What does 'yield' keyword do in flutter?

Question

What does the yield keyword actually do in Dart?

Answer 1

The accepted answer's link is broken, here is an official link about async* sync* yield* yield.

If you have some experiences with other languages, you might stuck at these keywords. Here are some tips for getting over keywords.

1. async* sync* yield* yield are called generator functions. You might use these mostly in Bloc pattern.

2. async* is also a async, you could use Asynchronous as usual.

3. sync* cannot be used as sync, you will receive the error that noticed "The modifier sync must be followed by a star".

4. yield and yield* can only be used with generator functions (async* sync*).

And there are four combinations.

1. async* yield will return a Stream<dynamic>.

Stream<int> runToMax(int n) async* {
  int i = 0;
  while (i < n) {
    yield i;
    i++;
    await Future.delayed(Duration(seconds: 300));
  }
}

2. async* yield* will call a function and return Stream<dynamic>.

Stream<int> countDownFrom(int n) async* {
  if (n > 0) {
    yield n;
    yield* countDownFrom(n - 1);
  }
}

3. sync* yield will return a Iterable<dynamic>.

Iterable<int> genIterates(int max) sync* {
  var i = 0;
  while (i < max) {
    yield i;
    i++;
  }
}

4. sync* yield* will call a function and return Iterable<dynamic>.

Iterable<int> countDownFrom(int n) sync* {
  if (n > 0) {
    yield n;
    yield* countDownFrom(n - 1);
  }
}

If there are any errors, please leave a comment to correct the answer.