audrey/src/ui/play_queue.vala

class Ui.PlayQueueStore : Object, ListModel, Gtk.SelectionModel {
    public ListStore inner = new ListStore (typeof (Song));
    public uint playing_index { get; private set; default = 0; }

    public signal void begin_playback (Song song);
    public signal void prepare_next (Song? song);
    public signal void playback_continues (Song song);
    public signal void stop_playback ();

    construct {
        this.inner.items_changed.connect ((position, removed, added) => {
            if (this.playing_index >= position) {
                if (this.playing_index < position+removed) {
                    this.playing_index = position;
                    if (this.playing_index < this.inner.get_n_items ()) {
                        this.begin_playback ((Song) this.inner.get_item (this.playing_index));
                        this.prepare_next ((Song?) this.inner.get_item (this.playing_index+1));
                    } else {
                        this.stop_playback ();
                    }
                } else {
                    this.playing_index += added;
                    this.playing_index -= removed;
                }
            } else {
                this.prepare_next ((Song?) this.inner.get_item (this.playing_index+1));
            }

            this.items_changed (position, removed, added);
            if (this.playing_index < this.inner.get_n_items ()) {
                this.selection_changed (this.playing_index, 1);
            }
        });
    }

    public void song_transition () {
        this.playing_index += 1;
        this.selection_changed (this.playing_index-1, 2);
        this.playback_continues ((Song) this.inner.get_item (this.playing_index));
        this.prepare_next ((Song?) this.inner.get_item (this.playing_index+1));
    }

    public void playback_finished () {
        this.playing_index += 1;
        assert (this.playing_index == this.inner.get_n_items ());
        this.selection_changed (this.playing_index-1, 1);
    }

    Gtk.Bitset get_selection_in_range (uint position, uint n_items) {
        var bitset = new Gtk.Bitset.empty ();
        if (this.playing_index < this.inner.get_n_items ()) {
            bitset.add (playing_index);
        }
        return bitset;
    }

    bool is_selected (uint position) {
        return position == this.playing_index;
    }

    bool select_all () {
        return false;
    }

    bool select_item (uint position, bool unselect_rest) {
        if (!unselect_rest) {
            return false;
        }
      
        var previous = this.playing_index;
        this.playing_index = position;

        if (previous < this.inner.get_n_items ()) {
            this.selection_changed (previous, 1);
        }
        this.selection_changed (position, 1);

        this.begin_playback ((Song) this.inner.get_item (this.playing_index));
        this.prepare_next ((Song) this.inner.get_item (this.playing_index+1));

        return true;
    }

    bool select_range (uint position, uint n_items, bool unselect_rest) {
        return false;
    }

    bool set_selection (Gtk.Bitset selected, Gtk.Bitset mask) {
        return false;
    }

    bool unselect_all () {
        return false;
    }

    bool unselect_item (uint position) {
        return false;
    }

    bool unselect_range (uint position, uint n_items) {
        return false;
    }

    Object? get_item (uint position) {
        return this.inner.get_item (position);
    }

    Type get_item_type () {
        return this.inner.get_item_type ();
    }

    uint get_n_items () {
        return this.inner.get_n_items ();
    }
}

[GtkTemplate (ui = "/eu/callcc/audrey/ui/play_queue.ui")]
public class Ui.PlayQueue : Adw.NavigationPage {
    [GtkChild] private unowned Gtk.ColumnView view;
    PlayQueueStore store = new PlayQueueStore ();

    public signal void begin_playback (Song song);
    public signal void prepare_next (Song? next_song);
    public signal void playback_continues (Song song);
    public signal void stop_playback ();

    public bool can_clear_all { get; private set; default = false; }

    construct {
        this.view.model = this.store;
    }

    [GtkCallback] public void clear () {
        this.store.inner.remove_all ();
        this.can_clear_all = false;

        this.store.begin_playback.connect ((song) => this.begin_playback (song));
        this.store.prepare_next.connect ((next_song) => this.prepare_next (next_song));
        this.store.playback_continues.connect ((song) => this.playback_continues (song));
        this.store.stop_playback.connect (() => this.stop_playback ());
    }

    public void queue (Song song) {
        this.store.inner.append (song);
    }

    internal void song_transition () {
        this.store.song_transition ();
    }

    internal void playback_finished () {
        this.store.playback_finished ();
    }

    internal void restart () {
        this.store.select_item (0, true);
    }

    public void skip_forward () {
        this.store.select_item (this.store.playing_index+1, true);
    }

    public void skip_backward () {
        if (this.store.playing_index >= 1) {
            this.store.select_item (this.store.playing_index-1, true);
        }
    }
}