Improve updating following and fix potential crash.

lib/utils/channel.dart

@@ -0,0 +1,58 @@
+import 'dart:async';
+import 'dart:collection';
+
+class Channel<T> {
+  final Queue<T> _queue;
+
+  final int size;
+
+  Channel(this.size) : _queue = Queue<T>();
+
+  Completer? _releaseCompleter;
+
+  Completer? _pushCompleter;
+
+  var currentSize = 0;
+
+  var isClosed = false;
+
+  Future<void> push(T item) async {
+    if (currentSize >= size) {
+      _releaseCompleter ??= Completer();
+      return _releaseCompleter!.future.then((_) {
+        if (isClosed) {
+          return;
+        }
+        _queue.addLast(item);
+        currentSize++;
+      });
+    }
+    _queue.addLast(item);
+    currentSize++;
+    _pushCompleter?.complete();
+    _pushCompleter = null;
+  }
+
+  Future<T?> pop() async {
+    while (_queue.isEmpty) {
+      if (isClosed) {
+        return null;
+      }
+      _pushCompleter ??= Completer();
+      await _pushCompleter!.future;
+    }
+    var item = _queue.removeFirst();
+    currentSize--;
+    if (_releaseCompleter != null && currentSize < size) {
+      _releaseCompleter!.complete();
+      _releaseCompleter = null;
+    }
+    return item;
+  }
+
+  void close() {
+    isClosed = true;
+    _pushCompleter?.complete();
+    _releaseCompleter?.complete();
+  }
+}