What 's the difference between async and async* in Dart?

In Dart, both the async and async* keywords are used for asynchronous operations, but they differ in their usage scenarios and return types.

async

When you use the async keyword in a function definition, it indicates that the function is asynchronous and returns a Future. Within such a function, you can use the await keyword to pause execution until an asynchronous operation completes and retrieve its result.

Example:

dart
Future<String> fetchUserOrder() async {
  var order = await fetchAsyncFromDatabase();
  return 'Your order is: $order';
}

In the above example, fetchAsyncFromDatabase() is an asynchronous function returning a Future of database query results. Using the await keyword, you can write code in a nearly synchronous manner, waiting for the result before proceeding.

async*

Functions defined with the async* keyword are generator functions that return a Stream. This allows the function body to use the yield and yield* keywords to provide multiple values, which are sent one by one over time.

Example:

dart
Stream<int> countStream(int to) async* {
  for (int i = 1; i <= to; i++) {
    await Future.delayed(Duration(seconds: 1));
    yield i; // Send numbers one by one
  }
}

In this example, the countStream function generates numbers every second until reaching the specified limit. Each yield operation effectively waits for the completion of a Future.delayed, simulating a time-consuming operation.

Summary

• Functions defined with the async keyword return a Future, suitable for a single asynchronous result.
• Functions defined with the async* keyword return a Stream, suitable for generating multiple asynchronous events.

In practical applications, choosing between async and async* depends on whether you need a single or multiple asynchronous results from the function.