扩展一个只有一个工厂构造函数的类 [英] Extending a class with only one factory constructor

问题描述

我想知道哪个是扩展 CustomEvent 类的最好方法，这个类只有一个工厂构造函数。我试着做以下和遇到一个问题与超级构造函数：

  class MyExtendedEvent extends CustomEvent {
 int count; 
 
 factory MyExtendedEvent（num count）{
 return new MyExtendedEvent._internal（1）; 
} 
 
 MyExtendedEvent._internal（num count）{
 this.count = count; 
} 
} 
  

但我无法使用。我总是遇到：

未解决的对超级构造函数CustomEvent（）的隐式调用

如果我尝试内部构造函数：

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

我最后得到：

$ b $

I'b 我不知道我在做什么 - 但我想问题是 CustomEvent 只有一个构造函数是一个工厂构造函数（如doc说 - http://api.dartlang.org/docs/releases/latest/dart_html/CustomEvent.html ）

扩展 CustomEvent 或此表单的任何类别的最佳方法是什么？

解决方案

您不能使用工厂构造函数直接延伸类。

例如：

$ b $

  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：我在这里使用反射来简化代码片段。一个更好的实现（关于性能）将定义所有的方法，例如 method1 => _delegate.method1（）

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?

解决方案

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()

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