流动的观察者模式

// 被观察者
class Subject {
  List<Observer> _observers;
  Subject([List<Observer> observers]) {
    _observers = observers ?? [];
  }

  // 注册观察者
  void registerObserver(Observer observer) {
    _observers.add(observer);
  }
  
  // 解注册观察者
  void unregisterObserver(Observer observer) {
    _observers.remove(observer)
  }

  // 通知观察者
  void notifyobservers(Notification notification) {
    for (var observer in _observers) {
      observer.notify(notification);
    }
  }
}

// 观察者
class Observer {
  String name;
  
  Observer(this.name);

  void notify(Notification notification) {
    print("[${notification.timestamp.toIso8601String()}] Hey $name, ${notification.message}!");
  }
}

// 具体的被观察者 CoffeeMaker
// 每当 Coffee 制作完成发出通知给观察者。
class CoffeeMaker extends Subject {
  CoffeeMaker([List<Observer> observers]) : super(observers);
  
  void brew() {
    print("Brewing the coffee...");
    notifyobservers(Notification.forNow("coffee's done"));
  }
}

void main() {
  var me = Observer("Tyler");
  var mrCoffee = CoffeeMaker(List.from([me]));
  var myWife = Observer("Kate");
  mrCoffee.registerObserver(myWife);
  mrCoffee.brew();
}

var controller = new StreamController<int>();

controller.add(1); // 将数据放入流中

var controller = new StreamController<int>();

controller.stream.listen((item) => print(item)); // 数据观察者函数

controller.add(1);
controller.add(2);
controller.add(3);

1
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3

var controller = new StreamController<String>();

StreamSubscription subscription = controller.stream.listen((item) => print(item));

controller.add(1);
controller.add(2);
controller.add(3);

await Future.delayed(Duration(milliseconds: 500));

subscription.cancel();

class ChangeNotifier implements Listenable {
  LinkedList<_ListenerEntry>? _listeners = LinkedList<_ListenerEntry>();

  @protected
  bool get hasListeners {
    return _listeners!.isNotEmpty;
  }
  
  @override
  void addListener(VoidCallback listener) {
    _listeners!.add(_ListenerEntry(listener));
  }

  @override
  void removeListener(VoidCallback listener) {
    for (final _ListenerEntry entry in _listeners!) {
      if (entry.listener == listener) {
        entry.unlink();
        return;
      }
    }
  }

  @mustCallSuper
  void dispose() {
    _listeners = null;
  }

  @protected
  @visibleForTesting
  void notifyListeners() {
    if (_listeners!.isEmpty)
      return;
    final List<_ListenerEntry> localListeners = List<_ListenerEntry>.from(_listeners!);
    for (final _ListenerEntry entry in localListeners) {
      try {
        if (entry.list != null)
          entry.listener();
      } catch (exception, stack) {
        // ...
      }
    }
  }
}

class CartModel extends ChangeNotifier {
  final List<Item> _items = [];

  UnmodifiableListView<Item> get items => UnmodifiableListView(_items);

  int get totalPrice => _items.length * 42;

  void add(Item item) {
    _items.add(item);
    notifyListeners();
  }

  void removeAll() {
    _items.clear();
    notifyListeners();
  }
}

test('adding item increases total cost', () {
  final cart = CartModel();
  final startingPrice = cart.totalPrice;
  cart.addListener(() {
    expect(cart.totalPrice, greaterThan(startingPrice));
  });
  cart.add(Item('Dash'));
});

void main() {
  runApp(
    ChangeNotifierProvider(
      create: (context) => CartModel(),
      child: const MyApp(),
    ),
  );
}

return Consumer<CartModel>(
  builder: (context, cart, child) {
    return Text("Total price: ${cart.totalPrice}");
  },
);

 Widget build(BuildContext context) {
    return new MaterialApp(
      navigatorObservers: [new MyNavigatorObserver()],
      home: new Scaffold(
        body: new MyPage(),
      ),
    );
  }

class MyRouteObserver extends RouteObserver<PageRoute<dynamic>> {

  // 监听导航器的 push 操作
  @override
  void didPush(Route<dynamic> route, Route<dynamic> previousRoute) {
    super.didPush(route, previousRoute);
    if (previousRoute is PageRoute && route is PageRoute) {
      print('${previousRoute.settings.name} => ${route.settings.name}');
    }
  }

  // 监听导航器的 replace 操作
  @override
  void didReplace({Route<dynamic> newRoute, Route<dynamic> oldRoute}) {
    super.didReplace(newRoute: newRoute, oldRoute: oldRoute);
    if (newRoute is PageRoute) {
      print('${oldRoute.settings.name} => ${oldRoute.settings.name}');
    }
  }

  // 监听导航器的 pop 操作
  @override
  void didPop(Route<dynamic> route, Route<dynamic> previousRoute) {
    super.didPop(route, previousRoute);
    if (previousRoute is PageRoute && route is PageRoute) {
      print('${route.settings.name} => ${previousRoute.settings.name}');
    }
  }
}

 Future<T> push<T extends Object>(Route<T> route) {
  // ...
  for (NavigatorObserver observer in widget.observers)
    observer.didPush(route, oldRoute);
	// ...
}