jQuery.Callbacks之源码解读

　　在上一篇jQuery.Callbacks之demo主要说了Callbacks对象初始化常见的选项，这一篇主要分析下Callbacks对象的源代码，对给出两个较为繁琐的demo

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

// Convert String-formatted options into Object-formatted ones and store in cache
/*
这个函数主要将传入的options字符串封装成对象
比如将传入的'once memory'封装成
optionsCache['once memory'] = {
    once : true,
    memory : true
}
这样方便下次同样的options复用和判断
*/
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)
        //大多数情况下这个变量是包含两个元素的数组，[0]表示上次调用的对象，[1]表示上次调用的参数
        memory,
        // Flag to know if list was already fired
        //标识是否执行过回调函数，主要用来实现once
        fired,
        // Flag to know if list is currently firing
        //当前是否在firing，可以参考多线编程中锁的概念，主要用在调用回调函数时，对callbacks对象进行add、remove或者fire，后面会有两个单独的例子说明这种情况
        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
        //所有的回调会被push到这个数组
        list = [],
        // Stack of fire calls for repeatable lists
        //结合firing使用，如果有once选项没什么作用，否则当firing为true时将add或者fire的操作临时存入这个变量，以便于循环完list时继续处理这个变量里面的函数队列
        stack = !options.once && [],
        // Fire callbacks
        fire = function( data ) {
            //如果设置memory为true，则将本次的参数data缓存到memory中，用于下次调用
            memory = options.memory && data;
            fired = true;
            //如果options.memory为true，firingStart为上一次Callbacks.add后回调列表的length值
            firingIndex = firingStart || 0;
            firingStart = 0;
            firingLength = list.length;
            firing = true;
            for ( ; list && firingIndex < firingLength; firingIndex++ ) {
                //如果stopOnFalse为true且本次执行的回调函数返回值为false，则终止回调函数队列的执行
                if ( list[ firingIndex ].apply( data[ 0 ], data[ 1 ] ) === false && options.stopOnFalse ) {
                    //设置memory为false，防止调用add时会被fire（这个分支是在stopOnFalse memory时被触发）
                    memory = false; // To prevent further calls using add
                    break;
                }
            }
            firing = false;
            if ( list ) {
                //options.once为false（stack的作用见上）
                if ( stack ) {
                    //存在递归的可能，所以不用使用while
                    if ( stack.length ) {
                        fire( stack.shift() );
                    }
                //memory = true, memory = true的情况
                } else if ( memory ) {
                    list = [];
                } else {
                    //once = true， memory = false的情况
                    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" ) {
                                //实现unique（回调不唯一 或 唯一且不存在，则push）
                                if ( !options.unique || !self.has( arg ) ) {
                                    list.push( arg );
                                }
                            //如果arg是数组，递归添加回调
                            } 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
                    //如果memory不是false，则直接每次add的时候都自动fire
                    } 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
                            //如果在执行Callbacks.remove操作的状态为firing时则更新firingLength和firingIndex的值
                            if ( firing ) {
                                if ( index <= firingLength ) {
                                    firingLength--;
                                }
                                //特殊处理，如果移除的回调的索引小于当前正在执行回调的索引，则firingIdex--
                                //后面未执行的回调则得以正常执行
                                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;
};

　　需要特殊注意的是有一个firing这个变量，下面给出这个变量的应用场景：

　　1、在Callbacks.add中firing为true的情况

// 定义三个将要增加到回调列表的回调函数fn1,fn2,fn3
function fn1(val){
    console.log( 'fn1 says ' + val );
    //此时Callbacks函数内部的firingLength会自动加1，虽然初始化的Callbacks对象有memory选项，
    //但add并不会立即执行fn2，而是等执行完add前的函数队列之后再执行fn2
    cbs.add(fn2);
}
function fn2(val){
    console.log( 'fn2 says ' + val );
}
function fn3(val){
    console.log( 'fn3 says ' + val );
}

// Callbacks传递了memory
// 也可以这样使用$.Callbacks({ memory: true });
var cbs = $.Callbacks('memory');

// 将fn1增加到回调列表中，因为在fn1中有执行了add(fn2)操作，因此回调列表中的回调为fn1,fn2
cbs.add(fn1);

//fn1 says foo
//fn2 says foo
cbs.fire('foo');

//将之前fire的参数传递给最近增加的回调fn3，并执行fn3
//fn3 says foo
cbs.add(fn3);

//再执行一次fire，注意此时回调列表中的回调依次是fn1,fn2,fn3,fn2
//fn1 says bar
//fn2 says bar
//fn3 says bar
//fn2 says bar
cbs.fire('bar');

　　2、在Callbacks.fireWith中firing为true的情况

function fn1(val){
    console.log( 'fn1 says ' + val );
}
function fn2(val){
    console.log( 'fn2 says ' + val );
    //此时并不会立即触发cbs里面的回调，而是先把[window, ['bar']]放入stack里面
    //等执行完fireWith前的函数队列之后才执行
    cbs.fireWith(window, ['bar']);
    //firingLength会减一，一定要将当前的函数remove掉，否则会导致死循环
    cbs.remove(fn2);
}

var cbs = $.Callbacks();
cbs.add(fn1);
cbs.add(fn2);
//fn1 says bar
//fn2 says bar
//fn1 says bar
cbs.fire('bar');