Video optimizations (#18)

* use events for `VideoTrackRenderer`

* logger

* implemented visibility detection

* optimize code with 1 param debounce func

* stop pending visibility event when disposed

* clean up

* mark internal

* report to track

* call dispose on pub and track

* use internal events to receive visibility events

* clean up visibility for disposed track

* listen to track visibility events on pub

* re-design

* quickly re-enable

* clean up

* clean up

lib/src/managers/event.dart

@@ -1,6 +1,7 @@
 import 'dart:async';
 
 import 'package:synchronized/synchronized.dart' as sync;
+import 'package:meta/meta.dart';
 
 import '../exceptions.dart';
 import '../extensions.dart';
@@ -30,9 +31,13 @@ class EventsEmitter<T> extends EventsListenable<T> {
   @override
   EventsEmitter<T> get emitter => this;
 
+  @internal
   void emit(T event) {
     // do nothing if already closed
-    if (streamCtrl.isClosed) return;
+    if (streamCtrl.isClosed) {
+      logger.warning('failed to emit event ${event} on a disposed emitter');
+      return;
+    }
     // emit the event
     streamCtrl.add(event);
   }
@@ -56,25 +61,31 @@ abstract class EventsListenable<T> extends Disposable {
   // the emitter to listen to
   EventsEmitter<T> get emitter;
 
-  bool synchronized;
+  final bool synchronized;
   // keep track of listeners to cancel later
   final _listeners = <StreamSubscription<T>>[];
   final _syncLock = sync.Lock();
 
+  List<StreamSubscription<T>> get listeners => _listeners;
+
   EventsListenable({
     required this.synchronized,
   }) {
     onDispose(() async {
-      if (_listeners.isNotEmpty) {
-        // Stop listening to all events
-        logger.fine('${objectId} cancelling ${_listeners.length} listeners(s)');
-        for (final listener in _listeners) {
-          await listener.cancel();
-        }
-      }
+      await cancelAll();
     });
   }
 
+  Future<void> cancelAll() async {
+    if (_listeners.isNotEmpty) {
+      // Stop listening to all events
+      logger.fine('${objectId} cancelling ${_listeners.length} listeners(s)');
+      for (final listener in _listeners) {
+        await listener.cancel();
+      }
+    }
+  }
+
   // @override
   // @mustCallSuper
   // Future<bool> dispose() async {