jquery源码之低调的回调函数队列--Callbacks

jQuery中有一个很实用的函数队列，可能我们很少用到，但他在jQuery内部却有着举足轻重的地位。

他就是Callbacks. jQuery作者用它构建了很多非常重要的模块。比如说$.Deferred。

Callbacks 说白了就是个数组，里面存了很多函数对象。然而他真的 just so so么？

好吧，爱因斯坦也只是个人，但他真的仅仅是个普普通通的人吗？Callbacks也不是。

不说废话了，见源码。

// String to Object options format cache
var optionsCache = {};

// Convert String-formatted options into Object-formatted ones and store in cache
function createOptions( options ) {
	var object = optionsCache[ options ] = {};
	jQuery.each( options.split( core_rspace ), function( _, flag ) {
		object[ flag ] = true;
	});
	return object;
}

/*
 * Create a callback list using the following parameters:
 *
 *	options: an optional list of space-separated options that will change how
 *			the callback list behaves or a more traditional option object
 *
 * By default a callback list will act like an event callback list and can be
 * "fired" multiple times.
 *
 * Possible options:
 *
 *	once:			will ensure the callback list can only be fired once (like a Deferred)
 *
 *	memory:			will keep track of previous values and will call any callback added
 *					after the list has been fired right away with the latest "memorized"
 *					values (like a Deferred)
 *
 *	unique:			will ensure a callback can only be added once (no duplicate in the list)
 *
 *	stopOnFalse:	interrupt callings when a callback returns false
 *
 */
 //
jQuery.Callbacks = function( options ) {

	// Convert options from String-formatted to Object-formatted if needed
	// (we check in cache first)
	options = typeof options === "string" ?
		( optionsCache[ options ] || createOptions( options ) ) :
		jQuery.extend( {}, options );

	var // Last fire value (for non-forgettable lists)
		memory,
		// Flag to know if list was already fired
		fired,
		// Flag to know if list is currently firing
		firing,
		// First callback to fire (used internally by add and fireWith)
		firingStart,
		// End of the loop when firing
		firingLength,
		// Index of currently firing callback (modified by remove if needed)
		firingIndex,
		// Actual callback list
		list = [],
		// Stack of fire calls for repeatable lists
		stack = !options.once && [],
		// Fire callbacks
		fire = function( data ) {
			memory = options.memory && data;
			fired = true;
			firingIndex = firingStart || 0;
			firingStart = 0;
			firingLength = list.length;
			firing = true;
			for ( ; list && firingIndex < firingLength; firingIndex++ ) {
				if ( list[ firingIndex ].apply( data[ 0 ], data[ 1 ] ) === false && options.stopOnFalse ) {
					memory = false; // To prevent further calls using add
					break;
				}
			}
			firing = false;
			if ( list ) {
				if ( stack ) {
					if ( stack.length ) {
						fire( stack.shift() );
					}
				} else if ( memory ) {
					list = [];
				} else {
					self.disable();
				}
			}
		},
		// Actual Callbacks object
		self = {
			// Add a callback or a collection of callbacks to the list
			add: function() {
				if ( list ) {
					// First, we save the current length
					var start = list.length;
					(function add( args ) {
						jQuery.each( args, function( _, arg ) {
							var type = jQuery.type( arg );
							if ( type === "function" ) {
								if ( !options.unique || !self.has( arg ) ) {
									list.push( arg );
								}
							} else if ( arg && arg.length && type !== "string" ) {
								// Inspect recursively
								add( arg );
							}
						});
					})( arguments );
					// Do we need to add the callbacks to the
					// current firing batch?
					if ( firing ) {
						firingLength = list.length;
					// With memory, if we‘re not firing then
					// we should call right away
					} else if ( memory ) {
						firingStart = start;
						fire( memory );
					}
				}
				return this;
			},
			// Remove a callback from the list
			remove: function() {
				if ( list ) {
					jQuery.each( arguments, function( _, arg ) {
						var index;
						while( ( index = jQuery.inArray( arg, list, index ) ) > -1 ) {
							list.splice( index, 1 );
							// Handle firing indexes
							if ( firing ) {
								if ( index <= firingLength ) {
									firingLength--;
								}
								if ( index <= firingIndex ) {
									firingIndex--;
								}
							}
						}
					});
				}
				return this;
			},
			// Control if a given callback is in the list
			has: function( fn ) {
				return jQuery.inArray( fn, list ) > -1;
			},
			// Remove all callbacks from the list
			empty: function() {
				list = [];
				return this;
			},
			// Have the list do nothing anymore
			disable: function() {
				list = stack = memory = undefined;
				return this;
			},
			// Is it disabled?
			disabled: function() {
				return !list;
			},
			// Lock the list in its current state
			lock: function() {
				stack = undefined;
				if ( !memory ) {
					self.disable();
				}
				return this;
			},
			// Is it locked?
			locked: function() {
				return !stack;
			},
			// Call all callbacks with the given context and arguments
			fireWith: function( context, args ) {
				args = args || [];
				args = [ context, args.slice ? args.slice() : args ];
				if ( list && ( !fired || stack ) ) {
					if ( firing ) {
						stack.push( args );
					} else {
						fire( args );
					}
				}
				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;
};

代码只有仅仅200行不到，但真正看起来却又点绕，

《think in java》中有这么一句，理解一个程序最好的方法，就是把它看做一个服务的提供者。

那他提供了那些服务：

首先我们看看返回的self对象

{
	// 添加方法
	add: function() {},
	// 删除
	remove: function() {},
	// 是否包含
	has: function() {},
	// 清空
	empty: function() {},
	// 禁用
	disable: function() {},
	// 加锁
	lock: function() {},
	// 是否加锁
	locked: function() {},
	// 触发
	fireWith: function(){},
	fire: function() {},
	// 是否触发
	fired: function() {}
}

　　用途都十分清晰，那我们再看看参数，程序是服务的提供者，那么参数作为程序的入口的携带者，一般会用来装配一些属性。

显然这里就是这样。