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:
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
1 var Stateful = (function(){ 2 'use strict'; 3 4 var attributes = {}; 5 6 function _getNameAttrs(name){ 7 return attributes[name] || {}; 8 } 9 10 function _setNameAttrs(name) { 11 if (!attributes[name]) { 12 attributes[name] = { 13 s: '_' + name + 'Setter', 14 g: '_' + name + 'Getter', 15 wcbs: [] 16 } 17 } 18 } 19 20 21 function _setNameValue(name, value){ 22 _setNameAttrs(name); 23 var attrs = _getNameAttrs(name); 24 var oldValue = _getNameValue.call(this, name); 25 //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。 26 if (this[attrs.s]){ 27 this[attrs.s].call(this, value); 28 } else { 29 this[name] = value; 30 } 31 32 if (attrs.wcbs && attrs.wcbs.length > 0){ 33 var wcbs = attrs.wcbs; 34 for (var i = 0, len = wcbs.length; i < len; i++) { 35 wcbs[i](name, oldValue, value); 36 } 37 } 38 }; 39 40 function _getNameValue(name) { 41 _setNameAttrs(name); 42 var attrs = _getNameAttrs(name); 43 44 var oldValue = null; 45 // 如果拥有_nameGetter方法则调用该方法,否则直接从对象中获取。 46 if (this[attrs.g]) { 47 oldValue = this[attrs.g].call(this, name); 48 } else { 49 oldValue = this[name]; 50 } 51 52 return oldValue; 53 }; 54 55 function ST(){}; 56 57 ST.prototype.set = function(name, value){ 58 //每次调用set方法时都将name存储到attributes中 59 if (typeof name === 'string'){ 60 _setNameValue.call(this, name, value); 61 } else if (typeof name === object) { 62 for (var p in name) { 63 _setNameValue.call(this, p, name[p]); 64 } 65 } 66 67 return this; 68 }; 69 70 ST.prototype.get = function(name) { 71 if (typeof name === 'string') { 72 return _getNameValue.call(this, name); 73 } 74 }; 75 76 ST.prototype.watch = function(name, wcb) { 77 var attrs = null; 78 if (typeof name === 'string') { 79 _setNameAttrs(name); 80 attrs = _getNameAttrs(name); 81 attrs.wcbs.push(wcb); 82 83 return { 84 remove: function(){ 85 for (var i = 0, len = attrs.wcbs.length; i < len; i++) { 86 if (attrs.wcbs[i] === wcb) { 87 break; 88 } 89 } 90 91 attrs.wcbs.splice(i, 1); 92 } 93 } 94 } else if (typeof name === 'function'){ 95 for (var p in attributes) { 96 attrs = attributes[p]; 97 attrs.wcbs.splice(0,0, wcb); //将所有的callback添加到wcbs数组中 98 } 99 100 return { 101 remove: function() { 102 for (var p in attributes) { 103 var attrs = attributes[p]; 104 for (var i = 0, len = attrs.wcbs.length; i < len; i++) { 105 if (attrs.wcbs[i] === wcb) { 106 break; 107 } 108 } 109 110 attrs.wcbs.splice(i, 1); 111 } 112 } 113 } 114 } 115 }; 116 117 return ST; 118 })()
测试工作:
1 console.log(Stateful); 2 var stateful = new Stateful(); 3 4 function A(name){ 5 this.name = name; 6 }; 7 A.prototype = stateful; 8 A.prototype._NameSetter = function(n) { 9 this.name = n; 10 }; 11 A.prototype._NameGetter = function() { 12 return this.name; 13 } 14 15 function B(name) { 16 this.name = name; 17 }; 18 B.prototype = stateful; 19 B.prototype._NameSetter = function(n) { 20 this.name = n; 21 }; 22 B.prototype._NameGetter = function() { 23 return this.name; 24 }; 25 26 var a = new A(); 27 var handle = a.watch('Name', function(name, oldValue, newValue){ 28 console.log(name + 'be changed from ' + oldValue + ' to ' + newValue); 29 }); 30 a.set('Name', 'AAA'); 31 console.log(a.name); 32 33 var b = new B(); 34 b.set('Name', 'BBB'); 35 console.log(b.get('Name')); 36 37 handle.remove(); 38 a.set('Name', 'new AAA'); 39 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:
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
1 var Stateful = (function(){ 2 'use strict'; 3 4 var attributes = {}; 5 6 function _getNameAttrs(name){ 7 return attributes[name] || {}; 8 } 9 10 function _setNameAttrs(name) { 11 if (!attributes[name]) { 12 attributes[name] = { 13 s: '_' + name + 'Setter', 14 g: '_' + name + 'Getter'/*, 15 wcbs: []*/ 16 } 17 } 18 } 19 20 21 function _setNameValue(name, value){ 22 if (name === '_watchCallbacks') { 23 return; 24 } 25 _setNameAttrs(name); 26 var attrs = _getNameAttrs(name); 27 var oldValue = _getNameValue.call(this, name); 28 29 if (this[attrs.s]){ 30 this[attrs.s].call(this, value); 31 } else { 32 this[name] = value; 33 } 34 35 if (this._watchCallbacks){ 36 this._watchCallbacks(name, oldValue, value); 37 } 38 }; 39 40 function _getNameValue(name) { 41 _setNameAttrs(name); 42 var attrs = _getNameAttrs(name); 43 44 var oldValue = null; 45 if (this[attrs.g]) { 46 oldValue = this[attrs.g].call(this, name); 47 } else { 48 oldValue = this[name]; 49 } 50 51 return oldValue; 52 }; 53 54 function ST(obj){ 55 for (var p in obj) { 56 _setNameValue.call(this, p, obj[p]); 57 } 58 }; 59 60 ST.prototype.set = function(name, value){ 61 if (typeof name === 'string'){ 62 _setNameValue.call(this, name, value); 63 } else if (typeof name === 'object') { 64 for (var p in name) { 65 _setNameValue.call(this, p, name[p]); 66 } 67 } 68 69 return this; 70 }; 71 72 ST.prototype.get = function(name) { 73 if (typeof name === 'string') { 74 return _getNameValue.call(this, name); 75 } 76 }; 77 78 ST.prototype.watch = function(name, wcb) { 79 var attrs = null; 80 81 var callbacks = this._watchCallbacks; 82 if (!callbacks) { 83 callbacks = this._watchCallbacks = function(n, ov, nv) { 84 var execute = function(cbs){ 85 if (cbs && cbs.length > 0) { 86 for (var i = 0, len = cbs.length; i < len; i++) { 87 cbs[i](n, ov, nv); 88 } 89 } 90 } 91 //在函数作用域链中可以访问到callbacks变量 92 execute(callbacks['_' + n]); 93 execute(callbacks['*']);// 通配符 94 } 95 } 96 97 var _name = ''; 98 if (typeof name === 'string') { 99 var _name = '_' + name; 100 } 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 101 _name = '*'; 102 wcb = name; 103 } 104 callbacks[_name] = callbacks[_name] ? callbacks[_name] : []; 105 callbacks[_name].push(wcb); 106 107 return { 108 remove: function(){ 109 var idx = callbacks[_name].indexOf(wcb); 110 if (idx > -1) { 111 callbacks[_name].splice(idx, 1); 112 } 113 } 114 }; 115 }; 116 117 return ST; 118 })()
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。