gdk.EventScroll event) {
        return false;
    }
    
    protected virtual bool on_mousewheel_down(Gdk.EventScroll event) {
        return false;
    }
    
    protected virtual bool on_mousewheel_left(Gdk.EventScroll event) {
        return false;
    }
    
    protected virtual bool on_mousewheel_right(Gdk.EventScroll event) {
        return false;
    }
    
    protected virtual bool on_context_keypress() {
        return false;
    }
    
    protected virtual bool on_context_buttonpress(Gdk.EventButton event) {
        return false;
    }
    
    protected virtual bool on_context_invoked() {
        return true;
    }

    protected bool popup_context_menu(Gtk.Menu? context_menu,
        Gdk.EventButton? event = null) {

        if (context_menu == null || !on_context_invoked())
            return false;

        context_menu.popup_at_pointer(event);

        return true;
    }

    protected void on_event_source_realize() {
        assert(event_source.get_window() != null); // the realize event means the Widget has a window

        if (event_source.get_window().get_cursor() != null) {
            last_cursor = event_source.get_window().get_cursor().get_cursor_type();
            return;
        }

        // no custom cursor defined, check parents
        Gdk.Window? parent_window = event_source.get_window();
        do {
            parent_window = parent_window.get_parent();
        } while (parent_window != null && parent_window.get_cursor() == null);
        
        if (parent_window != null)
            last_cursor = parent_window.get_cursor().get_cursor_type();
    }

    public void set_cursor_hide_time(int hide_time) {
        cursor_hide_msec = hide_time;
    }

    public void start_cursor_hiding() {
        check_cursor_hiding();
    }

    public void stop_cursor_hiding() {
        if (last_timeout_id != 0)
            Source.remove(last_timeout_id);
    }

    public void suspend_cursor_hiding() {
        cursor_hide_time_cached = cursor_hide_msec;

        if (last_timeout_id != 0)
            Source.remove(last_timeout_id);

        cursor_hide_msec = 0;
    }

    public void restore_cursor_hiding() {
        cursor_hide_msec = cursor_hide_time_cached;
        check_cursor_hiding();
    }

    // Use this method to set the cursor for a page, NOT window.set_cursor(...)
    protected virtual void set_page_cursor(Gdk.CursorType cursor_type) {
        last_cursor = cursor_type;

        if (!cursor_hidden && event_source != null) {
            var display = event_source.get_window ().get_display ();
            event_source.get_window().set_cursor(new Gdk.Cursor.for_display(display, cursor_type));
        }
    }

    private void check_cursor_hiding() {
        if (cursor_hidden) {
            cursor_hidden = false;
            set_page_cursor(last_cursor);
        }

        if (cursor_hide_msec != 0) {
            if (last_timeout_id != 0)
                Source.remove(last_timeout_id);
            last_timeout_id = Timeout.add(cursor_hide_msec, on_hide_cursor);
        }
    }

    private bool on_hide_cursor() {
        cursor_hidden = true;

        if (event_source != null) {
            var display = event_source.get_window().get_display ();
            event_source.get_window().set_cursor(new Gdk.Cursor.for_display(display, Gdk.CursorType.BLANK_CURSOR));
        }

        // We remove the timeout so reset the id
        last_timeout_id = 0;

        return false;
    }

    protected void update_menu_item_label (string id,
                                         string new_label) {
        AppWindow.get_instance().update_menu_item_label (id, new_label);
    }

    protected GLib.MenuModel? find_extension_point (GLib.MenuModel model,
                                                    string extension_point) {
        var items = model.get_n_items ();
        GLib.MenuModel? section = null;

        for (int i = 0; i < items && section == null; i++) {
            string? name = null;
            model.get_item_attribute (i, "id", "s", out name);
            if (name == extension_point) {
                section = model.get_item_link (i, GLib.Menu.LINK_SECTION);
            } else {
                var subsection = model.get_item_link (i, GLib.Menu.LINK_SECTION);

                if (subsection == null)
                    continue;

                // Recurse into submenus
                var sub_items = subsection.get_n_items ();
                for (int j = 0; j < sub_items && section == null; j++) {
                    var submenu = subsection.get_item_link
                                                (j, GLib.Menu.LINK_SUBMENU);
                    if (submenu != null) {
                        section = this.find_extension_point (submenu,
                                                             extension_point);
                    }
                }
            }
        }

        return section;
    }

}

public abstract class CheckerboardPage : Page {
    private const int AUTOSCROLL_PIXELS = 50;
    private const int AUTOSCROLL_TICKS_MSEC = 50;
    
    private CheckerboardLayout layout;
    private string item_context_menu_path = null;
    private string page_context_menu_path = null;
    private Gtk.Viewport viewport = new Gtk.Viewport(null, null);
    protected CheckerboardItem anchor = null;
    protected CheckerboardItem cursor = null;
    private CheckerboardItem current_hovered_item = null;
    private bool autoscroll_scheduled = false;
    private CheckerboardItem activated_item = null;
    private Gee.ArrayList<CheckerboardItem> previously_selected = null;

    public enum Activator {
        KEYBOARD,
        MOUSE
    }

    public struct KeyboardModifiers {
        public KeyboardModifiers(Page page) {
            ctrl_pressed = page.get_ctrl_pressed();
            alt_pressed = page.get_alt_pressed();
            shift_pressed = page.get_shift_pressed();
            super_pressed = page.get_super_pressed();
        }

        public bool ctrl_pressed;
        public bool alt_pressed;
        public bool shift_pressed;
        public bool super_pressed;
    }

    protected CheckerboardPage(string page_name) {
        base (page_name);
        
        layout = new CheckerboardLayout(get_view());
        layout.set_name(page_name);
        
        set_event_source(layout);

        set_border_width(0);
        set_shadow_type(Gtk.ShadowType.NONE);
        
        viewport.set_border_width(0);
        viewport.set_shadow_type(Gtk.ShadowType.NONE);
        
        viewport.add(layout);
        
        // want to set_adjustments before adding to ScrolledWindow to let our signal handlers
        // run first ... otherwise, the thumbnails draw late
        layout.set_adjustments(get_hadjustment(), get_vadjustment());
        
        add(viewport);
        
        // need to monitor items going hidden when dealing with anchor/cursor/highlighted items
        get_view().items_hidden.connect(on_items_hidden);
        get_view().contents_altered.connect(on_contents_altered);
        get_view().items_state_changed.connect(on_items_state_changed);
        get_view().items_visibility_changed.connect(on_items_visibility_changed);
        
        // scrollbar policy
        set_policy(Gtk.PolicyType.AUTOMATIC, Gtk.PolicyType.AUTOMATIC);
    }
    
    public void init_item_context_menu(string path) {
        item_context_menu_path = path;
    }

    public void init_page_context_menu(string path) {
        page_context_menu_path = path;
    }
    
    public Gtk.Menu? get_context_menu() {
        // show page context menu if nothing is selected
        return (get_view().get_selected_count() != 0) ? get_item_context_menu() :
            get_page_context_menu();
    }
    
    private Gtk.Menu item_context_menu;
    public virtual Gtk.Menu? get_item_context_menu() {
        if (item_context_menu == null) {
            var model = this.builder.get_object (item_context_menu_path)
                as GLib.MenuModel;
            item_context_menu = new Gtk.Menu.from_model (model);
            item_context_menu.attach_to_widget (this, null);
        }

        return item_context_menu;
    }
    
    private Gtk.Menu page_context_menu;
    public override Gtk.Menu? get_page_context_menu() {
        if (page_context_menu_path == null)
            return null;

        if (page_context_menu == null) {
            var model = this.builder.get_object (page_context_menu_path)
                as GLib.MenuModel;
            page_context_menu = new Gtk.Menu.from_model (model);
            page_context_menu.attach_to_widget (this, null);
        }

        return page_context_menu;
    }
    
    protected override bool on_context_keypress() {
        return popup_context_menu(get_context_menu());
    }
    
    protected virtual string get_view_empty_message() {
        return _("No photos/videos");
    }

    protected virtual string get_filter_no_match_message() {
        return _("No photos/videos found which match the current filter");
    }

    protected virtual void on_item_activated(CheckerboardItem item, Activator activator, 
        KeyboardModifiers modifiers) {
    }
    
    public CheckerboardLayout get_checkerboard_layout() {
        return layout;
    }
    
    // Gets the search view filter for this page.
    public abstract SearchViewFilter get_search_view_filter();
    
    public virtual Core.ViewTracker? get_view_tracker() {
        return null;
    }
    
    public override void switching_from() {
        layout.set_in_view(false);
        get_search_view_filter().refresh.disconnect(on_view_filter_refresh);

        // unselect everything so selection won't persist after page loses focus 
        get_view().unselect_all();
        
        base.switching_from();
    }
    
    public override void switched_to() {
        layout.set_in_view(true);
        get_search_view_filter().refresh.connect(on_view_filter_refresh);
        on_view_filter_refresh();

        if (get_view().get_selected_count() > 0) {
            CheckerboardItem? item = (CheckerboardItem?) get_view().get_selected_at(0);

            // if item is in any way out of view, scroll to it
            Gtk.Adjustment vadj = get_vadjustment();
            if (!(get_adjustment_relation(vadj, item.allocation.y) == AdjustmentRelation.IN_RANGE
                && (get_adjustment_relation(vadj, item.allocation.y + item.allocation.height) == AdjustmentRelation.IN_RANGE))) {

                    // scroll to see the new item
                    int top = 0;
                    if (item.allocation.y < vadj.get_value()) {
                        top = item.allocation.y;
                        top -= CheckerboardLayout.ROW_GUTTER_PADDING / 2;
                    } else {
                        top = item.allocation.y + item.allocation.height - (int) vadj.get_page_size();
                        top += CheckerboardLayout.ROW_GUTTER_PADDING / 2;
                    }

                    vadj.set_value(top);

                }
        }

        base.switched_to();
    }
    
    private void on_view_filter_refresh() {
        update_view_filter_message();
    }
    
    private void on_contents_altered(Gee.Iterable<DataObject>? added,
        Gee.Iterable<DataObject>? removed) {
        update_view_filter_message();
    }
    
    private void on_items_state_changed(Gee.Iterable<DataView> changed) {
        update_view_filter_message();
    }
    
    private void on_items_visibility_changed(Gee.Collection<DataView> changed) {
        update_view_filter_message();
    }
    
    private void update_view_filter_message() {
        if (get_view().are_items_filtered_out() && get_view().get_count() == 0) {
            set_page_message(get_filter_no_match_message());
        } else if (get_view().get_count() == 0) {
            set_page_message(get_view_empty_message());
        } else {
            unset_page_message();
        }
    }
    
    public void set_page_message(string message) {
        layout.set_message(message);
        if (is_in_view())
            layout.queue_draw();
    }
    
    public void unset_page_message() {
        layout.unset_message();
        if (is_in_view())
            layout.queue_draw();
    }
    
    public override void set_page_name(string name) {
        base.set_page_name(name);
        
        layout.set_name(name);
    }
    
    public CheckerboardItem? get_item_at_pixel(double x, double y) {
        return layout.get_item_at_pixel(x, y);
    }
    
    private void on_items_hidden(Gee.Iterable<DataView> hidden) {
        foreach (DataView view in hidden) {
            CheckerboardItem item = (CheckerboardItem) view;
            
            if (anchor == item)
                anchor = null;
            
            if (cursor == item)
                cursor = null;
            
            if (current_hovered_item == item)
                current_hovered_item = null;
        }
    }

    protected override bool key_press_event(Gdk.EventKey event) {
        bool handled = true;

        // mask out the modifiers we're interested in
        uint state = event.state & Gdk.ModifierType.SHIFT_MASK;
        
        switch (Gdk.keyval_name(event.keyval)) {
            case "Up":
            case "KP_Up":
                move_cursor(CompassPoint.NORTH);
                select_anchor_to_cursor(state);
            break;
            
            case "Down":
            case "KP_Down":
                move_cursor(CompassPoint.SOUTH);
                select_anchor_to_cursor(state);
            break;
            
            case "Left":
            case "KP_Left":
                move_cursor(CompassPoint.WEST);
                select_anchor_to_cursor(state);
            break;
            
            case "Right":
            case "KP_Right":
                move_cursor(CompassPoint.EAST);
                select_anchor_to_cursor(state);
            break;
            
            case "Home":
            case "KP_Home":
                CheckerboardItem? first = (CheckerboardItem?) get_view().get_first();
                if (first != null)
                    cursor_to_item(first);
                select_anchor_to_cursor(state);
            break;
            
            case "End":
            case "KP_End":
                CheckerboardItem? last = (CheckerboardItem?) get_view().get_last();
                if (last != null)
                    cursor_to_item(last);
                select_anchor_to_cursor(state);
            break;
            
            case "Return":
            case "KP_Enter":
                if (get_view().get_selected_count() == 1)
                    on_item_activated((CheckerboardItem) get_view().get_selected_at(0),
                        Activator.KEYBOARD, KeyboardModifiers(this));
                else
                    handled = false;
            break;
            
            case "space":
                Marker marker = get_view().mark(layout.get_cursor());
                get_view().toggle_marked(marker);
            break;
            
            default:
                handled = false;
            break;
        }
        
        if (handled)
            return true;
        
        return (base.key_press_event != null) ? base.key_press_event(event) : true;
    }
    
    protected override bool on_left_click(Gdk.EventButton event) {
        // only interested in single-click and double-clicks for now
        if ((event.type != Gdk.EventType.BUTTON_PRESS) && (event.type != Gdk.EventType.2BUTTON_PRESS))
            return false;
        
        // mask out the modifiers we're interested in
        uint state = event.state & (Gdk.ModifierType.CONTROL_MASK | Gdk.ModifierType.SHIFT_MASK);
        
        // use clicks for multiple selection and activation only; single selects are handled by
        // button release, to allow for multiple items to be selected then dragged ...
        CheckerboardItem item = get_item_at_pixel(event.x, event.y);
        if (item != null) {
            // ... however, there is no dragging if the user clicks on an interactive part of the
            // CheckerboardItem (e.g. a tag)
            if (layout.handle_left_click(item, event.x, event.y, event.state))
                return true;

            switch (state) {
                case Gdk.ModifierType.CONTROL_MASK:
                    // with only Ctrl pressed, multiple selections are possible ... chosen item
                    // is toggled
                    Marker marker = get_view().mark(item);
                    get_view().toggle_marked(marker);

                    if (item.is_selected()) {
                        anchor = item;
                        cursor = item;
                    }
                break;
                
                case Gdk.ModifierType.SHIFT_MASK:
                    get_view().unselect_all();
                    
                    if (anchor == null)
                        anchor = item;
                    
                    select_between_items(anchor, item);
                    
                    cursor = item;
                break;
                
                case Gdk.ModifierType.CONTROL_MASK | Gdk.ModifierType.SHIFT_MASK:
                    // Ticket #853 - Make Ctrl + Shift + Mouse Button 1 able to start a new run
                    // of contiguous selected items without unselecting previously-selected items
                    // a la Nautilus.
                    // Same as the case for SHIFT_MASK, but don't unselect anything first.
                    if (anchor == null)
                        anchor = item;
                    
                    select_between_items(anchor, item);
                    
                    cursor = item;
                break;
                
                default:
                    if (event.type == Gdk.EventType.2BUTTON_PRESS) {
                        activated_item = item;
                    } else {
                        // if the user has selected one or more items and is preparing for a drag,
                        // don't want to blindly unselect: if they've clicked on an unselected item
                        // unselect all and select that one; if they've clicked on a previously
                        // selected item, do nothing
                        if (!item.is_selected()) {
                            Marker all = get_view().start_marking();
                            all.mark_many(get_view().get_selected());
                            
                            get_view().unselect_and_select_marked(all, get_view().mark(item));
                        }
                    }

                    anchor = item;
                    cursor = item;
                break;
            }
            layout.set_cursor(item);
        } else {
            // user clicked on "dead" area; only unselect if control is not pressed
            // do we want similar behavior for shift as well?
            if (state != Gdk.ModifierType.CONTROL_MASK)
                get_view().unselect_all();

            // grab previously marked items
            previously_selected = new Gee.ArrayList<CheckerboardItem>();
            foreach (DataView view in get_view().get_selected())
                previously_selected.add((CheckerboardItem) view);

            layout.set_drag_select_origin((int) event.x, (int) event.y);

            return true;
        }

        // need to determine if the signal should be passed to the DnD handlers
        // Return true to block the DnD handler, false otherwise

        return get_view().get_selected_count() == 0;
    }
    
    protected override bool on_left_released(Gdk.EventButton event) {
        previously_selected = null;

        // if drag-selecting, stop here and do nothing else
        if (layout.is_drag_select_active()) {
            layout.clear_drag_select();
            anchor = cursor;

            return true;
        }
        
        // only interested in non-modified button releases
        if ((event.state & (Gdk.ModifierType.CONTROL_MASK | Gdk.ModifierType.SHIFT_MASK)) != 0)
            return false;
        
        // if the item was activated in the double-click, report it now
        if (activated_item != null) {
            on_item_activated(activated_item, Activator.MOUSE, KeyboardModifiers(this));
            activated_item = null;
            
            return true;
        }
        
        CheckerboardItem item = get_item_at_pixel(event.x, event.y);
        if (item == null) {
            // released button on "dead" area
            return true;
        }

        if (cursor != item) {
            // user released mouse button after moving it off the initial item, or moved from dead
            // space onto one.  either way, unselect everything
            get_view().unselect_all();
        } else {
            // the idea is, if a user single-clicks on an item with no modifiers, then all other items
            // should be deselected, however, if they single-click in order to drag one or more items,
            // they should remain selected, hence performing this here rather than on_left_click
            // (item may not be selected if an unimplemented modifier key was used)
            if (item.is_selected())
                get_view().unselect_all_but(item);
        }

        return true;
    }
    
    protected override bool on_right_click(Gdk.EventButton event) {
        // only interested in single-clicks for now
        if (event.type != Gdk.EventType.BUTTON_PRESS)
            return false;
        
        // get what's right-clicked upon
        CheckerboardItem item = get_item_at_pixel(event.x, event.y);
        if (item != null) {
            // mask out the modifiers we're interested in
            switch (event.state & (Gdk.ModifierType.CONTROL_MASK | Gdk.ModifierType.SHIFT_MASK)) {
                case Gdk.ModifierType.CONTROL_MASK:
                    // chosen item is toggled
                    Marker marker = get_view().mark(item);
                    get_view().toggle_marked(marker);
                break;
                
                case Gdk.ModifierType.SHIFT_MASK:
                    // TODO
                break;
                
                case Gdk.ModifierType.CONTROL_MASK | Gdk.ModifierType.SHIFT_MASK:
                    // TODO
                break;
                
                default:
                    // if the item is already selected, proceed; if item is not selected, a bare right
                    // click unselects everything else but it
                    if (!item.is_selected()) {
                        Marker all = get_view().start_marking();
                        all.mark_many(get_view().get_selected());
                        
                        get_view().unselect_and_select_marked(all, get_view().mark(item));
                    }
                break;
            }
        } else {
            // clicked in "dead" space, unselect everything
            get_view().unselect_all();
        }
       
        Gtk.Menu context_menu = get_context_menu();
        return popup_context_menu(context_menu, event);
    }
    
    protected virtual bool on_mouse_over(CheckerboardItem? item, int x, int y, Gdk.ModifierType mask) {
        if (item != null)
            layout.handle_mouse_motion(item, x, y, mask);

        // if hovering over the last hovered item, or both are null (nothing highlighted and
        // hovering over empty space), do nothing
        if (item == current_hovered_item)
            return true;
        
        // either something new is highlighted or now hovering over empty space, so dim old item
        if (current_hovered_item != null) {
            current_hovered_item.handle_mouse_leave();
            current_hovered_item = null;
        }
        
        // if over empty space, done
        if (item == null)
            return true;
        
        // brighten the new item
        current_hovered_item = item;
        current_hovered_item.handle_mouse_enter();
        
        return true;
    }
    
    protected override bool on_motion(Gdk.EventMotion event, int x, int y, Gdk.ModifierType mask) {
        // report what item the mouse is hovering over
        if (!on_mouse_over(get_item_at_pixel(x, y), x, y, mask))
            return false;
        
        // go no further if not drag-selecting
        if (!layout.is_drag_select_active())
            return false;
        
        // set the new endpoint of the drag selection
        layout.set_drag_select_endpoint(x, y);
        
        updated_selection_band();

        // if out of bounds, schedule a check to auto-scroll the viewport
        if (!autoscroll_scheduled 
            && get_adjustment_relation(get_vadjustment(), y) != AdjustmentRelation.IN_RANGE) {
            Timeout.add(AUTOSCROLL_TICKS_MSEC, selection_autoscroll);
            autoscroll_scheduled = true;
        }

        // return true to stop a potential drag-and-drop operation
        return true;
    }
    
    private void updated_selection_band() {
        assert(layout.is_drag_select_active());
        
        // get all items inside the selection
        Gee.List<CheckerboardItem>? intersection = layout.items_in_selection_band();
        if (intersection == null)
            return;
        
        Marker to_unselect = get_view().start_marking();
        Marker to_select = get_view().start_marking();

        // mark all selected items to be unselected
        to_unselect.mark_many(get_view().get_selected());

        // except for the items that were selected before the drag began
        assert(previously_selected != null);
        to_unselect.unmark_many(previously_selected);        
        to_select.mark_many(previously_selected);   
        
        // toggle selection on everything in the intersection and update the cursor
        cursor = null;
        
        foreach (CheckerboardItem item in intersection) {
            if (to_select.toggle(item))
                to_unselect.unmark(item);
            else
                to_unselect.mark(item);

            if (cursor == null)
                cursor = item;
        }
        
        get_view().select_marked(to_select);
        get_view().unselect_marked(to_unselect);
    }
    
    private bool selection_autoscroll() {
        if (!layout.is_drag_select_active()) { 
            autoscroll_scheduled = false;
            
            return false;
        }
        
        // as the viewport never scrolls horizontally, only interested in vertical
        Gtk.Adjustment vadj = get_vadjustment();
        
        int x, y;
        Gdk.ModifierType mask;
        get_event_source_pointer(out x, out y, out mask);
        
        int new_value = (int) vadj.get_value();
        switch (get_adjustment_relation(vadj, y)) {
            case AdjustmentRelation.BELOW:
                // pointer above window, scroll up
                new_value -= AUTOSCROLL_PIXELS;
                layout.set_drag_select_endpoint(x, new_value);
            break;
            
            case AdjustmentRelation.ABOVE:
                // pointer below window, scroll down, extend selection to bottom of page
                new_value += AUTOSCROLL_PIXELS;
                layout.set_drag_select_endpoint(x, new_value + (int) vadj.get_page_size());
            break;
            
            case AdjustmentRelation.IN_RANGE:
                autoscroll_scheduled = false;
                
                return false;
            
            default:
                warn_if_reached();
            break;
        }
        
        // It appears that in GTK+ 2.18, the adjustment is not clamped the way it was in 2.16.
        // This may have to do with how adjustments are different w/ scrollbars, that they're upper
        // clamp is upper - page_size ... either way, enforce these limits here
        vadj.set_value(new_value.clamp((int) vadj.get_lower(), 
            (int) vadj.get_upper() - (int) vadj.get_page_size()));
        
        updated_selection_band();
        
        return true;
    }
    
    public void cursor_to_item(CheckerboardItem item) {
        assert(get_view().contains(item));

        cursor = item;
        
        if (!get_ctrl_pressed()) {
            get_view().unselect_all();
            Marker marker = get_view().mark(item);
            get_view().select_marked(marker);
        }
        layout.set_cursor(item);
        
        // if item is in any way out of view, scroll to it
        Gtk.Adjustment vadj = get_vadjustment();
        if (get_adjustment_relation(vadj, item.allocation.y) == AdjustmentRelation.IN_RANGE
            && (get_adjustment_relation(vadj, item.allocation.y + item.allocation.height) == AdjustmentRelation.IN_RANGE))
            return;

        // scroll to see the new item
        int top = 0;
        if (item.allocation.y < vadj.get_value()) {
            top = item.allocation.y;
            top -= CheckerboardLayout.ROW_GUTTER_PADDING / 2;
        } else {
            top = item.allocation.y + item.allocation.height - (int) vadj.get_page_size();
            top += CheckerboardLayout.ROW_GUTTER_PADDING / 2;
        }
        
        vadj.set_value(top);
    }
    
    public void move_cursor(CompassPoint point) {
        // if no items, nothing to do
        if (get_view().get_count() == 0)
            return;
            
        // if there is no better starting point, simply select the first and exit
        // The right half of the or is related to Bug #732334, the cursor might be non-null and still not contained in
        // the view, if the user dragged a full screen Photo off screen
        if (cursor == null && layout.get_cursor() == null || cursor != null && !get_view().contains(cursor)) {
            CheckerboardItem item = layout.get_item_at_coordinate(0, 0);
            cursor_to_item(item);
            anchor = item;

            return;
        }

        if (cursor == null) {
            cursor = layout.get_cursor() as CheckerboardItem;
        }
               
        // move the cursor relative to the "first" item
        CheckerboardItem? item = layout.get_item_relative_to(cursor, point);
        if (item != null)
            cursor_to_item(item);
   }

    public void set_cursor(CheckerboardItem item) {
        Marker marker = get_view().mark(item);
        get_view().select_marked(marker);
            
        cursor = item;
        anchor = item;
   }

    public void select_between_items(CheckerboardItem item_start, CheckerboardItem item_end) {
        Marker marker = get_view().start_marking();

        bool passed_start = false;
        bool passed_end = false;

        foreach (DataObject object in get_view().get_all()) {
            CheckerboardItem item = (CheckerboardItem) object;
            
            if (item_start == item)
                passed_start = true;

            if (item_end == item)
                passed_end = true;

            if (passed_start || passed_end)
                marker.mark((DataView) object);

            if (passed_start && passed_end)
                break;
        }
        
        get_view().select_marked(marker);
    }

    public void select_anchor_to_cursor(uint state) {
        if (cursor == null || anchor == null)
            return;

        if (state == Gdk.ModifierType.SHIFT_MASK) {
            get_view().unselect_all();
            select_between_items(anchor, cursor);
        } else {
            anchor = cursor;
        }
    }

    protected virtual void set_display_titles(bool display) {
        get_view().freeze_notifications();
        get_view().set_property(CheckerboardItem.PROP_SHOW_TITLES, display);
        get_view().thaw_notifications();
    }

    protected virtual void set_display_comments(bool display) {
        get_view().freeze_notifications();
        get_view().set_property(CheckerboardItem.PROP_SHOW_COMMENTS, display);
        get_view().thaw_notifications();
    }
}

public abstract class SinglePhotoPage : Page {
    public const Gdk.InterpType FAST_INTERP = Gdk.InterpType.NEAREST;
    public const Gdk.InterpType QUALITY_INTERP = Gdk.InterpType.BILINEAR;
    public const int KEY_REPEAT_INTERVAL_MSEC = 200;
    
    public enum UpdateReason {
        NEW_PIXBUF,
        QUALITY_IMPROVEMENT,
        RESIZED_CANVAS
    }
    
    protected Gtk.DrawingArea canvas = new Gtk.DrawingArea();
    protected Gtk.Viewport viewport = new Gtk.Viewport(null, null);
    
    private bool scale_up_to_viewport;
    private TransitionClock transition_clock;
    private int transition_duration_msec = 0;
    private Cairo.Surface pixmap = null;
    private Cairo.Context pixmap_ctx = null;
    private Cairo.Context text_ctx = null;
    private Dimensions pixmap_dim = Dimensions();
    private Gdk.Pixbuf unscaled = null;
    private Dimensions max_dim = Dimensions();
    private Gdk.Pixbuf scaled = null;
    private Gdk.Pixbuf old_scaled = null; // previous scaled image
    private Gdk.Rectangle scaled_pos = Gdk.Rectangle();
    private ZoomState static_zoom_state;
    private bool zoom_high_quality = true;
    private ZoomState saved_zoom_state;
    private bool has_saved_zoom_state = false;
    private uint32 last_nav_key = 0;
    
    protected SinglePhotoPage(string page_name, bool scale_up_to_viewport) {
        base(page_name);
        
        this.scale_up_to_viewport = scale_up_to_viewport;
        
        transition_clock = TransitionEffectsManager.get_instance().create_null_transition_clock();
        
        // With the current code automatically resizing the image to the viewport, scrollbars
        // should never be shown, but this may change if/when zooming is supported
        set_policy(Gtk.PolicyType.AUTOMATIC, Gtk.PolicyType.AUTOMATIC);

        set_border_width(0);
        set_shadow_type(Gtk.ShadowType.NONE);
        
        viewport.set_shadow_type(Gtk.ShadowType.NONE);
        viewport.set_border_width(0);
        viewport.add(canvas);
        
        add(viewport);

        canvas.add_events(Gdk.EventMask.EXPOSURE_MASK | Gdk.EventMask.STRUCTURE_MASK 
            | Gdk.EventMask.SUBSTRUCTURE_MASK);
        
        viewport.size_allocate.connect(on_viewport_resize);
        canvas.draw.connect(on_canvas_exposed);
        
        set_event_source(canvas);
        Config.Facade.get_instance().colors_changed.connect(on_colors_changed);
    }

    ~SinglePhotoPage() {
        Config.Facade.get_instance().colors_changed.disconnect(on_colors_changed);
    }
    
    public bool is_transition_in_progress() {
        return transition_clock.is_in_progress();
    }
    
    public void cancel_transition() {
        if (transition_clock.is_in_progress())
            transition_clock.cancel();
    }
    
    public void set_transition(string effect_id, int duration_msec) {
        cancel_transition();
        
        transition_clock = TransitionEffectsManager.get_instance().create_transition_clock(effect_id);
        if (transition_clock == null)
            transition_clock = TransitionEffectsManager.get_instance().create_null_transition_clock();
        
        transition_duration_msec = duration_msec;
    }
    
    // This method includes a call to pixmap_ctx.paint().
    private void render_zoomed_to_pixmap(ZoomState zoom_state) {
        assert(is_zoom_supported());
        
        Gdk.Rectangle view_rect = zoom_state.get_viewing_rectangle_wrt_content();
        
        Gdk.Pixbuf zoomed;
        if (get_zoom_buffer() != null) {
            zoomed = (zoom_high_quality) ? get_zoom_buffer().get_zoomed_image(zoom_state) :
                get_zoom_buffer().get_zoom_preview_image(zoom_state);
        } else {
            Gdk.Rectangle view_rect_proj = zoom_state.get_viewing_rectangle_projection(unscaled);
            
            Gdk.Pixbuf proj_subpixbuf = new Gdk.Pixbuf.subpixbuf(unscaled, view_rect_proj.x,
                view_rect_proj.y, view_rect_proj.width, view_rect_proj.height);
            
            zoomed = proj_subpixbuf.scale_simple(view_rect.width, view_rect.height,
                Gdk.InterpType.BILINEAR);
        }

        if (zoomed == null) {
            return;
        }
        
        int draw_x = (pixmap_dim.width - view_rect.width) / 2;
        draw_x = draw_x.clamp(0, int.MAX);

        int draw_y = (pixmap_dim.height - view_rect.height) / 2;
        draw_y = draw_y.clamp(0, int.MAX);
        paint_pixmap_with_background(pixmap_ctx, zoomed, draw_x, draw_y);
    }

    protected void on_interactive_zoom(ZoomState interactive_zoom_state) {
        assert(is_zoom_supported());
        
        set_source_color_from_string(pixmap_ctx, "#000");
        pixmap_ctx.paint();
        
        bool old_quality_setting = zoom_high_quality;
        zoom_high_quality = false;
        render_zoomed_to_pixmap(interactive_zoom_state);
        zoom_high_quality = old_quality_setting;
        
        canvas.queue_draw();
    }

    protected void on_interactive_pan(ZoomState interactive_zoom_state) {
        assert(is_zoom_supported());
        
        set_source_color_from_string(pixmap_ctx, "#000");
        pixmap_ctx.paint();
        
        bool old_quality_setting = zoom_high_quality;
        zoom_high_quality = true;
        render_zoomed_to_pixmap(interactive_zoom_state);
        zoom_high_quality = old_quality_setting;
        
        canvas.queue_draw();
    }

    protected virtual bool is_zoom_supported() {
        return false;
    }

    protected virtual void cancel_zoom() {
        if (pixmap != null) {
            set_source_color_from_string(pixmap_ctx, "#000");
            pixmap_ctx.paint();
        }
    }

    protected virtual void save_zoom_state() {
        saved_zoom_state = static_zoom_state;
        has_saved_zoom_state = true;
    }
    
    protected virtual void restore_zoom_state() {
        if (!has_saved_zoom_state)
            return;

        static_zoom_state = saved_zoom_state;
        repaint();
        has_saved_zoom_state = false;
    }
    
    protected virtual ZoomBuffer? get_zoom_buffer() {
        return null;
    }
    
    protected ZoomState get_saved_zoom_state() {
        return saved_zoom_state;
    }

    protected void set_zoom_state(ZoomState zoom_state) {
        assert(is_zoom_supported());

        static_zoom_state = zoom_state;
    }

    protected ZoomState get_zoom_state() {
        assert(is_zoom_supported());

        return static_zoom_state;
    }

    public override void switched_to() {
        base.switched_to();
        
        if (unscaled != null)
            repaint();
    }
    
    public override void set_container(Gtk.Window container) {
        base.set_container(container);
        
        // scrollbar policy in fullscreen mode needs to be auto/auto, else the pixbuf will shift
        // off the screen
        if (container is FullscreenWindow)
            set_policy(Gtk.PolicyType.AUTOMATIC, Gtk.PolicyType.AUTOMATIC);
    }
    
    // max_dim represents the maximum size of the original pixbuf (i.e. pixbuf may be scaled and
    // the caller capable of producing larger ones depending on the viewport size).  max_dim
    // is used when scale_up_to_viewport is set to true.  Pass a Dimensions with no area if
    // max_dim should be ignored (i.e. scale_up_to_viewport is false).
    public void set_pixbuf(Gdk.Pixbuf unscaled, Dimensions max_dim, Direction? direction = null) {
        static_zoom_state = ZoomState(max_dim, pixmap_dim,
            static_zoom_state.get_interpolation_factor(),
            static_zoom_state.get_viewport_center());

        cancel_transition();
        
        this.unscaled = unscaled;
        this.max_dim = max_dim;
        this.old_scaled = scaled;
        scaled = null;
        
        // need to make sure this has happened
        canvas.realize();
        
        repaint(direction);
    }
    
    public void blank_display() {
        unscaled = null;
        max_dim = Dimensions();
        scaled = null;
        pixmap = null;
        
        // this has to have happened
        canvas.realize();
        
        // force a redraw
        invalidate_all();
    }
    
    public Cairo.Surface? get_surface() {
        return pixmap;
    }
    
    public Dimensions get_surface_dim() {
        return pixmap_dim;
    }
    
    public Cairo.Context get_cairo_context() {
        return pixmap_ctx;
    }
    
    public void paint_text(Pango.Layout pango_layout, int x, int y) {
        text_ctx.move_to(x, y);
        Pango.cairo_show_layout(text_ctx, pango_layout);
    }
    
    public Scaling get_canvas_scaling() {
        return (get_container() is FullscreenWindow) ? Scaling.for_screen(AppWindow.get_instance(), scale_up_to_viewport)
            : Scaling.for_widget(viewport, scale_up_to_viewport);
    }

    public Gdk.Pixbuf? get_unscaled_pixbuf() {
        return unscaled;
    }
    
    public Gdk.Pixbuf? get_scaled_pixbuf() {
        return scaled;
    }
    
    // Returns a rectangle describing the pixbuf in relation to the canvas
    public Gdk.Rectangle get_scaled_pixbuf_position() {
        return scaled_pos;
    }
    
    public bool is_inside_pixbuf(int x, int y) {
        return coord_in_rectangle(x, y, scaled_pos);
    }
    
    public void invalidate(Gdk.Rectangle rect) {
        if (canvas.get_window() != null)
            canvas.get_window().invalidate_rect(rect, false);
    }
    
    public void invalidate_all() {
        if (canvas.get_window() != null)
            canvas.get_window().invalidate_rect(null, false);
    }
    
    private void on_viewport_resize() {
        // do fast repaints while resizing
        internal_repaint(true, null);
    }
    
    protected override void on_resize_finished(Gdk.Rectangle rect) {
        base.on_resize_finished(rect);
       
        // when the resize is completed, do a high-quality repaint
        repaint();
    }

    private bool on_canvas_exposed(Cairo.Context exposed_ctx) {
        // draw pixmap onto canvas unless it's not been instantiated, in which case draw black
        // (so either old image or contents of another page is not left on screen)
        if (pixmap != null)
            exposed_ctx.set_source_surface(pixmap, 0, 0);
        else
            set_source_color_from_string(exposed_ctx, "#000");
        
        exposed_ctx.rectangle(0, 0, get_allocated_width(), get_allocated_height());
        exposed_ctx.paint();
        
        return true;
    }
    
    protected virtual void new_surface(Cairo.Context ctx, Dimensions ctx_dim) {
    }
    
    protected virtual void updated_pixbuf(Gdk.Pixbuf pixbuf, UpdateReason reason, Dimensions old_dim) {
    }

    protected virtual void paint(Cairo.Context ctx, Dimensions ctx_dim) {
        if (is_zoom_supported() && (!static_zoom_state.is_default())) {
            set_source_color_from_string(ctx, "#000");
            ctx.rectangle(0, 0, pixmap_dim.width, pixmap_dim.height);
            ctx.fill();
            
            render_zoomed_to_pixmap(static_zoom_state);
        } else if (!transition_clock.paint(ctx, ctx_dim.width, ctx_dim.height)) {
            // transition is not running, so paint the full image on a black background
            set_source_color_from_string(ctx, "#000");

            ctx.rectangle(0, 0, pixmap_dim.width, pixmap_dim.height);
            ctx.fill();

            paint_pixmap_with_background(ctx, scaled, scaled_pos.x, scaled_pos.y);
        }
    }
    
    private void repaint_pixmap() {
        if (pixmap_ctx == null)
            return;
        
        paint(pixmap_ctx, pixmap_dim);
        invalidate_all();
    }
    
    public void repaint(Direction? direction = null) {
        internal_repaint(false, direction);
    }
    
    private void internal_repaint(bool fast, Direction? direction) {
        // if not in view, assume a full repaint needed in future but do nothing more
        if (!is_in_view()) {
            pixmap = null;
            scaled = null;
            
            return;
        }
        
        // no image or window, no painting
        if (unscaled == null || canvas.get_window() == null)
            return;
        
        Gtk.Allocation allocation;
        viewport.get_allocation(out allocation);
        
        int width = allocation.width;
        int height = allocation.height;
        
        if (width <= 0 || height <= 0)
            return;
            
        bool new_pixbuf = (scaled == null);
        
        // save if reporting an image being rescaled
        Dimensions old_scaled_dim = Dimensions.for_rectangle(scaled_pos);
        Gdk.Rectangle old_scaled_pos = scaled_pos;

        // attempt to reuse pixmap
        if (pixmap_dim.width != width || pixmap_dim.height != height)
            pixmap = null;
        
        // if necessary, create a pixmap as large as the entire viewport
        bool new_pixmap = false;
        if (pixmap == null) {
            init_pixmap(width, height);
            new_pixmap = true;
        }
        
        if (new_pixbuf || new_pixmap) {
            Dimensions unscaled_dim = Dimensions.for_pixbuf(unscaled);
            
            // determine scaled size of pixbuf ... if a max dimensions is set and not scaling up,
            // respect it
            Dimensions scaled_dim = Dimensions();
            if (!scale_up_to_viewport && max_dim.has_area() && max_dim.width < width && max_dim.height < height)
                scaled_dim = max_dim;
            else
                scaled_dim = unscaled_dim.get_scaled_proportional(pixmap_dim);
            
            assert(width >= scaled_dim.width);
            assert(height >= scaled_dim.height);

            // center pixbuf on the canvas
            scaled_pos.x = (width - scaled_dim.width) / 2;
            scaled_pos.y = (height - scaled_dim.height) / 2;
            scaled_pos.width = scaled_dim.width;
            scaled_pos.height = scaled_dim.height;
        }
        
        Gdk.InterpType interp = (fast) ? FAST_INTERP : QUALITY_INTERP;
        
        // rescale if canvas rescaled or better quality is requested
        if (scaled == null) {
            scaled = resize_pixbuf(unscaled, Dimensions.for_rectangle(scaled_pos), interp);
            
            UpdateReason reason = UpdateReason.RESIZED_CANVAS;
            if (new_pixbuf)
                reason = UpdateReason.NEW_PIXBUF;
            else if (!new_pixmap && interp == QUALITY_INTERP)
                reason = UpdateReason.QUALITY_IMPROVEMENT;

            static_zoom_state = ZoomState(max_dim, pixmap_dim,
                static_zoom_state.get_interpolation_factor(),
                static_zoom_state.get_viewport_center());

            updated_pixbuf(scaled, reason, old_scaled_dim);
        }

        zoom_high_quality = !fast;
        
        if (direction != null && !transition_clock.is_in_progress()) {
            Spit.Transitions.Visuals visuals = new Spit.Transitions.Visuals(old_scaled,
                old_scaled_pos, scaled, scaled_pos, parse_color("#000"));
            
            transition_clock.start(visuals, direction.to_transition_direction(), transition_duration_msec,
                repaint_pixmap);
        }
        
        if (!transition_clock.is_in_progress())
            repaint_pixmap();
    }
    
    private void init_pixmap(int width, int height) {
        assert(unscaled != null);
        assert(canvas.get_window() != null);
        
        // Cairo backing surface (manual double-buffering)
        pixmap = new Cairo.ImageSurface(Cairo.Format.ARGB32, width, height);
        pixmap_dim = Dimensions(width, height);
        
        // Cairo context for drawing on the pixmap
        pixmap_ctx = new Cairo.Context(pixmap);
        
        // need a new pixbuf to fit this scale
        scaled = null;
        
        // Cairo context for drawing text on the pixmap
        text_ctx = new Cairo.Context(pixmap);
        set_source_color_from_string(text_ctx, "#fff");
        
        
        // no need to resize canvas, viewport does that automatically
        
        new_surface(pixmap_ctx, pixmap_dim);
    }

    protected override bool on_context_keypress() {
        return popup_context_menu(get_page_context_menu());
    }
    
    protected virtual void on_previous_photo() {
    }
    
    protected virtual void on_next_photo() {
    }
    
    public override bool key_press_event(Gdk.EventKey event) {
        // if the user holds the arrow keys down, we will receive a steady stream of key press
        // events for an operation that isn't designed for a rapid succession of output ... 
        // we staunch the supply of new photos to under a quarter second (#533)
        bool nav_ok = (event.time - last_nav_key) > KEY_REPEAT_INTERVAL_MSEC;
        
        bool handled = true;
        switch (Gdk.keyval_name(event.keyval)) {
            case "Left":
            case "KP_Left":
            case "BackSpace":
                if (nav_ok) {
                    on_previous_photo();
                    last_nav_key = event.time;
                }
            break;
            
            case "Right":
            case "KP_Right":
            case "space":
                if (nav_ok) {
                    on_next_photo();
                    last_nav_key = event.time;
                }
            break;
            
            default:
                handled = false;
            break;
        }
        
        if (handled)
            return true;
        
        return (base.key_press_event != null) ? base.key_press_event(event) : true;
    }

    private void on_colors_changed() {
        invalidate_transparent_background();
        repaint();
    }
}

//
// DragAndDropHandler attaches signals to a Page to properly handle drag-and-drop requests for the
// Page as a DnD Source.  (DnD Destination handling is handled by the appropriate AppWindow, i.e.
// LibraryWindow and DirectWindow). Assumes the Page's ViewCollection holds MediaSources.
//
public class DragAndDropHandler {
    private enum TargetType {
        XDS,
        MEDIA_LIST
    }
    
    private const Gtk.TargetEntry[] SOURCE_TARGET_ENTRIES = {
        { "XdndDirectSave0", Gtk.TargetFlags.OTHER_APP, TargetType.XDS },
        { "shotwell/media-id-atom", Gtk.TargetFlags.SAME_APP, TargetType.MEDIA_LIST }
    };
    
    private static Gdk.Atom? XDS_ATOM = null;
    private static Gdk.Atom? TEXT_ATOM = null;
    private static uint8[]? XDS_FAKE_TARGET = null;
    
    private weak Page page;
    private Gtk.Widget event_source;
    private File? drag_destination = null;
    private ExporterUI exporter = null;
    
    public DragAndDropHandler(Page page) {
        this.page = page;
        this.event_source = page.get_event_source();
        assert(event_source != null);
        assert(event_source.get_has_window());
        
        // Need to do this because static member variables are not properly handled
        if (XDS_ATOM == null)
            XDS_ATOM = Gdk.Atom.intern_static_string("XdndDirectSave0");
        
        if (TEXT_ATOM == null)
            TEXT_ATOM = Gdk.Atom.intern_static_string("text/plain");
        
        if (XDS_FAKE_TARGET == null)
            XDS_FAKE_TARGET = string_to_uchar_array("shotwell.txt");
        
        // register what's available on this DnD Source
        Gtk.drag_source_set(event_source, Gdk.ModifierType.BUTTON1_MASK, SOURCE_TARGET_ENTRIES,
            Gdk.DragAction.COPY);
        
        // attach to the event source's DnD signals, not the Page's, which is a NO_WINDOW widget
        // and does not emit them
        event_source.drag_begin.connect(on_drag_begin);
        event_source.drag_data_get.connect(on_drag_data_get);
        event_source.drag_end.connect(on_drag_end);
        event_source.drag_failed.connect(on_drag_failed);
    }
    
    ~DragAndDropHandler() {
        if (event_source != null) {
            event_source.drag_begin.disconnect(on_drag_begin);
            event_source.drag_data_get.disconnect(on_drag_data_get);
            event_source.drag_end.disconnect(on_drag_end);
            event_source.drag_failed.disconnect(on_drag_failed);
        }
        
        page = null;
        event_source = null;
    }
    
    private void on_drag_begin(Gdk.DragContext context) {
        debug("on_drag_begin (%s)", page.get_page_name());
        
        if (page == null || page.get_view().get_selected_count() == 0 || exporter != null)
            return;
        
        drag_destination = null;
        
        // use the first media item as the icon
        ThumbnailSource thumb = (ThumbnailSource) page.get_view().get_selected_at(0).get_source();
        
        try {
            Gdk.Pixbuf icon = thumb.get_thumbnail(AppWindow.DND_ICON_SCALE);
            Gtk.drag_source_set_icon_pixbuf(event_source, icon);
        } catch (Error err) {
            warning("Unable to fetch icon for drag-and-drop from %s: %s", thumb.to_string(),
                err.message);
        }
        
        // set the XDS property to indicate an XDS save is available
#if VALA_0_20
        Gdk.property_change(context.get_source_window(), XDS_ATOM, TEXT_ATOM, 8, Gdk.PropMode.REPLACE,
            XDS_FAKE_TARGET, 1);
#else
        Gdk.property_change(context.get_source_window(), XDS_ATOM, TEXT_ATOM, 8, Gdk.PropMode.REPLACE,
            XDS_FAKE_TARGET);
#endif
    }
    
    private void on_drag_data_get(Gdk.DragContext context, Gtk.SelectionData selection_data,
        uint target_type, uint time) {
        debug("on_drag_data_get (%s)", page.get_page_name());
        
        if (page == null || page.get_view().get_selected_count() == 0)
            return;
        
        switch (target_type) {
            case TargetType.XDS:
                // Fetch the XDS property that has been set with the destination path
                uchar[] data = new uchar[4096];
                Gdk.Atom actual_type;
                int actual_format = 0;
                bool fetched = Gdk.property_get(context.get_source_window(), XDS_ATOM, TEXT_ATOM,
                    0, data.length, 0, out actual_type, out actual_format, out data);
                
                // the destination path is actually for our XDS_FAKE_TARGET, use its parent
                // to determine where the file(s) should go
                if (fetched && data != null && data.length > 0)
                    drag_destination = File.new_for_uri(uchar_array_to_string(data)).get_parent();
                
                debug("on_drag_data_get (%s): %s", page.get_page_name(),
                    (drag_destination != null) ? drag_destination.get_path() : "(no path)");
                
                // Set the property to "S" for Success or "E" for Error
                selection_data.set(XDS_ATOM, 8,
                    string_to_uchar_array((drag_destination != null) ? "S" : "E"));
            break;
            
            case TargetType.MEDIA_LIST:
                Gee.Collection<MediaSource> sources =
                    (Gee.Collection<MediaSource>) page.get_view().get_selected_sources();
                
                // convert the selected media sources to Gdk.Atom-encoded sourceID strings for
                // internal drag-and-drop
                selection_data.set(Gdk.Atom.intern_static_string("SourceIDAtom"), (int) sizeof(Gdk.Atom),
                    serialize_media_sources(sources));
            break;
            
            default:
                warning("on_drag_data_get (%s): unknown target type %u", page.get_page_name(),
                    target_type);
            break;
        }
    }
    
    private void on_drag_end() {
        debug("on_drag_end (%s)", page.get_page_name());

        if (page == null || page.get_view().get_selected_count() == 0 || drag_destination == null
            || exporter != null) {
            return;
        }

        debug("Exporting to %s", drag_destination.get_path());
        
        // drag-and-drop export doesn't pop up an export dialog, so use what are likely the
        // most common export settings (the current -- or "working" -- file format, with
        // all transformations applied, at the image's original size).
        if (drag_destination.get_path() != null) {
            exporter = new ExporterUI(new Exporter(
                (Gee.Collection<Photo>) page.get_view().get_selected_sources(),
                drag_destination, Scaling.for_original(), ExportFormatParameters.current()));
            exporter.export(on_export_completed);
        } else {
            AppWindow.error_message(_("Photos cannot be exported to this directory."));
        }
        
        drag_destination = null;
    }
    
    private bool on_drag_failed(Gdk.DragContext context, Gtk.DragResult drag_result) {
        debug("on_drag_failed (%s): %d", page.get_page_name(), (int) drag_result);
        
        if (page == null)
            return false;
        
        drag_destination = null;
        
        return false;
    }
    
    private void on_export_completed() {
        exporter = null;
    }

}