如何在 Javascript 中使用 mixin

Question

大家好我有摘要class电脑：

class Computer {
    constructor(manufacturer, processorSpeed, ram, hardDiskSpace) {
        if (new.target === Computer) {
            throw new Error("Cannot instantiate directly.");
        }
        this.manufacturer = manufacturer;
        this.processorSpeed = Number(processorSpeed);
        this.ram = Number(ram);
        this.hardDiskSpace = Number(hardDiskSpace);
    }
}

和桌面 class 扩展计算机 Class。我正在尝试将 mixin 功能连接到计算机 Class，如下所示：

computerQualityMixin.call(Computer.prototype);

并将其与 class 桌面的对象一起使用。这是我的混合代码；

function computerQualityMixin() {
    let ram = this.ram;
    let processorSpeed = this.processorSpeed;
    let hardDiskSpace = this.hardDiskSpace;
    this.getQuality = () => {
        return processorSpeed
            * ram
            * hardDiskSpace;
    };
    this.isFast = () => {
        return processorSpeed > ram / 4;
    };
    this.isRoomy = () => {
        return hardDiskSpace > Math.floor(ram * processorSpeed);
    };
}

问题是我得到了我尝试获得的所有属性的 'undefined'：'this.ram' 例如在我的 mixin 中，当我调用某个函数时：

let desktop = new Desktop("JAR Computers", 3.3, 8, 1);
console.log(desktop.getQuality());//Returns NaN because try to make Math operations with 'undefined'

有人可以帮助我理解 mixins 吗？谢谢。

Answer 1

这些评论提出了很好的问题，即您是否真的想在这里使用 mixins。但如果你这样做，你可能想看看 Angus Croll and Reg Braithwaite

的文章

使用前者的技术，您可以重写为

const asComputerQuality = function() {
  this.getQuality = function() {
    return this.processorSpeed
           * this.ram
           * this.hardDiskSpace;
  };
  this.isFast = function() {
    return this.processorSpeed > this.ram / 4;
  };
  this.isRoomy = function() {
    return this.hardDiskSpace > Math.floor(this.ram * this.processorSpeed);
  };
}

asComputerQuality.call(Computer.prototype);

那么您应该能够在 Computer 个实例上调用这些方法。

Answer 2

Mixins 应该被视为一种方便的代码重用形式。

描述对象特定行为的代码，以及往往被一遍又一遍地复制，可能会被考虑曾经 collected/stored 成为混音。

使用 JavaScript 中基于 mixin/trait 模式的函数还可以利用其提供的有状态变体人们可能会如何安排的更多可能性 type/object 个架构。

正如已经指出的那样，OP 的示例不太好根据适合 mixin/trait 的构图选择。

下一个给定的代码块仍然会稍微尝试一下改变变体以展示不同的方法在 JavaScript ...

中应用基于函数的 mixin/trait 模式

function withObjectBaseIntrospection(state) { // - mixin that preserves injected
    var                                       //   local state by creating a
        object = this;                        //   closure at call/apply time.

    object.valueOf = function () {
        return Object.assign({}, state);
    };
    object.toString = function () {
        return JSON.stringify(state);
    };
}


function withHardwareStandardGetters(state) { // - mixin that preserves injected
    var                                       //   local state by creating a
        hardware = this;                      //   closure at call/apply time.

    Object.defineProperty(hardware, "manufacturer", {
        get: function () { return state.manufacturer; }
    });
    Object.defineProperty(hardware, "processorSpeed", {
        get: function () { return state.processorSpeed; }
    });
    Object.defineProperty(hardware, "ram", {
        get: function () { return state.ram; }
    });
    Object.defineProperty(hardware, "hardDiskSpace", {
        get: function () { return state.hardDiskSpace; }
    });
}
function withDesktopSpecificGetters(state) {  // - mixin that preserves injected
    var                                       //   local state by creating a
        hardware = this;                      //   closure at call/apply time.

    Object.defineProperty(hardware, "bodyLength", {
        get: function () { return state.bodyLength; }
    });
    Object.defineProperty(hardware, "bodyWidth", {
        get: function () { return state.bodyWidth; }
    });
    Object.defineProperty(hardware, "bodyHeight", {
        get: function () { return state.bodyHeight; }
    });
}


function withHardwareSpecificQuality() {  // - generic function based mixin pattern.
    this.getQuality = function() {
        return (this.processorSpeed * this.ram * this.hardDiskSpace);
    };
    this.isFast = function () {
        return (this.processorSpeed > (this.ram / 4));
    };
    this.isRoomy = function () {
        return (this.hardDiskSpace > Math.floor(this.ram * this.processorSpeed));
    };
}
function withDesktopSpecificMeasures() {  // - generic function based mixin pattern.
    this.getBodyVolume = function() {
        return (this.bodyLength * this.bodyWidth * this.bodyHeight);
    };
}


class Computer {
    constructor(state) {

        withObjectBaseIntrospection.call(this, state);  // - applying 2 "stateful mixin"
        withHardwareStandardGetters.call(this, state);  //   at instance/object level.
    }
}
withHardwareSpecificQuality.call(Computer.prototype);   // - making use of inheritance via the
                                                        //   constructor's prototype, but enriching the
                                                        //   latter by a more generic mixin (at "class level").
class Desktop extends Computer {  // - newly available
    constructor(state) {          //   syntactic sugar for the more
                                  //   "class like" inheritance pattern.
        super(state);
        withDesktopSpecificGetters.call(this, state);   // - applying a "stateful mixin"
    }                                                   //   at instance/object level.
}
withDesktopSpecificMeasures.call(Desktop.prototype);    // - making use of inheritance via the
                                                        //   constructor's prototype, but enriching the
                                                        //   latter by a more generic mixin (at "class level").
let
    desktop = new Desktop({

        manufacturer: "JAR Computers",
        processorSpeed: 3.3,
        ram: 8,
        hardDiskSpace: 1,

        bodyWidth: 300,
        bodyHeight: 40,
        bodyLength: 300
    });

console.log("Desktop.prototype : ", Desktop.prototype);
console.log("Computer.prototype : ", Computer.prototype);

console.log("(desktop instanceof Desktop) ? ", (desktop instanceof Desktop));
console.log("(desktop instanceof Computer) ? ", (desktop instanceof Computer));

console.log("desktop.manufacturer : ", desktop.manufacturer);
console.log("desktop.processorSpeed : ", desktop.processorSpeed);
console.log("desktop.ram : ", desktop.ram);
console.log("desktop.hardDiskSpace : ", desktop.hardDiskSpace);

console.log("desktop.getQuality() : ", desktop.getQuality());
console.log("desktop.getBodyVolume() : ", desktop.getBodyVolume());

console.log("desktop.valueOf() : ", desktop.valueOf());
console.log("desktop.toString() : ", desktop.toString());

人们也可以考虑看一个更好的 JavaScript 示例，它也试图证明 when to use inheritance via class extension and when not, when to use just mixin/trait based composition and also when to use both。

旁注 - JavaScript

中关于基于 Mixins/Traits/Taltens 的函数的推荐资源

A fresh look at JavaScript Mixins 安格斯·克罗尔自 2011 年 5 月起
The many talents of JavaScript for generalizing Role Oriented Programming approaches like Traits and Mixins 自 2014 年 4 月起。

此外，我确实建议阅读我在 SO 上给出的一些列出的答案，它们也与该主题相关。

ES 6 Classes - Mixins
How to use mixins properly in Javascript
Traits in javascript

如何在 Javascript 中使用 mixin

How to use mixin in Javascript

javascript

oop

ecmascript-6

mixins

composition