What does 'yield' keyword do in flutter?

yield adds a value to the output stream of the surrounding async* function. It's like return, but doesn't terminate the function.

See https://dart.dev/guides/language/language-tour#generators

Stream asynchronousNaturalsTo(n) async* {
  int k = 0;
  while (k < n) yield k++;
}

When the yield statement executes, it adds the result of evaluating its expression to the stream. It doesn’t necessarily suspend (though in the current implementations it does).

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.