playSource method
Make source the active source and start playback. If a different
source was active, it is implicitly paused via adapter.load.
Implementation
Future<void> playSource(String source) async {
if (_activeSource != source) {
await adapter.load(source);
_activeSource = source;
_position = Duration.zero;
_duration = null;
notifyListeners();
}
await adapter.play();
}