Understanding async and async*/yield in Dart

In Dart, the keywords async and async* are used to handle asynchronous operations, but they serve different purposes. This article explains the differences between them - with some practical examples included.

What is async?

The async keyword is used to mark a function as asynchronous. It allows the use of await within the function and returns a Future.

Example of async:

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1Future<int> doSomeLongTask() async {
2  await Future.delayed(const Duration(seconds: 1));
3  return 42;
4}
5
6void main() async {
7  int result = await doSomeLongTask();
8  print(result); // prints '42' after waiting 1 second
9}

What is async*?

The async* keyword marks a function as an asynchronous generator, which returns a Stream of values instead of a single value.

Example of async*:

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1Stream<int> countForOneMinute() async* {
2  for (int i = 1; i <= 60; i++) {
3    await Future.delayed(const Duration(seconds: 1));
4    yield i;
5  }
6}
7
8void main() async {
9  await for (int i in countForOneMinute()) {
10    print(i); // prints 1 to 60, one integer per second
11  }
12}

Key Differences

• async functions return a Future and are useful for single asynchronous operations.
• async* functions return a Stream and are used to emit multiple asynchronous values over time.
• yield is used in async* functions to emit values, while return is used in async functions.

Best Practices

• Use async for functions that perform a single asynchronous task and need to return a value.
• Use async* for functions that need to emit multiple values over time, such as in data streams or periodic updates.