JavaScript中getter/setter的实现

jopen 9年前

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。

 来源:木的树的博客