What does 'yield' keyword do in flutter?

Question

What does the yield keyword actually do in Dart?

Answer 1

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).

Answer 2

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.