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

 ...
 // 以下變量和函數 外部無法修改，只能通過self暴露的方法去修改和訪問
 var // Flag to know if list is currently firing
 firing,

 // Last fire value for non-forgettable lists
 // 保存上一次觸發callback的參數，調用add之后并用該參數觸發
 memory,

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

 // Flag to prevent firing
 // locked==true fire無效 若memory非空則同時add無效
 locked,

 // Actual callback list
 // callback函數數組
 list = [],

 // Queue of execution data for repeatable lists
 // 保存各個callback執行時的context和傳入的參數
 queue = [],

 // Index of currently firing callback (modified by add/remove as needed)
 // 當前正觸發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 ]; // :前為args是數組，: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非空且非正在觸發，在queue中保存memory的值,說明add后要執行fire
 // 將firingIndex移至list末尾 下一次fire從新add進來的函數開始
 if ( memory && !firing ) {
 firingIndex = list.length - 1;
 queue.push( memory );
 }

 ( function add( args ) {
 jQuery.each( args, function( _, arg ) {
 // 傳參方式為add(fn)或add(fn1,fn2)
 if ( jQuery.isFunction( arg ) ) {
 /**
 * options.unique==false
 * 或
 * options.unique==true&&self中沒有arg
 */
 if ( !options.unique || !self.has( arg ) ) {
 list.push( arg );
 }
 } else if ( arg && arg.length && jQuery.type( arg ) !== "string" ) {
 // 傳參方式為add([fn...]) 遞歸
 // Inspect recursively
 add( arg );
 }
 } );
 } )( arguments ); //arguments為參數數組 所以add的第一步是each遍歷

 //添加到list后若memory真則fire，此時firingIndex為回調隊列的最后一個函數
 if ( memory && !firing ) {
 fire();
 }
 }
 return this;
 }

// Fire callbacks
 fire = function() {

 // Enforce single-firing
 // 執行單次觸發
 locked = locked || options.once;

 // Execute callbacks for all pending executions,
 // respecting firingIndex overrides and runtime changes
 // 標記已觸發和正在觸發
 fired = firing = true;
 // 循環調用list中的回調函數
 // 循環結束之后 firingIndex賦-1 下一次fire從list的第一個開始 除非firingIndex被修改過
 // 若設置了memory，add的時候會修改firingIndex并調用fire
 // queue在fireWith函數內被更新，保存了觸發函數的context和參數
 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]是參數
 if ( list[ firingIndex ].apply( memory[ 0 ], memory[ 1 ] ) === false &&
 options.stopOnFalse ) {
 
 // Jump to end and forget the data so .add doesn't re-fire
 // 當前執行函數范圍false且options.stopOnFalse==true 直接跳至list尾 終止循環
 firingIndex = list.length;
 memory = false;
 }
 }
 }

 // 沒設置memory時不保留參數
 // 設置了memory時 參數仍保留在其中
 // 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 = "";
 }
 }
 },