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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 AAAAAANamebe changed from undefined to BBBBBBnew 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 AAAAAABBB* foo undefined FOO* goo undefined GOOGOONamebe changed from AAA to AAA+* Name AAA AAA+* Name AAA+ new AAAnew AAA BBB 以上代码就是的原理。
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