Extending a class with only one factory constructor

Question

I was wondering which is the best way to extend the CustomEvent class, a class which has only one factory constructor. I tried doing the following and ran into an issue with the super constructor :

class MyExtendedEvent extends CustomEvent {
  int count;

  factory MyExtendedEvent(num count) {
    return new MyExtendedEvent._internal(1);
  }

  MyExtendedEvent._internal(num count) {
    this.count = count;
  }
}

but I can't get it working. I always run into :

unresolved implicit call to super constructor 'CustomEvent()'

If i try chaning the internal constructor to :

MyExtendedEvent._internal(num count) : super('MyCustomEvent') {
  this.count = count;
}

I end up with :

'resolved implicit call to super constructor 'CustomEvent()''.

I'm not sure what I'm doing wrong - but I guess the problem is that the CustomEvent has only one constructor which is a factory constructor (as doc says - http://api.dartlang.org/docs/releases/latest/dart_html/CustomEvent.html)

What is the best way to extend a CustomEvent, or any class of this form?

It seems like it isn't possible at all to extend CustomEvent. I tried to outsmart it, and ended up with a runtime error instead: Internal error: '[filename]': Error: line 3 pos 7: class 'MyEvent' is trying to extend a native fields class, but library '[same filename]' has no native resolvers". I don't know if one can work around this or not, sorry. — MarioP, Sep 02 '13 at 08:21

Alexandre Ardhuin · Answer 1 · 2013-09-02T09:02:50.383

23

You can't directly extend a class with a factory constructor. However you can implement the class and use delegation to simulate the extension.

For instance :

class MyExtendedEvent implements CustomEvent {
  int count;
  final CustomEvent _delegate;

  MyExtendedEvent(this.count, String type) :
    _delegate = new CustomEvent(type);

  noSuchMethod(Invocation invocation) =>
      reflect(_delegate).delegate(invocation);
}

NB : I used reflection here to simplify the code snippet. A better implementation (in regard of performances) would have been to define all methods like method1 => _delegate.method1()

edited Sep 02 '13 at 09:02

answered Sep 02 '13 at 08:49

Alexandre Ardhuin

71,959
15
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How will this affect performance? I can imagine that reflection is an expensive operation. – Christophe Herreman Sep 02 '13 at 08:58
3

I used reflection here to simplify the code snippet. A better implementation would have been to define all methods like `method1 => _delegate.method1()` – Alexandre Ardhuin Sep 02 '13 at 09:00
Dart/Flutter documentation states that reflection is not used because it's not convenient when using tree shaking – funder7 Jan 07 '21 at 20:21

score 5 · Accepted Answer · answered Sep 02 '13 at 18:04

Unfortunately, you can't extend a class if it only has factory constructors, you can only implement it. That won't work well with CustomEvent though since it's a DOM type, which is also why it only has factory constructors: the browser has to produce these instances, the Dart object is just a wrapper. If you try to implement CustomElement and fire one, you'll probably get an error.

spydon · Answer 3 · 2020-09-25T11:01:23.373

You can now use the extension feature of Dart (from Dart 2.7) to get a similar behaviour if you just want to add some methods to the existing class.

extension MyExtensionEvent on CustomEvent {
  void increaseCount() => count++;
}

and then when you want to use your newly created extension you just import it and then call an instance of the original class but with one of your newly created methods:

import event_extension.dart

final event = CustomEvent(1);
event.increaseCount();

dilshan · Answer 4 · 2021-04-21T17:05:50.763

0

If the class has only factory constructors(named or unnamed) cannot be used for extending. (But you can use this class to implement subclasses)

So if you want to extend a class by a superclass, The superclass must be needed to have at least one generative constructor.

edited Apr 21 '21 at 17:05

answered Apr 21 '21 at 05:33

dilshan

2,045
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Extending a class with only one factory constructor

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