Event Handling in Dart

events.dart

Type typeof<T>() => T;
typedef void DelegateHandler<T>(T response);

abstract class IHandler {
    void run(dynamic response);
    bool isType<T>(T type);
    dynamic get();
}

class Handler<T> implements IHandler {
    T type;
    DelegateHandler<T> _handler;
    Handler(DelegateHandler<T> handler) {
        _handler = handler;
    }

    T cast<T>(x) => x is T ? x : null;
    void run(dynamic response) {
        _handler(cast<T>(response));
    }

    bool isType<T2>(T2 type) {
        if (type is T) {
            return true;
        }
        return false;
    }

    DelegateHandler<T> get() {
        return _handler;
    }
}

abstract class IDelegate<T> {
    void subscribe(DelegateHandler<T> handler);
    void remove(DelegateHandler<T> handler);
    void invoke(T response);
}

class Delegate<T> implements IDelegate<T> {
    final _handlers = new List<IHandler>();

    void subscribe(DelegateHandler<T> handler) {
        _handlers.add(Handler(handler));
    }

    void invoke(T response) {
        _handlers.forEach((element) {
            if (element.isType(response)) {
                element.run(response);
            }
        });
    }

    void remove(DelegateHandler<T> handler) {
        int indexSplice;
        for (var i = 0; i < _handlers.length; i++) {
            if (_handlers[i].get() == handler) {
                indexSplice = i;
            }
        }
        _handlers.removeAt(indexSplice);
    }
}

example.dart

// use

class MyClass {
  final IDelegate<String> errorDelegate = Delegate();

  void testError(String message) {
    errorDelegate.invoke(message);
  }
}

class MyParentClass {
  final MyClass dep = MyClass();
  
  MyParentClass() {
    dep.errorDelegate.subscribe((message) {
       print("ANONYMOUS $message");
    });
    dep.errorDelegate.subscribe(subscribeError);
    dep.testError("test 123");
    dep.errorDelegate.remove(subscribeError);
    dep.testError("test 123 again");
  }
  
  void subscribeError(String message) {
    print("SUBSCRIBE ERROR: $message");
  }
}