red-block-7846

readme.md

red-block-7846

This enormous Dart program contains some errors and warnings, and demonstrates use of async features. This code has no associated html or css.

Find this at dartpad.dartlang.org/?source=e4d8b5ab-7eb8-4a6b-95e0-28590a2c309b.

Created with <3 with dartpad.dartlang.org.

main.dart

import 'dart:async';


main() {
  var completer = new SafeCompleter();
  completer.complete('foo');
  completer.getFuture().then((val) => print('Complete! $val'));
}

class SafeCompleter<T> {
  bool _getFutureCalled = false;
  Completer<T> _completer = new Completer<T>();

  bool get isCompleted => _completer == null ? true : _completer.isCompleted;

  void complete([FutureOr<T> value]) {
    if (_completer == null) {
      throw new StateError('Future already completed');
    }
    _completer.complete(value);
  }

  void completeError(Object error, [StackTrace stackTrace]) {
    if (_completer == null) {
      throw new StateError('Future already completed');
    }
    _completer.completeError(error, stackTrace);
  }

  /// Returns the future that will contain the result provided to this completer.
  ///
  /// This method can only be called once for this instance; subsequent calls will throw a [StateError].
  ///
  /// This restriction prevents this completer from leaking the completed value or errors,
  /// while still allowing users to check [isCompleted].
  Future<T> getFuture() {
    if (_getFutureCalled) {
      throw new StateError('getFuture can only be called once; the Future is no longer available');
    }
    _getFutureCalled = true;
    
    _completer.future.then((_) {
      _completer = null;
    });
    
    // TODO: this may be annoying in practice, but would help prevent leaks caused by getFuture never being called.
    // TODO: Is there a better way to check this? Is it even necessary to protect against this case?
    // This does not cause issues if `.getFuture()` is called after `.complete()`
    // but within the same tick, due to the async nature of `.complete`.
    assert(_completer == null || !_completer.isCompleted,
        'getFuture must be called before the completer completes');

    return _completer.future;
  }
}