Dart-试图了解“工厂”构造函数的价值 [英] Dart - Trying to understand the value of 'factory' constructor

问题描述

如果我正确理解：

"A factory constructor affords an abstract class to be 
    instantiated by another class, despite being abstract." 

例如：

abstract class Animal {
   String makeNoise(String sound);
   String chooseColor(String color);
   factory Animal() => new Cat(); 
}

class Cat implements Animal {
   String makeNoise(String noise) => noise;
   String chooseColor(color) => color;
}

以上内容使我可以做到这一点：

Cat cat = new Animal();
var catSound = cat.makeNoise('Meow');
var catColor = cat.chooseColor('black');
print(catSound); // Meow

这也阻止了我这样做：

class Dog implements Animal {
 int age;
 String makeNoise(String noise) => noise;
 String chooseColor(color) => color;
}

Dog dog = new Animal(); // <-- Not allowed because of the factory constructor

，我被问到为什么要为Animal加上额外的代码？

So if I am correct with all this, I am led to ask why the extra code for Animal?

如果您打算为动物使用工厂构造函数（仅创建猫），为什么不只拥有Cat？

If you intend on using a factory constructor for animal, which creates only cats, why not just have a Cat class with the required methods/properties?

或者，动物类的目的是使用上面的工厂构造函数，真的是专门为Cat类设计的接口吗？

Or, is the purpose of the Animal class with a factory constructor like above, really an interface specifically designed for Cat class only?

推荐答案

我认为工厂中的问题 code>。

您的代码最初是错误的。

Your code initially was wrong.

看下面的代码并得出结论

Look at this code and make your conclusions.

Locomotive locomotive = new SomethingWithSmokeFromChimney();

现在看下面的代码。

Plant plant = new SomethingWithSmokeFromChimney();

您错误地认为地球上的所有动物（甚至是狗）都是猫。

You mistakenly believe that all animals on the earth (even the dogs) are cats.

Cat cat = new Animal();

如果您要这样做。

Cat cat = new Animal();
Dog dog = new Animal();

然后（如果我正确理解你的话），你也想要这个。

Then (if I correctly understand you) you also want this.

// Cat cat = new Animal();
// Dog dog = new Animal(); 
Dog dog = new Cat();

PS

相同的错误结论，但没有工厂。

The same wrong conclusions but without factory.

void main() {
  Cat cat = new Animal();
  Dog dog = new Animal();
}

class Animal {
}

class Cat implements Animal {
}

class Dog implements Animal {
}

但是此代码（取决于文档）可能被认为是正确的。 / p>

But this code (depending on documenation) may be considered correct.

void main() {
  Cat cat = new Animal("cat");
  Dog dog = new Animal("dog");
}

abstract class Animal {
  factory Animal(String type) {
    switch(type) {
      case "cat":
        return new Cat();
      case "dog":
        return new Dog();
      default:
        throw "The '$type' is not an animal";
    }
  }
}

class Cat implements Animal {
}

class Dog implements Animal {
}

抽象类的工厂构造函数可以（默认）返​​回该抽象类的一些默认实现

The factory constructor of the abstract class can return (by default) some default implementation of this abstract class.

abstract class Future<T> {
   factory Future(computation()) {
    _Future result = new _Future<T>();
    Timer.run(() {
      try {
        result._complete(computation());
      } catch (e, s) {
        result._completeError(e, s);
      }
    });
    return result;
  }
}

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