Javascript自己动手实现getter/setter

　　虽然ES5中为我们提供了Object.defineProperty方法来设置getter与setter，但此原生方法使用起来并不方便，我们何不自己来实现一个类，只要继承该类并遵循一定的规范就可以拥有媲美原生的getter与setter。

　　现在我们定义以下规范：

　　取值器跟设值器遵循格式：_xxxGetter/_xxxSetter，xxx代表需要被控制的属性。例如，如果要控制foo属性，则对象需要提供_fooGetter/_fooSetter方法来作为实际的取值器与控制器，这样我们可以带代码中调用obj.get('foo')和obj.set('foo', value)来进行取值与设值；否则调用get与set方法相当于代码：obj.foo和obj.foo = value;

　　提供watch函数：obj.watch(attr, function(name, oldValue, newValue){});每次调用set方法时，便会触发fucntion参数。 function中name代表被改变的属性，oldValue是上一次该属性的值，newValue代表该属性的最新值。该方法返回一个handle对象，拥有remove方法，调用remove将function参数从函数链中移除。

　　首先使用闭包模式，使用attributes变量作为私有属性存放所有属性的getter与setter：

var Stateful = (function(){
    'use strict';

    var attributes = {
        Name: {
            s: '_NameSetter',
            g: '_NameGetter',
            wcbs: []
        }
    };

    var ST = function(){};

    return ST;
})()

　　其中wcbs用来存储调用watch(name, callback)时所有的callback。

　　第一版实现代码如下：

 var Stateful = (function(){
     'use strict';

     var attributes = {};

     function _getNameAttrs(name){
         return attributes[name] || {};
     }

     function _setNameAttrs(name) {
         if (!attributes[name]) {
             attributes[name] = {
                 s: '_' + name + 'Setter',
                 g: '_' + name + 'Getter',
                 wcbs: []
             }
         }
     }

     function _setNameValue(name, value){
         _setNameAttrs(name);
         var attrs = _getNameAttrs(name);
         var oldValue = _getNameValue.call(this, name);
         //如果对象拥有_nameSetter方法则调用该方法，否则直接在对象上赋值。
         if (this[attrs.s]){
             this[attrs.s].call(this, value);
         } else {
             this[name] = value;
         }

         if (attrs.wcbs && attrs.wcbs.length > 0){
             var wcbs = attrs.wcbs;
             for (var i = 0, len = wcbs.length; i < len; i++) {
                 wcbs[i](name, oldValue, value);
             }
         }
     };

     function _getNameValue(name) {
         _setNameAttrs(name);
         var attrs = _getNameAttrs(name);

         var oldValue = null;
         // 如果拥有_nameGetter方法则调用该方法，否则直接从对象中获取。
         if (this[attrs.g]) {
             oldValue = this[attrs.g].call(this, name);
         } else {
             oldValue = this[name];
         }

         return oldValue;
     };

     function ST(){};

     ST.prototype.set = function(name, value){
         //每次调用set方法时都将name存储到attributes中
         if (typeof name === 'string'){
             _setNameValue.call(this, name, value);
         } else if (typeof name === object) {
             for (var p in name) {
                 _setNameValue.call(this, p, name[p]);
             }
         }

         return this;
     };

     ST.prototype.get = function(name) {
         if (typeof name === 'string') {
             return _getNameValue.call(this, name);
         }
     };

     ST.prototype.watch = function(name, wcb) {
         var attrs = null;
         if (typeof name === 'string') {
             _setNameAttrs(name);
             attrs = _getNameAttrs(name);
             attrs.wcbs.push(wcb);

             return {
                 remove: function(){
                     for (var i = 0, len = attrs.wcbs.length; i < len; i++) {
                         if (attrs.wcbs[i] === wcb) {
                             break;
                         }
                     }

                     attrs.wcbs.splice(i, 1);
                 }
             }
         } else if (typeof name === 'function'){
             for (var p in attributes) {
                 attrs = attributes[p];
                 attrs.wcbs.splice(0,0, wcb); //将所有的callback添加到wcbs数组中
             }

             return {
                 remove: function() {
                     for (var p in attributes) {
                         var attrs = attributes[p];
                         for (var i = 0, len = attrs.wcbs.length; i < len; i++) {
                             if (attrs.wcbs[i] === wcb) {
                                 break;
                             }
                         }

                         attrs.wcbs.splice(i, 1);
                     }
                 }
             }
         }
     };

     return ST;
 })()

　　测试工作：

 console.log(Stateful);
     var stateful = new Stateful();

     function A(name){
         this.name = name;
     };
     A.prototype = stateful;
     A.prototype._NameSetter = function(n) {
         this.name = n;
     };
     A.prototype._NameGetter = function() {
         return this.name;
     }

     function B(name) {
         this.name = name;
     };
     B.prototype = stateful;
     B.prototype._NameSetter = function(n) {
         this.name = n;
     };
     B.prototype._NameGetter = function() {
         return this.name;
     };

     var a = new A();
     var handle = a.watch('Name', function(name, oldValue, newValue){
         console.log(name + 'be changed from ' + oldValue + ' to ' + newValue);
     });
     a.set('Name', 'AAA');
     console.log(a.name);

     var b = new B();
     b.set('Name', 'BBB');
     console.log(b.get('Name'));

     handle.remove();
     a.set('Name', 'new AAA');
     console.log(a.get('Name'), b.get('Name'))

　　输出：

function ST(){}
Namebe changed from undefined to AAA
AAA
Namebe changed from undefined to BBB
BBB
new AAA BBB

　　可以看到将所有watch函数存放于wcbs数组中，所有子类重名的属性访问的都是同一个wcbs数组。有什么方法可以既保证每个实例拥有自己的watch函数链又不发生污染？可以考虑这种方法：为每个实例添加一个_watchCallbacks属性，该属性是一个函数，将所有的watch函数链都存放到该函数上，主要代码如下：

ST.prototype.watch = function(name, wcb) {
        var attrs = null;

        var callbacks = this._watchCallbacks;
        if (!callbacks) {
            callbacks = this._watchCallbacks = function(n, ov, nv) {
                var execute = function(cbs){
                    if (cbs && cbs.length > 0) {
                        for (var i = 0, len = cbs.length; i < len; i++) {
                            cbs[i](n, ov, nv);
                        }
                    }
                }
                //在函数作用域链中可以访问到callbacks变量
                execute(callbacks['_' + n]);
                execute(callbacks['*']);// 通配符
            }
        }

        var _name = '';
        if (typeof name === 'string') {
            var _name = '_' + name;
        } else if (typeof name === 'function') {//如果name是函数，则所有属性改变时都会调用该函数
            _name = '*';
            wcb = name;
        }
        callbacks[_name] = callbacks[_name] ? callbacks[_name] : [];
        callbacks[_name].push(wcb);

        return {
            remove: function(){
                var idx = callbacks[_name].indexOf(wcb);
                if (idx > -1) {
                    callbacks[_name].splice(idx, 1);
                }
            }
        };
    };

　　经过改变后整体代码如下：

 var Stateful = (function(){
     'use strict';

     var attributes = {};

     function _getNameAttrs(name){
         return attributes[name] || {};
     }

     function _setNameAttrs(name) {
         if (!attributes[name]) {
             attributes[name] = {
                 s: '_' + name + 'Setter',
                 g: '_' + name + 'Getter'/*,
                 wcbs: []*/
             }
         }
     }

     function _setNameValue(name, value){
         if (name === '_watchCallbacks') {
             return;
         }
         _setNameAttrs(name);
         var attrs = _getNameAttrs(name);
         var oldValue = _getNameValue.call(this, name);

         if (this[attrs.s]){
             this[attrs.s].call(this, value);
         } else {
             this[name] = value;
         }

         if (this._watchCallbacks){
             this._watchCallbacks(name, oldValue, value);
         }
     };

     function _getNameValue(name) {
         _setNameAttrs(name);
         var attrs = _getNameAttrs(name);

         var oldValue = null;
         if (this[attrs.g]) {
             oldValue = this[attrs.g].call(this, name);
         } else {
             oldValue = this[name];
         }

         return oldValue;
     };

     function ST(obj){
         for (var p in obj) {
             _setNameValue.call(this, p, obj[p]);
         }
     };

     ST.prototype.set = function(name, value){
         if (typeof name === 'string'){
             _setNameValue.call(this, name, value);
         } else if (typeof name === 'object') {
             for (var p in name) {
                 _setNameValue.call(this, p, name[p]);
             }
         }

         return this;
     };

     ST.prototype.get = function(name) {
         if (typeof name === 'string') {
             return _getNameValue.call(this, name);
         }
     };

     ST.prototype.watch = function(name, wcb) {
         var attrs = null;

         var callbacks = this._watchCallbacks;
         if (!callbacks) {
             callbacks = this._watchCallbacks = function(n, ov, nv) {
                 var execute = function(cbs){
                     if (cbs && cbs.length > 0) {
                         for (var i = 0, len = cbs.length; i < len; i++) {
                             cbs[i](n, ov, nv);
                         }
                     }
                 }
                 //在函数作用域链中可以访问到callbacks变量
                 execute(callbacks['_' + n]);
                 execute(callbacks['*']);// 通配符
             }
         }

         var _name = '';
         if (typeof name === 'string') {
             var _name = '_' + name;
         } else if (typeof name === 'function') {//如果name是函数，则所有属性改变时都会调用该函数
             _name = '*';
             wcb = name;
         }
         callbacks[_name] = callbacks[_name] ? callbacks[_name] : [];
         callbacks[_name].push(wcb);

         return {
             remove: function(){
                 var idx = callbacks[_name].indexOf(wcb);
                 if (idx > -1) {
                     callbacks[_name].splice(idx, 1);
                 }
             }
         };
     };

     return ST;
 })()

　　测试：

console.log(Stateful);
    var stateful = new Stateful();

    function A(name){
        this.name = name;
    };
    A.prototype = stateful;
    A.prototype._NameSetter = function(n) {
        this.name = n;
    };
    A.prototype._NameGetter = function() {
        return this.name;
    }

    function B(name) {
        this.name = name;
    };
    B.prototype = stateful;
    B.prototype._NameSetter = function(n) {
        this.name = n;
    };
    B.prototype._NameGetter = function() {
        return this.name;
    };

    var a = new A();
    var handle = a.watch('Name', function(name, oldValue, newValue){
        console.log(name + 'be changed from ' + oldValue + ' to ' + newValue);
    });
    a.set('Name', 'AAA');
    console.log(a.name);

    var b = new B();
    b.set('Name', 'BBB');
    console.log(b.get('Name'));

    a.watch(function(name, ov, nv) {
        console.log('* ' + name + ' ' + ov + ' ' + nv);
    });

    a.set({
        foo: 'FOO',
        goo: 'GOO'
    });

    console.log(a.get('goo'));

    a.set('Name', 'AAA+');

    handle.remove();
    a.set('Name', 'new AAA');
    console.log(a.get('Name'), b.get('Name'))

　　输出：

function ST(obj){
        for (var p in obj) {
            _setNameValue.call(this, p, obj[p]);
        }
    }
Namebe changed from undefined to AAA
AAA
BBB
* foo undefined FOO
* goo undefined GOO
GOO
Namebe changed from AAA to AAA+
* Name AAA AAA+
* Name AAA+ new AAA
new AAA BBB

　　以上代码就是dojo/Stateful的原理。