audrey/src/playbin.vala

enum PlaybinState {
    STOPPED,
    PAUSED,
    PLAYING,
}

class Playbin : Object {
    // dynamic: undocumented vala feature
    // lets us access the about-to-finish signal
    private dynamic Gst.Element playbin = Gst.ElementFactory.make ("playbin3", null);

    // cubic: recommended for media player volume sliders?
    public double volume {
        get {
            return ((Gst.Audio.StreamVolume) this.playbin).get_volume (Gst.Audio.StreamVolumeFormat.CUBIC);
        }
        set {
            ((Gst.Audio.StreamVolume) this.playbin).set_volume (Gst.Audio.StreamVolumeFormat.CUBIC, value);
        }
    }

    public bool mute {
        get { return this.playbin.mute; }
        set { this.playbin.mute = value; }
    }

    public PlaybinState state { get; private set; default = PlaybinState.STOPPED; }

    private bool update_position = false;

    public int64 position { get; private set; }
    public int64 duration { get; private set; }

    public Subsonic api { get; set; }
    private ListModel play_queue;

    public signal void now_playing (uint index, Song song, int64 duration);
    private uint playing_index;

    private bool next_gapless;

    private void source_setup (Gst.Element playbin, dynamic Gst.Element source) {
        source.user_agent = "audrey/linux";
    }

    public Playbin (ListModel play_queue) {
        this.play_queue = play_queue;

        this.next_uri = new AsyncQueue<string> ();
        this.next_uri.push ("");

        // gstreamer docs: GNOME-based applications, for example, will usually
        // want to create gconfaudiosink and gconfvideosink elements and make
        // playbin3 use those, so that output happens to whatever the user has
        // configured in the GNOME Multimedia System Selector configuration dialog.
        this.playbin.audio_sink = Gst.ElementFactory.make ("gconfaudiosink", null);

        this.playbin.source_setup.connect (this.source_setup);
        this.playbin.about_to_finish.connect (this.about_to_finish);

        play_queue.items_changed.connect ((play_queue, position, removed, added) => {
            if (this.state == PlaybinState.STOPPED) {
                return;
            }

            if (this.playing_index >= position) {
                if (this.playing_index < position+removed) {
                    // current track was removed, start playing something else
                    // TODO check if it was actually reordered

                    this.begin_playback (position);
                } else {
                    // unaffected
                    // fix up playing index though
                    this.playing_index += added;
                    this.playing_index -= removed;
                }
            } else if (this.playing_index+1 == position) {
                // next track was changed
                // try to fix up gapless transition
                string? next_uri = this.next_uri.try_pop ();
                if (next_uri != null) {
                    // we're in luck, about-to-finish hasn't been triggered yet
                    // we can get away with replacing it
                    if (this.playing_index+1 < play_queue.get_n_items ()) {
                        Song song = (Song) play_queue.get_item (this.playing_index+1);
                        this.next_uri.push (this.api.stream_uri (song.id));
                    } else {
                        this.next_uri.push ("");
                    }
                } else {
                    // about-to-finish already triggered
                    // we'll need to stop the new track when it starts playing
                    assert (false); // TODO
                }
            }
        });

        // regularly update position/duration
        Timeout.add (500, () => {
            if (this.update_position) {
                int64 new_position;
                if (this.playbin.query_position (Gst.Format.TIME, out new_position)) {
                    this.position = new_position < this.duration ? new_position : this.duration;
                } else {
                    this.position = 0;
                }
            }

            // keep rerunning
            return true;
        });

        var bus = this.playbin.get_bus ();
        bus.add_signal_watch ();

        bus.message["error"].connect ((message) => {
            Error err;
            string? debug;
            message.parse_error (out err, out debug);
            error ("gst playbin bus error: %s", err.message);
        });

        bus.message["warning"].connect ((message) => {
            Error err;
            string? debug;
            message.parse_error (out err, out debug);
            warning ("gst playbin bus warning: %s", err.message);
        });

        bus.message["stream-start"].connect ((message) => {
            int64 new_duration;
            if (this.playbin.query_duration (Gst.Format.TIME, out new_duration)) {
                this.duration = new_duration;
            } else {
                warning ("could not obtain new stream duration");
            }

            this.position = 0;

            if (this.next_gapless) {
                // advance position in play queue
                this.playing_index += 1;
            } else {
                this.next_gapless = true;
            }

            this.now_playing (this.playing_index, (Song) play_queue.get_item (this.playing_index), this.duration);

            if (this.playing_index+1 < play_queue.get_n_items ()) {
                Song song = (Song) play_queue.get_item (this.playing_index+1);
                this.next_uri.push (this.api.stream_uri (song.id));
            } else {
                this.next_uri.push ("");
            }
        });

        bus.message["state-changed"].connect ((message) => {
            if (message.src != this.playbin) return;

            Gst.State new_state;
            message.parse_state_changed (null, out new_state, null);

            this.update_position = new_state == Gst.State.PLAYING;
        });
        
        bus.message["eos"].connect ((message) => {
            assert (false); // TODO
        });
    }

    public void seek (int64 position) {
        if (!this.playbin.seek_simple (Gst.Format.TIME, Gst.SeekFlags.FLUSH, position)) {
            warning ("could not seek");
        }
    }

    public void begin_playback (uint position) {
        this.state = PlaybinState.PLAYING;

        this.playing_index = position;
        this.playbin.set_state (Gst.State.READY);
        this.playbin.uri = this.api.stream_uri (((Song) this.play_queue.get_item (position)).id);
        this.playbin.set_state (Gst.State.PLAYING);
        this.next_gapless = false;

        string? next_uri = this.next_uri.try_pop ();
        if (next_uri != null) {
            // we're in luck, about-to-finish hasn't been triggered yet
        } else {
            // about-to-finish already triggered
            // we'll need to stop the new track when it starts playing
            assert (false); // TODO
        }
    }

    // ASSUMPTION: about-to-finish will be signalled exactly once per track
    // even if seeking backwards after
    AsyncQueue<string?> next_uri;

    // called when uri can be switched for gapless playback
    // need async queue because this might be called from a gstreamer thread
    private void about_to_finish (dynamic Gst.Element playbin) {
        print ("about to finish\n");

        // will block if the next uri isn't ready yet
        // leaves the queue empty as per the ASSUMPTION above
        string? next_uri = this.next_uri.pop ();

        if (next_uri != "") {
            playbin.uri = next_uri;
        }
    }

    public void pause () {
        assert (this.state != PlaybinState.STOPPED);
        this.playbin.set_state (Gst.State.PAUSED);
        this.state = PlaybinState.PAUSED;
    }

    public void play () {
        assert (this.state != PlaybinState.STOPPED);
        this.playbin.set_state (Gst.State.PLAYING);
        this.state = PlaybinState.PLAYING;
    }

    public void stop_playback() {
        this.playbin.set_state (Gst.State.READY);
        this.state = PlaybinState.STOPPED;
    }
}