Dart/Flutter - 从回调函数中产生

Question

I need to make a function call which does not return anything ( void ). Only way to get notified about function completion is sending a callback function.
Now I an using BLoC pattern along with ReDux , When an event is dispatched, I dispatch another action to store of redux , After action is completed it calls the callback function. And now inside callback function, I want to update the state of bloc . Below is my implementation,

if (event is Login) {
  yield currentState.copyWith(formProcessing: true);
  store.dispatch(authActions.login(
    currentState.username,
    currentState.password,
    (error, data) {
      print(error);
      print(data);
      // I want to yield here.
      yield currentState.copyWith(formProcessing: false);
    },
  ));
}

As shown in the above code snippet, Inside the callback function, I want to yield .

Solution

Create a function that returns the future and makes callback function to store dispatch, Heres sample.

if (event is Login) {
  yield currentState.copyWith(formProcessing: true);

  try {
    dynamic result = await loginAction(store, currentState.username, currentState.password);
    print(result);
    yield currentState.copyWith(formProcessing: false);
  } catch (e) {
    print(e);
  }
}

Future loginAction(store, username, password) {
  var completer = new Completer();

  store.dispatch(authActions.login(
    username,
    password,
    (error, data) {
      if (error != null) {
        completer.completeError(error);
      } else if (data != null) {
        completer.complete(data);
      }
    },
  ));

  return completer.future;
}

Answer 1

You need to create other event , and dispatch this event in your callback function, then you can do what you want in the function that filter your events .

I don't know what is the purpose of your BLoC, but the name of this event depends on the use case, it can be UpdateForm , UpdateState , LoggedIn , LoggedOut , etc. You will find the most descriptive name for your use case.

Remember that you can also create this event with parameters, for example UpdateForm (bool isLoggedIn) , and yield different states according to your conditions.

For the example, the name of this event is OtherEvent .

if (event is Login) {
  yield currentState.copyWith(formProcessing: true);
  store.dispatch(authActions.login(
    currentState.username,
    currentState.password,
    (error, data) {
      print(error);
      print(data);

      dispatch(OtherEvent());
    },
  ));
} else if (event is OtherEvent) {
   // You can yield here what you want
   yield currentState.copyWith(formProcessing: false);
}