大约在夏季，我们谈过ES6的Promise，其实在ES6前jQuery早就有了Promise，也就是我们所知道的Deferred对象，宗旨当然也和ES6的Promise一样，通过链式调用，避免层层嵌套，如下：
//jquery版本大于1.8 function runAsync(){ var def = $.Deferred(); setTimeout(function(){ console.log('I am done'); def.resolve('whatever'); }, 1000); return def; } runAsync().then(function(msg){ console.log(msg);//=>打印'whatever' }).done(function(msg){ console.log(msg);//=>打印'undefined' });
注：从jQuery1.8版本开始，then方法会返回一个新的受限制的deferred对象，即deferred.promise()—后续源码解读中我们会更加全面地了解到。因此，上述代码done中会打印'undefined'。
好了，通过上述示例代码，短暂的回顾了jQuery的Deferred使用后，我们一起来看看jQuery是怎么实现Deferred，当然解读jQuery的版本是大于1.8。
二、jQuery之Deferred源码剖析
整体架构，如下：
jQuery.extend( { Deferred: function( func ) { var tuples = [ // action, add listener, listener list, final state [ "resolve", "done", jQuery.Callbacks( "once memory" ), "resolved" ], [ "reject", "fail", jQuery.Callbacks( "once memory" ), "rejected" ], [ "notify", "progress", jQuery.Callbacks( "memory" ) ] ], state = "pending", promise = { state: function() {...}, always: function() {...}, then: function() {...}, promise: function( obj ) {...} }, deferred = {}; // Keep pipe for back-compat promise.pipe = promise.then; // Add list-specific methods jQuery.each( tuples, function( i, tuple ) {} ); // Make the deferred a promise promise.promise( deferred ); // Call given func if any if ( func ) { func.call( deferred, deferred ); } // All done! return deferred; } }
整体架构上，如果你了解设计模式中的工厂模式，那么不难看出，jQuery.Deferred就是一个工厂，每次执行jQuery.Deferred时，都会返回一个加工好的deferred对象。
接下来，我们再一步一步剖析上述代码。
首先，是数组tuples：
tuples = [ // action, add listener, listener list, final state [ "resolve", "done", jQuery.Callbacks( "once memory" ), "resolved" ], [ "reject", "fail", jQuery.Callbacks( "once memory" ), "rejected" ], [ "notify", "progress", jQuery.Callbacks( "memory" ) ] ]
tuples一开始就为我们预先定义了三种状态—‘resolved'、'rejected'以及'pending'，以及它们所对应的一系列值和操作，值得注意的是每种状态中，都调用了一个jQuery.Callbacks方法，如下：
它是个什么玩意儿？
jQuery.Callbacks = function( options ) { // Convert options from String-formatted to Object-formatted if needed // (we check in cache first) options = typeof options === "string" ? createOptions( options ) : jQuery.extend( {}, options ); var // Flag to know if list is currently firing firing, // Last fire value for non-forgettable lists memory, // Flag to know if list was already fired fired, // Flag to prevent firing locked, // Actual callback list list = [], // Queue of execution data for repeatable lists queue = [], // Index of currently firing callback (modified by add/remove as needed) firingIndex = -1, // Fire callbacks fire = function() { // Enforce single-firing locked = options.once; // Execute callbacks for all pending executions, // respecting firingIndex overrides and runtime changes fired = firing = true; for ( ; queue.length; firingIndex = -1 ) { memory = queue.shift(); while ( ++firingIndex < list.length ) { // Run callback and check for early termination if ( list[ firingIndex ].apply( memory[ 0 ], memory[ 1 ] ) === false && options.stopOnFalse ) { // Jump to end and forget the data so .add doesn't re-fire firingIndex = list.length; memory = false; } } } // Forget the data if we're done with it if ( !options.memory ) { memory = false; } firing = false; // Clean up if we're done firing for good if ( locked ) { // Keep an empty list if we have data for future add calls if ( memory ) { list = []; // Otherwise, this object is spent } else { list = ""; } } }, // Actual Callbacks object self = { // Add a callback or a collection of callbacks to the list add: function() { if ( list ) { // If we have memory from a past run, we should fire after adding if ( memory && !firing ) { firingIndex = list.length - 1; queue.push( memory ); } ( function add( args ) { jQuery.each( args, function( _, arg ) { if ( jQuery.isFunction( arg ) ) { if ( !options.unique || !self.has( arg ) ) { list.push( arg ); } } else if ( arg && arg.length && jQuery.type( arg ) !== "string" ) { // Inspect recursively add( arg ); } } ); } )( arguments ); if ( memory && !firing ) { fire(); } } return this; }, // Remove a callback from the list remove: function() { jQuery.each( arguments, function( _, arg ) { var index; while ( ( index = jQuery.inArray( arg, list, index ) ) > -1 ) { list.splice( index, 1 ); // Handle firing indexes if ( index <= firingIndex ) { firingIndex--; } } } ); return this; }, // Check if a given callback is in the list. // If no argument is given, return whether or not list has callbacks attached. has: function( fn ) { return fn ? jQuery.inArray( fn, list ) > -1 : list.length > 0; }, // Remove all callbacks from the list empty: function() { if ( list ) { list = []; } return this; }, // Disable .fire and .add // Abort any current/pending executions // Clear all callbacks and values disable: function() { locked = queue = []; list = memory = ""; return this; }, disabled: function() { return !list; }, // Disable .fire // Also disable .add unless we have memory (since it would have no effect) // Abort any pending executions lock: function() { locked = true; if ( !memory ) { self.disable(); } return this; }, locked: function() { return !!locked; }, // Call all callbacks with the given context and arguments fireWith: function( context, args ) { if ( !locked ) { args = args || []; args = [ context, args.slice ? args.slice() : args ]; queue.push( args ); if ( !firing ) { fire(); } } return this; }, // Call all the callbacks with the given arguments fire: function() { self.fireWith( this, arguments ); return this; }, // To know if the callbacks have already been called at least once fired: function() { return !!fired; } }; return self; };
细细品味了上述jQuery.Callbacks源码，如果你了解设计模式中的发布订阅者模式，不难发现，就是一个”自定义事件”嘛
所以，我们精简jQuery.Callbacks后，核心代码如下：