start method
Starts listening for messages on this BroadcastReceiver.
Throws a StateError, if it is already listening.
Implementation
Future<void> start() async {
if (isListening) {
throw StateError('This BroadcastReceiver is already started.');
}
final stream = _BroadcastChannel.instance.startReceiver(this);
_subscription = stream.listen((event) {
_messages.add(event);
});
}