dart:async

Completer<T> class

A way to produce Future objects and to complete them later with a value or error.

Most of the time, the simplest way to create a future is to just use one of the Future constructors to capture the result of a single asynchronous computation:

Future(() { doSomething(); return result; });

or, if the future represents the result of a sequence of asynchronous computations, they can be chained using Future.then or similar functions on Future:

Future doStuff(){
  return someAsyncOperation().then((result) {
    return someOtherAsyncOperation(result);
  });
}

If you do need to create a Future from scratch — for example, when you're converting a callback-based API into a Future-based one — you can use a Completer as follows:

class AsyncOperation {
  final Completer _completer = new Completer();

  Future<T> doOperation() {
    _startOperation();
    return _completer.future; // Send future object back to client.
  }

  // Something calls this when the value is ready.
  void _finishOperation(T result) {
    _completer.complete(result);
  }

  // If something goes wrong, call this.
  void _errorHappened(error) {
    _completer.completeError(error);
  }
}

Constructors

Completer()

factory

Creates a new completer. [...]

Completer.sync()

factory

Completes the future synchronously. [...]

Properties

future → Future<T>

read-only

The future that is completed by this completer. [...]

hashCode → int

read-only, inherited

The hash code for this object. [...]

isCompleted → bool

read-only

Whether the future has been completed. [...]

runtimeType → Type

read-only, inherited

A representation of the runtime type of the object.

Methods

complete([FutureOr<T>? value]) → void

Completes future with the supplied values. [...]

completeError(Object error, [StackTrace? stackTrace]) → void

Complete future with an error. [...]

noSuchMethod(Invocation invocation) → dynamic

inherited

Invoked when a non-existent method or property is accessed. [...]

toString() → String

inherited

A string representation of this object. [...]

Operators

operator ==(Object other) → bool

inherited

The equality operator. [...]