Abstract methods in Dart API reference

Question

Many methods like complete in class Completer are marked "abstract", but in fact It can be directly invoked without being implemented. I'm really confused. Could anyone help me?

Answer 1

Yes, this can be a bit confusing. While abstract classes cannot be instantiated, it is possible to make them appear to be instantiable by defining a factory constructor. This is what Completer, Future and other abstract classes do:

abstract class Completer<T> {

  factory Completer() => new _CompleterImpl<T>();

  ...
}

You can then invoke methods on the object created by the factory constructor. In the example above, factory Completer() returns a new _CompleterImpl object. Look at the (truncated) code of that class:

class _CompleterImpl<T> implements Completer<T> {

  final _FutureImpl<T> _futureImpl;

  _CompleterImpl() : _futureImpl = new _FutureImpl() {}

  Future<T> get future {
    return _futureImpl;
  }

  void complete(T value) {
    _futureImpl._setValue(value);
  }
  ...
}

and you see complete(); that is the method being invoked.