Dart Future then()
Syntax & Examples

Future.then() method

The `then` method in Dart registers callbacks to be called when a future completes.

Syntax of Future.then()

The syntax of Future.then() method is:

 Future<R> then<R>(FutureOr<R> onValue(T value), { Function onError })

This then() method of Future register callbacks to be called when this future completes.

Parameters

ParameterOptional/RequiredDescription
onValuerequiredA callback function to be called when the future completes successfully, with the value of the future as an argument.
onErroroptionalA callback function to be called if the future completes with an error.

Return Type

Future.then() returns value of type Future<R>.


✐ Examples

1 Handling successful completion

In this example,

  1. We create a Future called futureInt that resolves to 42 after a delay of 2 seconds.
  2. We register a callback using then to print the value when the future completes successfully.

Dart Program

void main() {
  Future<int> futureInt = Future.delayed(Duration(seconds: 2), () => 42);
  futureInt.then((value) {
    print('Future completed with value: $value');
  });
}

Output

Future completed with value: 42

2 Handling completion with an error

In this example,

  1. We create a Future called futureString that throws an exception after a delay of 3 seconds.
  2. We register callbacks using then to print the value on success and the error on failure.

Dart Program

void main() {
  Future<String> futureString = Future.delayed(Duration(seconds: 3), () => throw Exception('Error!'));
  futureString.then((value) {
    print('Future completed with value: $value');
  }, onError: (error) {
    print('Future completed with error: $error');
  });
}

Output

Future completed with error: Exception: Error!

3 Handling completion with a list value

In this example,

  1. We create a Future> called futureList that resolves to [1, 2, 3] after a delay of 1 second.
  2. We register a callback using then to print the list when the future completes successfully.

Dart Program

void main() {
  Future<List<int>> futureList = Future.delayed(Duration(seconds: 1), () => [1, 2, 3]);
  futureList.then((value) {
    print('Future completed with value: $value');
  });
}

Output

Future completed with value: [1, 2, 3]

Summary

In this Dart tutorial, we learned about then() method of Future: the syntax and few working examples with output and detailed explanation for each example.