Add Loop functionality to player controls

Adding the Loop functionality, along with the Player changes to support
it, and a button to the player controls.

`Player` now has a `loopMode` attribute, which determines whether it
should loop. At the moment, only the following modes are supported:

* `LoopNone`: Disables loop.
* `LoopAll`: Enables loop for the entire playlist queue.

This has been designed as an enum, to support other modes in the future
(e.g. loop for the current track only).

It depends on the `loop` functionality provided by MPV (using the
[`loop-playlist` option](https://mpv.io/manual/master/#options-loop-playlist)),
so no custom logic is needed to reset the currently playing index.

player/player.go

@@ -58,6 +58,14 @@ type ReplayGainOptions struct {
 	// Fallback gain intentionally omitted
 }
 
+// The playback loop mode (LoopNone, LoopAll).
+type LoopMode int
+
+const (
+	LoopNone LoopMode = iota
+	LoopAll
+)
+
 // Information about a specific audio device.
 // Returned by ListAudioDevices.
 type AudioDevice struct {
@@ -99,6 +107,7 @@ type Player struct {
 	haveRGainOpts  bool
 	audioExclusive bool
 	status         Status
+	loopMode       LoopMode
 	seeking        bool
 	curPlaylistPos int64
 	prePausedState State
@@ -107,11 +116,12 @@ type Player struct {
 	bgCancel context.CancelFunc
 
 	// callbacks
-	onPaused      []func()
-	onStopped     []func()
-	onPlaying     []func()
-	onSeek        []func()
-	onTrackChange []func(int64)
+	onPaused          []func()
+	onStopped         []func()
+	onPlaying         []func()
+	onSeek            []func()
+	onLoopModeChanged []func(string)
+	onTrackChange     []func(int64)
 }
 
 // Returns a new player.
@@ -390,6 +400,47 @@ func (p *Player) PlayPause() error {
 	}
 }
 
+// Sets the loop mode of the player.
+func (p *Player) SetLoopMode(mode LoopMode) error {
+	if !p.initialized {
+		return ErrUnitialized
+	}
+
+	// Return early if player is already in specified mode
+	if mode == p.loopMode {
+		return nil
+	}
+
+	switch mode {
+	case LoopNone:
+		p.mpv.SetOptionString("loop-playlist", "no")
+	case LoopAll:
+		p.mpv.SetOptionString("loop-playlist", "inf")
+	}
+	p.loopMode = mode
+
+	defer func() {
+		for _, cb := range p.onLoopModeChanged {
+			cb(p.loopMode.String())
+		}
+	}()
+
+	return nil
+}
+
+// Changes the loop mode of the player to the next one.
+// Useful for toggling UI elements, to change modes without knowing the current player mode.
+func (p *Player) SetNextLoopMode() error {
+	switch p.loopMode {
+	case LoopNone:
+		return p.SetLoopMode(LoopAll)
+	case LoopAll:
+		return p.SetLoopMode(LoopNone)
+	default:
+		return nil
+	}
+}
+
 // Get the current status of the player.
 func (p *Player) GetStatus() Status {
 	if !p.initialized {
@@ -491,6 +542,11 @@ func (p *Player) OnSeek(cb func()) {
 	p.onSeek = append(p.onSeek, cb)
 }
 
+// Registers a callback which is invoked when the player enables queue repeat.
+func (p *Player) OnLoopModeChanged(cb func(string)) {
+	p.onLoopModeChanged = append(p.onLoopModeChanged, cb)
+}
+
 // Registers a callback which is invoked when the currently playing track changes,
 // or when playback begins at any time from the Stopped state.
 // Callback is invoked with the index of the currently playing track (zero-based).
@@ -590,3 +646,13 @@ func (s SeekMode) String() string {
 	}
 	return "UNKNOWN_SEEK_MODE"
 }
+
+func (l LoopMode) String() string {
+	switch l {
+	case LoopNone:
+		return "no"
+	case LoopAll:
+		return "all"
+	}
+	return "UNKNOWN_LOOP_MODE"
+}