在Dart中开启课程类型 [英] Switching on class type in Dart

问题描述

我希望在Dart超类中编写一个函数，该函数根据实际使用的子类采取不同的操作。像这样的东西：

I'm looking to write a function in a Dart superclass that takes different actions depending on which subclass is actually using it. Something like this:

class Foo {
  Foo getAnother(Foo foo) {
    var fooType = //some code here to extract fooType from foo;
    switch (fooType) {
      case //something about bar here:
        return new Bar();
      case //something about baz here:
        return new Baz();
    }
  }
}

class Bar extends Foo {}

class Baz extends Foo {}

其中的想法是我有一些对象，并且想要获得相同（子）类的新对象。

where the idea is that I have some object and want to get a new object of the same (sub)class.

主要问题是 fooType 应该是什么类型？我的第一个想法是Symbol，它会导致产生简单的案例陈述，例如 case #Bar：，但我不知道如何填充 fooType 和一个符号。我唯一想到的选择是做 Symbol fooType = new Symbol（foo.runtimeType.toString（））; 之类的事情，但是我的理解是 runtimeType.toString（）转换为javascript后将无法使用。您可以通过使用Mirrors来解决此问题，但这是一个轻量级的库，因此不在表中。 Object.runtimeType 返回 Type 类的内容，但是我不知道如何创建<$ c $的实例c> Type 我可以用于case语句。也许我错过了Dart库中其他更适合此功能的部分？

The main question is what type should fooType be? My first thought was Symbol, which leads to easy case statements like case #Bar:, but I don't know how I would populate fooType with a Symbol. The only options I can think of are to do something like Symbol fooType = new Symbol(foo.runtimeType.toString()); but my understanding is that runtimeType.toString() won't work when converted to javascript. You could get around that by using Mirrors, but this is meant to be a lightweight library, so those aren't on the table. Object.runtimeType returns something of the Type class, but I have no idea how to create instances of Type I could use for the case statements. Maybe I'm missing some other piece of the Dart library that is better suited for this?

推荐答案

您可以使用 runtimeType 在 switch 中：

class Foo {
  Foo getAnother(Foo foo) {
    switch (foo.runtimeType) {
      case Bar:
        return new Bar();
      case Baz:
        return new Baz();
    }
    return null;
  }
}

在情况下语句直接使用类名（也称为类文字）。这给出了与该类相对应的 Type 对象提到。因此，可以将 foo.runtimeType 与指定的类型进行比较。

In the case statements the class name is use directly (aka. class literal). This gives a Type object corresponding to the class mentionned. Thus foo.runtimeType can be compared with the specified type.

请注意您不能在类文字中暂时使用泛型。因此，不允许 case List< int>：。

Note that you can not use generics for now in class literals. Thus case List<int>: is not allowed.

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