Deffered.js的实现原理

 1 function Deferred () {
 2 　　return this.init();
 3 }
//设置默认的成功函数与失败函数
 4 Deferred.ok = function(x) {return x}
 5 Deferred.ng = function(x) {throw x}
 6 Deferred.isDeferred = function (obj) {
　　　　//判断是否是Deferred的实例
 7 　　return !!(obj && obj._id === Deferred.prototype._id);
 8 };
//next 的静态方法
 9 Deferred.next = function(fn){
10 var d = new Deferred();
11 var img = new Image();
12 var handler = function(){
13 d.canceller();
14 d.calls();
15 }
16 //利用img加载错误触发一个异步处理，常用的还有setTimeout等
17 img.addEventListener(‘error‘,handler,false);
18 d.canceller = function(){
19 img.removeEventListener(‘error‘,handler,false);
20 }
21 img.src = "data:image/png," + Math.random();
22 if(fn) d.callback.ok = fn;
23 return d;
24 }
25 Deferred.wait = function (n) {
26 var d = new Deferred(), t = new Date();
27 var id = setTimeout(function () {
28 d.calls((new Date()).getTime() - t.getTime());
29 }, n * 1000);
30 d.canceller = function () { clearTimeout(id) };
31 return d;
32 };
33 
34 
35 Deferred.prototype = {
36 _id : 8888, //用来判断实例的时候会有用到
37 init : function(){
38 this._next = null;
39 this.callback = {
40 ok : Deferred.ok,
41 ng : Deferred.ng
42 }
43 return this;
44 },
45 next : function (fun) { return this._post("ok", fun) },
46 
47 calls : function (val) { return this._fire("ok", val) },
48 
49 _post : function (okng, fun) {
50 this._next = new Deferred();
51 this._next.callback[okng] = fun;
52 return this._next;
53 },
54 
55 _fire : function (okng, value) {
56 var next = "ok";
57 
58 value = this.callback[okng](value);
59 
60 if (Deferred.isDeferred(value)) {
61 value._next = this._next;
62 } else {
63 if (this._next) this._next._fire(next, value);
64 }
65 return this;
66 }
67 }