如何在回调函数中产生收益？

Question

Please read this bloc fragment:

if (event is TapVariant) {
  final bool isVariantCorrect = (correctVariantIndex == event.index);
  if (isVariantCorrect) {  
    yield CorrectVariant();
  } else {
    yield IncorrectVariant();
    Future.delayed(Duration(seconds: 1), () { 
      yield CorrectVariant();
    });
  }
}

I need to yield CorrectVariant from nested function.

I solved it this way:

    yield IncorrectVariant();
    await Future.delayed(Duration(seconds: 1), () {});
    yield CorrectVariant();

But I'm curious.

Answer 1

You already presented the best way to do it and here is why:

• As you are in an async* function you have access to the await keyword, which allows you to handle future callbacks in the same scope.

• If you were using yield in a sync* function, you could not wait for callbacks anyway as you are not running asynchronous code.

---

Return from the callback

As you are dealing with Future 's, you can also return your value inside the callback like this:

yield 1;

// The following statement will yield "2" after one second.
yield await Future.delayed(Duration(seconds: 1), () {
  return 2;
});