避免在Dart中继承类 [英] Avoid inheritance of class in Dart

问题描述

在Dart中是否有避免继承的方法?我正在寻找

Is there a way to avoid inheritance in Dart? I'm looking for something like

final class MyJavaClass {
    ...
}

推荐答案

不直接，不.

您可以编写带有私有构造函数的类，并通过静态方法访问它们:

You could write a class with private constructors and access them via static methods:

class MyFinalClass {
  MyFinalClass._ctor1() {}
  MyFinalClass._ctor2(String param1, String param2) {}
  static MyFinalClass getInstance() {
    return new MyFinalClass._ctor1();
  }
  static MyFinalClass getInstanceWithParams(String param1, String param2) {
    return new MyFinalClass._ctor2(param1, param2);
  }
}

但这有多个问题:

• 同一库中的类仍然可以对其进行子类化-Dart中的可见性适用于库，而不是单个类.
• 很多代码，随构造函数的数量而扩展.
• 它引入了许多静态方法，所有这些方法都必须以不同的方式命名.
• 当然，您不能使用 new 关键字在类库之外实例化该类.
• 这仅防止扩展类.仍然可以实现最终"类.

• Classes inside the same library can still subclass it - visibility in Dart applies to libraries, not single classes.
• It's a lot of code, scaling with the amount of constructors.
• It introduces lots of static methods, which all have to be named differently.
• And of course, you can't instantiate the class outside its library with the new keyword.
• This only prevents extension of a class. Implementing the "final" class is still possible.

这些功能最终有很多缺点.由您决定是否真的值得.

Ultimately, these are quite a few drawbacks for this feature. It's up to you to decide if it is really worth it.

编辑

与我之前写的相反，工厂构造函数也可以工作，消除了无法使用 new 关键字实例化"的缺点.

Contrary to what I have written before, factory constructors would also work, eliminating the "unable to instantiate with new keyword" drawback.

class MyFinalClass {
  factory MyFinalClass.ctor1() {}
  factory MyFinalClass.ctor2(String param1, String param2) {}
  void method1() {}
  void method2() {}
}

另外，为了说明为什么不能实现最终类的能力是一个大问题:

Also, to illustrate why the ability to implement the not-so-final class is a big problem:

class Sub implements MyFinalClass {
  MyFinalClass _d;
  Sub.ctor1() {
    _d = new MyFinalClass.ctor1();
  }
  Sub.ctor2(String p1, String p2) {
    _d = new MyFinalClass.ctor2(p1,p2);
  }
  void method1() => _d.method1();
  void method2() {
    // do something completely different
  }
}

此伪子类化也可以与静态方法变体一起使用.

This pseudo-subclassing would work with the static method variant as well.

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