var cb = $.Callbacks('memory');
cb.add(function(val){
    console.log('1: ' + val)
})
cb.fire('callback')
cb.add(function(val){
    console.log('2: ' + val)
})
// console输出
1: callback
2: callback

    ...
    // 以下变量和函数 外部无法修改，只能通过self暴露的方法去修改和访问
    var // Flag to know if list is currently firing
        firing,

        // Last fire value for non-forgettable lists
        // 保存上一次触发callback的参数，调用add之后并用该参数触发
        memory,

        // Flag to know if list was already fired
        fired,

        // Flag to prevent firing
        // locked==true fire无效 若memory非空则同时add无效
        locked,

        // Actual callback list
        // callback函数数组
        list = [],

        // Queue of execution data for repeatable lists
        // 保存各个callback执行时的context和传入的参数
        queue = [],

        // Index of currently firing callback (modified by add/remove as needed)
        // 当前正触发callback的索引
        firingIndex = -1,

        // Fire callbacks
        fire = function() {
            ...
        },
        
        // Actual Callbacks object
        self = {
            // Add a callback or a collection of callbacks to the list
            add: function() {
                ...
            },
            ...
            // Call all callbacks with the given context and arguments
            fireWith: function( context, args ) {
                if ( !locked ) {
                    args = args || [];
                    args = [ context, args.slice ? args.slice() : args ]; // :前为args是数组，:后是string
                    queue.push( args );
                    if ( !firing ) {
                        fire();
                    }
                }
                return this;
            },

            // Call all the callbacks with the given arguments
            fire: function() {
                self.fireWith( this, arguments );
                return this;
            },
            ...
        }

            add: function() {
                if ( list ) {

                    // If we have memory from a past run, we should fire after adding
                    // 如果memory非空且非正在触发，在queue中保存memory的值,说明add后要执行fire
                    // 将firingIndex移至list末尾 下一次fire从新add进来的函数开始
                    if ( memory && !firing ) {
                        firingIndex = list.length - 1;
                        queue.push( memory );
                    }

                    ( function add( args ) {
                        jQuery.each( args, function( _, arg ) {
                            // 传参方式为add(fn)或add(fn1,fn2)
                            if ( jQuery.isFunction( arg ) ) {
                                /**
                                 * options.unique==false
                                 * 或
                                 * options.unique==true&&self中没有arg
                                 */
                                if ( !options.unique || !self.has( arg ) ) {
                                    list.push( arg );
                                }
                            } else if ( arg && arg.length && jQuery.type( arg ) !== "string" ) {
                                // 传参方式为add([fn...]) 递归
                                // Inspect recursively
                                add( arg );
                            }
                        } );
                    } )( arguments ); //arguments为参数数组 所以add的第一步是each遍历

                    //添加到list后若memory真则fire，此时firingIndex为回调队列的最后一个函数
                    if ( memory && !firing ) {
                        fire();
                    }
                }
                return this;
            }

// Fire callbacks
        fire = function() {

            // Enforce single-firing
            // 执行单次触发
            locked = locked || options.once;

            // Execute callbacks for all pending executions,
            // respecting firingIndex overrides and runtime changes
            // 标记已触发和正在触发
            fired = firing = true;
            // 循环调用list中的回调函数
            // 循环结束之后 firingIndex赋-1 下一次fire从list的第一个开始 除非firingIndex被修改过
            // 若设置了memory，add的时候会修改firingIndex并调用fire
            // queue在fireWith函数内被更新，保存了触发函数的context和参数
            for ( ; queue.length; firingIndex = -1 ) {
                memory = queue.shift();
                while ( ++firingIndex < list.length ) { 

                    // Run callback and check for early termination
                    // memory[0]是content memory[1]是参数
                    if ( list[ firingIndex ].apply( memory[ 0 ], memory[ 1 ] ) === false &&
                        options.stopOnFalse ) {
                        
                        // Jump to end and forget the data so .add doesn&#39;t re-fire
                        // 当前执行函数范围false且options.stopOnFalse==true 直接跳至list尾 终止循环
                        firingIndex = list.length;
                        memory = false;
                    }
                }
            }

            // 没设置memory时不保留参数
            // 设置了memory时 参数仍保留在其中
            // Forget the data if we&#39;re done with it
            if ( !options.memory ) {
                memory = false;
            }

            firing = false;

            // Clean up if we&#39;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 = "";
                }
            }
        },