雖然ES5中為我們提供了Object.defineProperty方法來設(shè)置getter與setter，但此原生方法使用起來并不方便，我們何不自己來實現(xiàn)一個類，只要繼承該類并遵循一定的規(guī)范就可以擁有媲美原生的getter與setter。

[[135999]]

現(xiàn)在我們定義以下規(guī)范：

取值器跟設(shè)值器遵循格式：_xxxGetter/_xxxSetter，xxx代表需要被控制的屬性。例如，如果要控制foo屬性，則對象需要提供 _fooGetter/_fooSetter方法來作為實際的取值器與控制器，這樣我們可以帶代碼中調(diào)用obj.get(‘foo’)和 obj.set(‘foo’, value)來進(jìn)行取值與設(shè)值；否則調(diào)用get與set方法相當(dāng)于代碼：obj.foo和obj.foo = value;

提供watch函數(shù)：obj.watch(attr, function(name, oldValue, newValue){});每次調(diào)用set方法時，便會觸發(fā)fucntion參數(shù)。 function中name代表被改變的屬性，oldValue是上一次該屬性的值，newValue代表該屬性的***值。該方法返回一個handle對 象，擁有remove方法，調(diào)用remove將function參數(shù)從函數(shù)鏈中移除。

首先使用閉包模式，使用attributes變量作為私有屬性存放所有屬性的getter與setter：

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

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

    var ST = function(){};

    return ST;
})()

其中wcbs用來存儲調(diào)用watch(name, callback)時所有的callback。

***版實現(xiàn)代碼如下：

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);
        //如果對象擁有_nameSetter方法則調(diào)用該方法，否則直接在對象上賦值。
        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;
        // 如果擁有_nameGetter方法則調(diào)用該方法，否則直接從對象中獲取。
        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){
        //每次調(diào)用set方法時都將name存儲到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); //將所有的callback添加到wcbs數(shù)組中
            }

            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;
})()

測試工作：

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'))

輸出：

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

可以看到將所有watch函數(shù)存放于wcbs數(shù)組中，所有子類重名的屬性訪問的都是同一個wcbs數(shù)組。有什么方法可以既保證每個實例擁有自己的 watch函數(shù)鏈又不發(fā)生污染？可以考慮這種方法：為每個實例添加一個_watchCallbacks屬性，該屬性是一個函數(shù)，將所有的watch函數(shù)鏈 都存放到該函數(shù)上，主要代碼如下：

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);
                        }
                    }
                }
                //在函數(shù)作用域鏈中可以訪問到callbacks變量
                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是函數(shù)，則所有屬性改變時都會調(diào)用該函數(shù)
            _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);
                }
            }
        };
    };

經(jīng)過改變后整體代碼如下：

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);
                        }
                    }
                }
                //在函數(shù)作用域鏈中可以訪問到callbacks變量
                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是函數(shù)，則所有屬性改變時都會調(diào)用該函數(shù)
            _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;
})()

測試：

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'))

輸出：

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

以上代碼就是dojo/Stateful的原理。