ProxyValueListenable<T> for Flutter

proxy_value_listenable.dart

import 'package:flutter/foundation.dart';

class ProxyValueListenable<T> implements ValueListenable<T> {
  ProxyValueListenable(this._parent) {
    _parent.addListener(_listener);
  }

  final _listeners = <VoidCallback>[];

  ValueListenable<T> _parent;

  ValueListenable<T> get parent => _parent;

  set parent(ValueListenable<T> value) {
    if (value == _parent) return;
    _parent.removeListener(_listener);

    final valueChanged = value.value != _parent.value;

    _parent = value;
    value.addListener(_listener);

    if (valueChanged) _listener();
  }

  @override
  T get value => parent.value;

  void _listener() {
    for (final listener in _listeners) {
      listener();
    }
  }

  @override
  void addListener(VoidCallback listener) {
    _listeners.add(listener);
  }

  @override
  void removeListener(VoidCallback listener) {
    _listeners.remove(listener);
  }
  
  void dispose() {
    _parent.removeListener(_listener);
  }
}