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Dart Asynchronous Flow

bookCleanup with whenComplete

When working with asynchronous operations in Dart, you often need to perform some cleanup or final actions after a Future completes. The whenComplete method provides a way to register such actions that will always run, whether the Future completes with a value or throws an error. This is different from then, which only runs if the Future completes successfully, and catchError, which only runs if there is an error.

Use whenComplete when you need to guarantee that some code runs after a Future ends, regardless of its outcome. This is especially useful for tasks like closing files, stopping timers, or printing status messages that should always happen after an operation.

main.dart

main.dart

copy

              
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import 'dart:math';

Future<void> performOperation() async {
  print('Starting operation...');
  // Randomly succeed or fail
  if (Random().nextBool()) {
    await Future.delayed(Duration(seconds: 1));
    print('Operation succeeded.');
  } else {
    await Future.delayed(Duration(seconds: 1));
    throw Exception('Operation failed!');
  }
}

void main() {
  performOperation()
      .whenComplete(() {
        print('Cleanup: Operation finished (success or error).');
      })
      .catchError((e) {
        print('Caught error: $e');
      });
}
question mark

Which statement best describes the role of whenComplete in a Dart Future chain?

Select the correct answer

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Sektion 2. Kapitel 2

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bookCleanup with whenComplete

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When working with asynchronous operations in Dart, you often need to perform some cleanup or final actions after a Future completes. The whenComplete method provides a way to register such actions that will always run, whether the Future completes with a value or throws an error. This is different from then, which only runs if the Future completes successfully, and catchError, which only runs if there is an error.

Use whenComplete when you need to guarantee that some code runs after a Future ends, regardless of its outcome. This is especially useful for tasks like closing files, stopping timers, or printing status messages that should always happen after an operation.

main.dart

main.dart

copy

              
1234567891011121314151617181920212223

            
import 'dart:math';

Future<void> performOperation() async {
  print('Starting operation...');
  // Randomly succeed or fail
  if (Random().nextBool()) {
    await Future.delayed(Duration(seconds: 1));
    print('Operation succeeded.');
  } else {
    await Future.delayed(Duration(seconds: 1));
    throw Exception('Operation failed!');
  }
}

void main() {
  performOperation()
      .whenComplete(() {
        print('Cleanup: Operation finished (success or error).');
      })
      .catchError((e) {
        print('Caught error: $e');
      });
}
question mark

Which statement best describes the role of whenComplete in a Dart Future chain?

Select the correct answer

Var alt klart?

Hvordan kan vi forbedre det?

Tak for dine kommentarer!

Sektion 2. Kapitel 2
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