How To

Use Generators in Dart

In Dart, generators are functions that generate a sequence of values on-demand (lazy evaluation). Dart's generators can be synchronous and asynchronous. Synchronous generators return an Iterable while asynchronous ones return a Stream. Asynchronous generators allow for asynchronous operations in their bodies (i.e. operations invoked with the await keyword), something that cannot be done in synchronous ones. Generators in Dart can also be used recursively.

Synchronous Generators

You can create a synchronous generator from a function using the sync* keyword:

Iterable<int> range(int num) sync* {
  while (num > 0) {
    yield --num;
  }
}

void main() {
  for (int value in range(10)) {
    print(value);
  }
}

Asynchronous Generators

You can create a asynchronous generator from a function using the async* keyword:

Stream<int> range(int num) async* {
  while (num > 0) {
    yield --num;
  }
}

void main() {
  range(10).listen((value) => print(value));
}

Recursive Generators

You can make generators recursive by using the yield* on the recursive function invocation:

Iterable<int> range(int num) sync* {
  if (num > 0) {
    yield --num;
    yield* range(num);
  }
}