javascript framework design and other factories_javascript skills

The emergence of classes and inheritance in JavaScript shows that JavaScript has reached the threshold of large-scale development. Before it was ECMAScript 4, it tried to introduce classes, modules and other things, but due to the excessive introduction of too many features, JavaScript became a mess. resulting in rejection. But it just delays the class to ES6. So far, JavaScript does not have a real class. However, we can simulate classes. In recent times, class factories have been the standard feature of frameworks. This chapter will introduce various class implementations to facilitate everyone to choose the style they like in their own framework.

1.javascript support for classes

In other languages, instances of classes must be created through the constructor new. As a language that deliberately imitates Java. There is a new operator in JavaScript, and all functions can be used as constructors. Constructors are no different from ordinary methods. In order to build its flourishing ecosystem, such as Node, Element, HTMLElement, and HTMLParagraphElement, the browser obviously uses inheritance relationships to facilitate the sharing of some methods or attributes, so JavaScript borrows the prototype mechanism from other languages. Prototype exists as a special object property on every function. When a function uses the new operator new to produce its "child" - "instance", this object named instance has all the members of the prototype object of this function, thereby realizing that all instance objects share a set of methods or properties. . The so-called "class" in JavaScript is to distinguish the native object from its other defined "classes" by modifying this Prototype object. In the browser, the node class is modified based on Object, while Element is based on Node, and HTMLElement is based on Element.... Compared with our work business, we can create our own classes to achieve reuse and sharing.

  function A(){

  }
  A.prototype = {
    aa:"aa",
    method:function(){
    }
  };
  var a = new A;
  var b = new A;
  console.log(a.aa === b.aa);
  console.log(a.method === b.method)

Generally, we call the method defined on the prototype a prototype method, which is shared by all instances. This is good and bad. In order to achieve differentiation, JavaScript allows us to specify its method directly in the constructor. This is called the privileged method. If it is an attribute, it is called a privileged attribute. Each instance of them has a copy and is not affected by each other. Therefore, we usually put the shared methods for operating data in the prototype, and put the private attributes in the privileged attributes. But if you put it on this, it can still be accessed at will, so put it in the scope of the function body. At this point it becomes a truly private property.

  function A() {
    var count = 0;
    this.aa = "aa";
    this.method = function() {
      return count;
    }
    this.obj = {}
  }
  A.prototype = {
    aa:"aa",
    method:function(){

    }
  };
  var a = new A;
  var b = new A;
  console.log(a.aa === b.aa);//true 由于aa的值为基本类型，比较值
  console.log(a.obj === b.obj) //false 引用类型，每次进入函数体都要重新创建，因此都不一样。
  console.log(a.method === b.method); //false

Privileged methods or attributes only cover the prototype methods or attributes, so as long as you delete the privileged method, you can access the prototype method or attribute with the same name.

  delete a.method;
  delete b.method;
  console.log(a.method === A.prototype.method);//true
  console.log(a.method === b.method); //true

In Java language, both prototype methods and privileged methods are attributes of instance methods. There is also something called class method and class attribute in Java. They are very simple to simulate using JavaScript, just define them directly on the function.

  A.method2 = function(){} //类方法
  var c = new A;
  console.log(c.method2); //undefined

Next, let’s look at the implementation of inheritance. As mentioned above, whatever is on the Prototype, its instance will have something, whether this attribute is added later or the entire Prototype is replaced. If we replace this prototype object with the prototype of another class, then it can easily obtain all prototype members of that class.

  function A() {};
  A.prototype = {
    aaa : 1
  }
  function B() {};
  B.prototype = A.prototype;
  var b = new B;
  console.log(b.aaa); //=> 1;
  A.prototype.bb = 2;
  console.log(b.bb) //=> 2;

Since it refers to the same object, this means that when we modify the prototype of class A, it is equivalent to modifying the prototype of class B. Therefore, we cannot assign an object to two classes. There are two ways to do this,

Method 1: Assign the prototype members of the parent class to the prototype of the subclass one by one through for in
Method 2 is: the prototype of the subclass is not obtained directly from the parent class. First, assign the prototype of the parent class to a function, and then use the instance of this function as the prototype of the subclass.

Method one, we usually have to implement a method like mixin, which some books call copy inheritance. The advantage is that it is simple and direct, but the disadvantage is that it cannot pass instanceof verification. The extend method of Prototype.js is used to do this.

  function extend (des, source) { //des = destination
    for (var property in source)
      des[property] = source[property];
    return des;
  }

Method two is to use your brain on the prototype, so it is called prototypal inheritance. Below is a template

  function A() {};
  A.prototype = {
    aa:function(){
      alert(1)
    }
  }
  function bridge() {

  };
  bridge.prototype = A.prototype;

  function B() {}
  B.prototype = new bridge();

  var a = new A;
  var b = new B;

  console.log(a == b) //false 证明成功分开原型
  console.log(A.prototype == B.prototype) //true 子类共享父类的原型方法
  console.log(a.aa === b.aa); //为父类动态添加新的方法
  A.prototype.bb = function () {
    alert(2)
  }
  //true，继承父类的方法
  B.prototype.cc = function (){
    alert(3)
  }
  //false 父类未必有子类的new实例
  console.log(a.cc === b.cc)
  //并且它能够正常通过javascript自带的验证机制instanceof
  console.log(b instanceof A) ;//true
  console.log(b instanceof B) ; //true

Method 2 can pass instanceof verification. es5 has built-in this method to implement prototype inheritance. It is Object.create. If the second parameter is not considered, it is approximately equal to the following code.

  Object.create = function (o) {
    function F() {}
    F.prototype = o;
    return new F();
  }

The above method requires passing in a prototype of the parent class as a parameter, and then returns the prototype of the subclass

However, we still miss something - the subclass not only inherits the inheritance of the parent class, but also has its own things. In addition, prototypal inheritance does not allow the subclass to inherit the members and privileged members of the parent class. We have to add these manually, such as class members, we can use the extend method above, and privileged members can be implemented in the subclass constructor through apply.

  function inherit(init, Parent, proto){
    function Son(){
      Parent.apply(this, argument); //先继承父类的特权成员
      init.apply(this, argument); //在执行自己的构造器
    }
  }
  //由于Object.create是我们伪造的，因此避免使用第二个参数
  Son.prototype = Object.create(Parent.prototype,{});
  Son.prototype.toString = Parent.prototype.toString; //处理IEbug
  Son.prototype.valueOf = Parent.prototype.valueOf; //处理IEbug
  Son.prototype.constructor = Son; //确保构造器正常指向，而不是Object
  extend(Son, proto) ;//添加子类的特有的原型成员
  return Son;

下面，做一组实验，测试下实例的回溯机制。当我们访问对象的一个属性，那么他先寻找其特权成员，如果有同名就返回，没有就找原型，再没有，就找父类的原型...我们尝试将它的原型临时修改下，看它的属性会变成那个。

  function A(){

  }
  A.prototype = {
    aa:1
  }
  var a = new A;
  console.log(a.aa) ; //=>1

  //将它的所有原型都替换掉
  A.prototype = {
    aa:2
  }
  console.log(a.aa); //=>1

  //于是我们想到每个实例都有一个constructor方法，指向其构造器
  //而构造器上面正好有我们的原型，javascript引擎是不是通过该路线回溯属性呢
  function B(){

  }
  B.prototype = {
    aa:3
  }
  a.constructor = B;
  console.log(a.aa) //1 表示不受影响

因此类的实例肯定通过另一条通道进行回溯，翻看ecma规范可知每一个对象都有一个内部属性[[prototype]],它保存这我们new它时的构造器所引用的Prototype对象。在标准浏览器与IE11里，它暴露了一个叫__proto__属性来访问它。因此，只要不动__proto__上面的代码怎么动，a.aa始终坚定不毅的返回1.

再看一下，new时操作发生了什么。

1.创建了一个空对象 instance
2.instance.__proto__ = intanceClass.prototype
3.将构造函数里面的this = instance
4.执行构造函数里的代码
5.判定有没有返回值，没有返回值就返回默认值为undefined,如果返回值为复合数据类型，则直接返回，否则返回this
于是有了下面的结果。

  function A(){
    console.log(this.__proto__.aa); //1
    this.aa = 2
  }
  A.prototype = {aa:1}
  var a = new A;
  console.log(a.aa)
  a.__proto__ = {
    aa:3
  }
  console.log(a.aa) //=>2
  delete a. aa; //删除特权属性，暴露原型链上的同名属性
  console.log(a.aa) //=>3

有了__proto__，我们可以将原型设计继承设计得更简单，我们还是拿上面的例子改一改，进行试验

  function A() {}
  A.prototype = {
    aa:1
  }
  function bridge() {}
  bridge.prototype = A.prototype;

  function B(){}
  B.prototype = new bridge();
  B.prototype.constructor = B;
  var b = new B;
  B.prototype.cc = function(){
    alert(3)
  }
  //String.prototype === new String().__proto__ => true
  console.log(B.prototype.__proto__ === A.prototype) //true
  console.log(b.__proto__ == B.prototype); //true 
  console.log(b.__proto__.__proto__ === A.prototype); //true 得到父类的原型对象

因为b.__proto__.constructor为B，而B的原型是从bridge中得来的，而bride.prototype = A.prototype,反过来，我们在定义时，B.prototype.__proto__ = A.prototype,就能轻松实现两个类的继承.

__proto__属性已经加入es6,因此可以通过防止大胆的使用

2.各种类工厂的实现。

上节我们演示了各种继承方式的实现，但都很凌乱。我们希望提供一个专门的方法，只要用户传入相应的参数，或按照一定简单格式就能创建一个类。特别是子类。

由于主流框架的类工厂太依赖他们庞杂的工具函数，而一个精巧的类工厂也不过百行左右

相当精巧的库，P.js

https://github.com/jiayi2/pjs

使用版：https://github.com/jiayi2/factoryjs

这是一个相当精巧的库，尤其调用父类的同名方法时，它直接将父类的原型抛在你面前，连_super也省了。

  var P = (function(prototype, ownProperty, undefined) {
 return function P(_superclass /* = Object */, definition) {
  // handle the case where no superclass is given
  if (definition === undefined) {
   definition = _superclass;
   _superclass = Object;
  }

  // C is the class to be returned.
  //
  // When called, creates and initializes an instance of C, unless
  // `this` is already an instance of C, then just initializes `this`;
  // either way, returns the instance of C that was initialized.
  //
  // TODO: the Chrome inspector shows all created objects as `C`
  //    rather than `Object`. Setting the .name property seems to
  //    have no effect. Is there a way to override this behavior?
  function C() {
   var self = this instanceof C ? this : new Bare;
   self.init.apply(self, arguments);
   return self;
  }

  // C.Bare is a class with a noop constructor. Its prototype will be
  // the same as C, so that instances of C.Bare are instances of C.
  // `new MyClass.Bare` then creates new instances of C without
  // calling .init().
  function Bare() {}
  C.Bare = Bare;

  // Extend the prototype chain: first use Bare to create an
  // uninitialized instance of the superclass, then set up Bare
  // to create instances of this class.
  var _super = Bare[prototype] = _superclass[prototype];
  var proto = Bare[prototype] = C[prototype] = C.p = new Bare;

  // pre-declaring the iteration variable for the loop below to save
  // a `var` keyword after minification
  var key;

  // set the constructor property on the prototype, for convenience
  proto.constructor = C;

  C.extend = function(def) { return P(C, def); }

  return (C.open = function(def) {
   if (typeof def === 'function') {
    // call the defining function with all the arguments you need
    // extensions captures the return value.
    def = def.call(C, proto, _super, C, _superclass);
   }

   // ...and extend it
   if (typeof def === 'object') {
    for (key in def) {
     if (ownProperty.call(def, key)) {
      proto[key] = def[key];
     }
    }
   }

   // if no init, assume we're inheriting from a non-Pjs class, so
   // default to using the superclass constructor.
   if (!('init' in proto)) proto.init = _superclass;

   return C;
  })(definition);
 }

 // as a minifier optimization, we've closured in a few helper functions
 // and the string 'prototype' (C[p] is much shorter than C.prototype)
})('prototype', ({}).hasOwnProperty);

我们尝试创建一个类：

  var Dog = P (function(proto, superProto){
    proto.init = function(name) { //构造函数
      this.name = name;
    }
    proto.move = function(meters){ //原型方法
      console.log(this.name + " moved " + meters + " m.")
    }
  });
  var a = new Dog("aaa")
  var b = new Dog("bbb"); //无实例变化
  a.move(1);
  b.move(2);

我们在现在的情况下，可以尝试创建更简洁的定义方式

  var Animal = P (function(proto, superProto){
    proto.init = function(name) { //构造函数
      this.name = name;
    }
    proto.move = function(meters){ //原型方法
      console.log(this.name + " moved " + meters + " m.")
    }
  });
  var a = new Animal("aaa")
  var b = new Animal("bbb"); //无实例变化
  a.move(1);
  b.move(2);
  //...............
  var Snake = P (Animal, function(snake, animal){
    snake.init = function(name, eyes){
      animal.init.call(this, arguments); //调运父类构造器
      this.eyes = 2;
    }
    snake.move = function() {
      console.log('slithering...');
      animal.move.call(this, 5); //调运父类同名方法
    }
  });
  var s = new Snake("snake", 1);
  s.move();
  console.log(s.name);
  console.log(s.eyes);

私有属性演示，由于放在函数体内集中定义，因此安全可靠！

  var Cobra = P (Snake, function(cobra){
    var age = 1;//私有属性
    //这里还可以编写私有方法
    cobra.glow = function(){ //长大
      return age++;
    }
  });
  var c = new Cobra("cobra");
  console.log(c.glow()); //1
  console.log(c.glow()); //2
  console.log(c.glow()); //3

以上所述就是本文的全部内容了，希望大家能够喜欢。