Learn Cleanup with whenComplete | Chaining and Coordination

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


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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');
      });
}

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Section 2. Chapter 2

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main.dart


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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');
      });
}

Everything was clear?

Thanks for your feedback!

Section 2. Chapter 2