用于ES6课程的Mixins，用babel编译

Question

根据各种来源（ 2ality ， esdiscuss ），应该能够将mixins添加到类中：

编辑发现类方法不可枚举，因此无法工作。 编辑了下面的代码，但仍然没有快乐

class CartoonCharacter {
  constructor(author) {
    this.author = author;
  }

  drawnBy() {
    console.log("drawn by", this.author);
  }
}

// THIS CANNOT WORK
// class methods are not enumerable
// class Human {
//  haveFun() {
//    console.log("drinking beer");
//  }
// }

let Human = Object.create({}, {
  haveFun:   {
    enumerable: true,
    value: function () {
      console.log("drinking beer");
    }
  }
});

class Simpson extends Object.assign(CartoonCharacter, Human) {
  constructor(author) {
    super(author);
  }
}


let homer = new Simpson("Matt Groening");
homer.drawnBy();  // expected: drawn by Matt Groening
homer.haveFun();  // expected: drinking beer

我得到了“由Matt Groening绘制”但不是“喝啤酒”我得到了一个错误

-> Uncaught TypeError: E.haveFun is not a function

---

Answer 1

Subclass Factory Mixins

还有另一种在Javascript中实现mixins的方法：使用子类工厂。

子类工厂是一个除了基类并返回此基类的扩展子类的函数：

const mixin = base => class extends base {
  /* properties to mix in */
}

子类工厂可能有两个原因：

• class es可以定义为表达式，是Javascript中的第一类*
• extends子句可以包含任意表达式

让我们应用这种模式：

 // superclass class CartoonCharacter { constructor(author) { this.author = author } drawnBy() { return "drawn by " + this.author } } // mixin 1 const Human = base => class extends base { haveFun() { return "drinking beer" } } // mixin 2 const Simpson = base => class extends base {} // composed subclass const Homer = Simpson(Human(CartoonCharacter)); // create an instance const homer = new Homer("Matt Groening") console.log(homer.drawnBy()); console.log(homer.haveFun()); 

sublcass工厂的mixins有什么优势？

• 自然属性优先级：稍后混合的属性将覆盖同名的先前属性
• mixins（ Human / Simpson ）和子类（ Homer ）都可以像往常一样使用super
• mixins可以有构造函数（有状态混合）
• 没有原型或实例的变异

---

有状态的混合

国家让事情变得艰难。 你应该尽可能避免国家。 但是，有时你的mixin需要它自己的状态，它通过构造函数传递：

 class CartoonCharacter { constructor(author) { this.author = author } drawnBy() { return "drawn by " + this.author } } const Human = base => class extends base { haveFun() { return "drinking beer" } } const Simpson = base => class extends base { constructor(arg, ...superArgs) { super(...superArgs); this.name = arg; } sayHey() { return "Hey, I am " + this.name } } const Homer = Simpson(Human(CartoonCharacter)); const homer = new Homer("Homer Simpson", "Matt Groening") console.log(homer.drawnBy()); console.log(homer.haveFun()); console.log(homer.sayHey()); 

_{*术语第一类函数意味着函数可以作为参数传递或返回值，如普通数据}

Answer 2

你的mixins有两个问题：

1. Object.assign仅复制对象的可枚举属性。 但是，类的方法和属性是不可枚举的。
2. 类的方法和属性未在构造函数上定义。 它们是在构造函数的原型上定义的。

这是使用mixins扩展类的方法：

class CartoonCharacter {
  constructor(author) {
    this.author = author;
  }

  drawnBy() {
    console.log("drawn by", this.author);
  }
}

class Human {
  haveFun() {
    console.log("drinking beer");
  }
}

mixin(CartoonCharacter, Human);

class Simpson extends CartoonCharacter {
  constructor(author) {
    super(author);
  }
}


let homer = new Simpson("Matt Groening");
homer.drawnBy();  // expected: drawn by Matt Groening
homer.haveFun();  // expected: drinking beer

function mixin(target, source) {
  target = target.prototype; source = source.prototype;

  Object.getOwnPropertyNames(source).forEach(function (name) {
    if (name !== "constructor") Object.defineProperty(target, name,
      Object.getOwnPropertyDescriptor(source, name));
  });
}

它在Babel： demo中按预期工作。

Answer 3

正如@mfeineis，@ aadit-m-shah和@gotofritz以及@ftor的最新帖子所展示的那样，人们总是需要仔细考虑巫婆类会瘫痪成“混合”而不是如何（重新）将它们组装成一个仍然可以被认为是一个好设计的层次结构。 这些例子只是表明，JavaScript可能需要比我们已经（强制）使用的更接近特征的东西。 就像是 ...

适用对象/类型

• 不要将他们的语法与类的语法混合，
• 因为它们应该只在对象/实例级别工作，因此也可以从内部（类）构造函数和其他适用类型的（函数）体内工作，...
• 因此，它只是Trait和Object Composition Rules的 容器 ，它们将在这样的特征/ ApplicableType的应用时间内执行。

为了在实践中证明这样一个概念，我确实将所有上述例子重构为特质的特征组成，并从类构造函数中的特征组成对象组合 。

我想讨论这种方法，它的优点和缺点，与目前使用的所有已经被接受的技术相比较。

 // - stateful applicable object/type proxy that does // link `this` context and injected local state. // const withExposeInternalStateProxy = Trait.create(function (use, applicator) { applicator(function (stateValue) { this.valueOf = function valueOf () { return Object.assign({}, stateValue); }; this.toString = function toString () { return JSON.stringify(stateValue); }; }); }); // - applicable object/type that just implements plain behavior. // - also known as: function based mixin ... kind of "FLight Mixin reloaded". // const withHumanBehavior = Trait.create(function (use, applicator) { function allowWeekness() { return "allow weekness"; } function showStrength() { return "show strength"; } function thinkFirst() { return "think first"; } function drinkingBeer() { return "drinking beer"; } applicator(function () { this.allowWeekness = allowWeekness; this.showStrength = showStrength; this.thinkFirst = thinkFirst; this.drinkingBeer = drinkingBeer; }); }); // - composite trait that uses its trait composition language ... trait from trait(s). // const withStrongBeerDrinkingHabit = Trait.create(function (use, applicator) { use(withHumanBehavior) .apply(withHumanBehavior, "drinkingBeer").as("haveFun"); //.apply.all.without("thinkFirst"); //.apply(withHumanBehavior).without("thinkFirst"); }); // - trait that defines a specialized use case of another behavior. // const withDoAcceptAlreadyExistingDrinkingHabit = Trait.create(function (use, applicator) { applicator(function () { var type = this; if (typeof type.haveFun !== "function") { withStrongBeerDrinkingHabit.call(type); } }); }); class CartoonCharacter { constructor(stateValue) { // linking `this` context and injected local state. withExposeInternalStateProxy.call(this, stateValue); } drawnBy() { return ("drawn by " + this.valueOf().author); } } class MaleSimpsonsType extends CartoonCharacter { constructor(stateValue) { super(stateValue); // object/type composition // withDoAcceptAlreadyExistingDrinkingHabit.call(super.constructor.prototype); // withStrongBeerDrinkingHabit.call(super.constructor.prototype); // do not always apply. // // withStrongBeerDrinkingHabit.call(this); // nope ...prototype is the far better choice. } sayHey() { return ("Hey, I am " + this.valueOf().name); } } const homer = new MaleSimpsonsType({ name: "Homer Simpson", author: "Matt Groening" }); console.log("\\n"); console.log('(homer + "") : ', (homer + "")); console.log('homer.valueOf() : ', homer.valueOf()); console.log("\\n"); console.log('homer.drawnBy() : ', homer.drawnBy()); console.log('homer.haveFun() : ', homer.haveFun()); console.log('homer.sayHey() : ', homer.sayHey()); console.log("\\n"); console.log('Object.keys(homer) : ', Object.keys(homer)); console.log('homer.constructor : ', homer.constructor); console.log('Object.keys(homer.constructor.prototype) : ', Object.keys(homer.constructor.prototype)); console.log('homer.constructor.prototype : ', homer.constructor.prototype); console.log('(homer instanceof MaleSimpsonsType) : ', (homer instanceof MaleSimpsonsType)); console.log('(homer instanceof CartoonCharacter) : ', (homer instanceof CartoonCharacter)); 

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为了备份上面的例子，还有另一个线程 - 重构传统的基于mixin的类层次结构 - 我也有机会重构OP的给定示例。 这其中确实有after和around的平原基于对象的行为改变。 这里使用的方法/工具集确实符合OP的所有要求。 现在可以使用类，免费获取封装的状态/数据，并且对任何类型的方法修改都有状态支持。