前言
Deferred是从1.5版本引入的一个核心特性之一，主要是为了解决Callback Hell，老生常谈的问题，这里就不多赘述了。本文旨在剖析Deferred的内部实现，让大家能够深入了解Deferred。
API
$.Deferred
通过调用$.Deferred()获取到一个Deferred实例，如下
var dd = $.Deferred();
调用的时候可以传入一个函数（可以简单的看作构造函数），如下
var dd = $.Deferred(function (newDefer) { newDefer.name = 'My Deferred';});console.log(dd.name);//My Deferred resolve/done reject/fail notify/progress
Deferred的实现使用了闭包，内部维护了三个$.Callbacks管理队列。
可以说Deferred的核心在于$.Callbacks。如果不熟悉$.Callbacks的请自行搜索或参考我的另一篇介绍文章。代码可以简化成下面的样子 var resolveCb = jQuery.Callbacks("once memory");var rejectCb = jQuery.Callbacks("once memory");var notifyCb = jQuery.Callbacks("memory");done = resolveCb.addresolve ~ resolveCb.fireresolveWith = resolveCb.fireWithfail = rejectCb.addreject ~ rejectCb.firerejectWith = rejectCb.fireWithprogress = notifyCb.addnotify ~ notifyCb.firenotifyWith = notifyCb.fireWith ~表示可以简单理解为对等。我们再看看下面的代码会不会有不一样的感觉了
var dd = $.Deferred();dd.done(function (name) { console.log(name, 1);}).done(function (name) { console.log(name, 2);});dd.resolve('Jacky'); 其实就相当于
resolveCb.add(function (name) { console.log(name, 1);}).add(function (name) { console.log(name, 2);});resolveCb.fire('Jacky'); 当Deferred执行完resolve以后，同时会调用rejectCb.disable和notifyCb.lock。
当Deferred执行完reject以后，同时会调用resolveCb.disable和notifyCb.lock。
notifyCb就是一个单纯的$.Callbacks，但是它的状态会受到resolve/reject的影响。
resolveCb和rejectCb两者之间是相互限制的，一旦两者中的某一个fire了，另一个就会被disable。通过这种方式来达到唯一状态。
resolve/resolveWith reject/rejectWith notify/notifyWith
就是fire和firewith的区别
state
返回Deferred的当前状态，总共有三种状态
执行resolve/reject前，返回值是pending
执行了resolve，返回值是resolved
-
执行了reject，返回值是rejected
var dd = $.Deferred();
console.log(dd.state());//pending dd.resolve(); console.log(dd.state());//resolved //dd.reject(); //console.log(dd.state());//rejected
always
看下源码大家就一清二楚了
还是调用的done和fail，往各自队列里添加回调。 promise
var dd = $.Deferred();var promise = dd.promise();console.log(dd);console.log(promise);
var obj = {name: 'xxx'};var dd = $.Deferred();var newObj = dd.promise(obj);console.log(newObj);console.log(newObj === obj); 
then/pipe
pipe是为了向下兼容留下的方法，pipe===then。then支持三个参数，返回值是一个新的promise对象。看几个例子
例子一
var dd = $.Deferred();dd.done(function (name) { console.log('dd', name);});dd.then(function (name) { console.log('then', name);});dd.resolve('Jacky');//dd Jacky//then Jacky 例子二
var dd = $.Deferred();dd.fail(function (name) { console.log('dd', name);});dd.then(null, function (name) { console.log('then', name);});dd.reject('Jacky');//dd Jacky//then Jacky 例子三
var dd = $.Deferred();dd.progress(function (name) { console.log('dd', name);});dd.then(null, null, function (name) { console.log('then', name);});dd.notify('Jacky');//dd Jacky//then Jacky 三个参数刚好分别添加到了内部的三个回调队列中。dd.then(fn)可以简单看作dd.done(fn)。前面说了then的返回值是一个新的promise对象，如果在新的Deferred对象上继续添加回调会怎么样呢？我们分两种情况来看。
then方法的返回值不是Deferred对象
var dd = $.Deferred();var newdd = dd.then(function (name) { console.log('then', name); return 'Mary';});newdd.done(function (name) { console.log('newDefer', name);});dd.resolve('Jacky');//then Jacky//newDefer Mary then的返回值会传递给done/fail的参数。而且无需我们手动调用newdd.resolve，内部帮我们调用了
返回值是Deferred对象
看个例子
var dd = $.Deferred();var returndd = $.Deferred();
var newdd = dd.then(function (name) { console.log('then', name); return returndd;});newdd.done(function (name) { console.log('newDefer', name);});dd.resolve('Jacky');//then Jacky newdd.done添加的函数没有马上执行了。我们手动调用下returndd.resolve
var dd = $.Deferred();var returndd = $.Deferred();
var newdd = dd.then(function (name) { console.log('then', name); return returndd;});newdd.done(function (name) { console.log('newDefer', name);});dd.resolve('Jacky');//then Jackyreturndd.resolve('Helen');//newDefer Helen 这里就是我们的promise编程了