JavaScript Accessor实现说明_javascript技巧

第一种算是比较常见了,通过闭包Store Value从而实现accessor,适用于所有浏览器.

function Sandy(val){
var value = val;
this.getValue = function(){
return value;
};
this.setValue = function(val){
value = val;
};
}
//usage
var sandy = new Sandy("test");
sandy.value
// => undefined
sandy.setValue("test2")
sandy.getValue

下面是JavaScript权威指南（中文第五版）中P152页使用闭包的一个例子.

function makeProperty(o, name, predicate) {
var value; //This is property value;

//The setter method simply returns the value
o['get' + name] = function() { return value;};

//The getter method stores the value or throws an exception if
//the predicate rejects the value
o['set' + name] = function(v) {
if (predicate && !predicate(v) {
throw 'set' + name + ': invalid value ' + v;
} else {
value = y;
}
}
}

//The following code demenstrates the makeProperty() method
var o = {}; // Here is an empty object

//Add property accessor methods getName and setName
//Ensure that only string values are allowed
makeProperty(o, 'Name', function(x) { return typeof x == 'string'; });

o.setName('Frank'); 　　//Set the property value;
print(o.getName()); 　　//Get the property value
o.setName(0); //Try to set a value of the wrong type

第二种方法是使用__defineSetter__与__defineGetter__来实现accessor,看下划线就知道它们并非标准,适用于Firefox 2.0+, Safari 3.0+, Google Chrome 1.0+ 和 Opera 9.5+ ,方法使用见MDN.

function Sandy(val){
var value = val,
_watch = function(newVal) {
console.log('val is Changed to : ' + newVal);
}

this.__defineGetter__("value", function(){
return value;
});

this.__defineSetter__("value", function(val){
value = val;
_watch(val);
});
}

var sandy = new Sandy("test");
sandy.value
// => test
sandy.value = "test2";
// => 'val is Changed to : test2'
sandy.value
// => "test2"

　除了__defineG/Setter__外, 你还可以使用'set'、'get'关键字在在原型对象上定义accessor,对于单个对象同样适用, 适用于Firefox 2.0+, Safari 3.0+, Google Chrome 1.0+ 和 Opera 9.5+.

function Sandy(val){
this.value = val;
}

Sandy.prototype = {
get value(){
return this._value;
},
set value(val){
this._value = val;
}
};

//Or

var sandy = {
'_value' : 'sandy',
get value() {
　　return this._value;
},
set value(val) {
　　this._value = val;
}
}

最后一种方法,用到了Object的静态方法defineProperty,作用于单个对象,该方法应该属于ES5的范畴了,目前似乎只有Chrome 支持这种方法，其实Ie8也支持,但操作对象仅限于Dom节点(Dom node),见IEBlog,该方法的使用见MDN.

var sandy = {}, rValue;
Object.defineProperty(sandy, 'value' ,
{
'set' : function(val) {
rValue = val;
},
'get' : function() {
return rValue;
},
'enumerable' : true,
'configurable' : true
}
)
//Ie8+
Object.defineProperty(document.body, "description", {
get : function () {
return this.desc;
},
set : function (val) {
this.desc = val;
}
});
document.body.description = "Content container";
// document.body.description will now return "Content container"

‘enumerable','configuralbe' 属于ES5规范中的Property Attributes(属性特性),在这里就不做讨论了,有兴趣的Google或者直接去看ES5的文档. ^ ^