add shared callback impl between mpv and jukebox players

backend/player/player.go

@@ -82,3 +82,68 @@ func (r ReplayGainMode) String() string {
 		return "no"
 	}
 }
+
+type BasePlayerCallbackImpl struct {
+	onPaused      []func()
+	onStopped     []func()
+	onPlaying     []func()
+	onSeek        []func()
+	onTrackChange []func()
+}
+
+// Registers a callback which is invoked when the player transitions to the Paused state.
+func (p *BasePlayerCallbackImpl) OnPaused(cb func()) {
+	p.onPaused = append(p.onPaused, cb)
+}
+
+// Registers a callback which is invoked when the player transitions to the Stopped state.
+func (p *BasePlayerCallbackImpl) OnStopped(cb func()) {
+	p.onStopped = append(p.onStopped, cb)
+}
+
+// Registers a callback which is invoked when the player transitions to the Playing state.
+func (p *BasePlayerCallbackImpl) OnPlaying(cb func()) {
+	p.onPlaying = append(p.onPlaying, cb)
+}
+
+// Registers a callback which is invoked whenever a seek event occurs.
+func (p *BasePlayerCallbackImpl) OnSeek(cb func()) {
+	p.onSeek = append(p.onSeek, 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).
+func (p *BasePlayerCallbackImpl) OnTrackChange(cb func()) {
+	p.onTrackChange = append(p.onTrackChange, cb)
+}
+
+func (p *BasePlayerCallbackImpl) InvokeOnPaused() {
+	for _, cb := range p.onPaused {
+		cb()
+	}
+}
+
+func (p *BasePlayerCallbackImpl) InvokeOnPlaying() {
+	for _, cb := range p.onPlaying {
+		cb()
+	}
+}
+
+func (p *BasePlayerCallbackImpl) InvokeOnStopped() {
+	for _, cb := range p.onStopped {
+		cb()
+	}
+}
+
+func (p *BasePlayerCallbackImpl) InvokeOnSeek() {
+	for _, cb := range p.onSeek {
+		cb()
+	}
+}
+
+func (p *BasePlayerCallbackImpl) InvokeOnTrackChange() {
+	for _, cb := range p.onTrackChange {
+		cb()
+	}
+}