Backbone事件模块源码分析
事件模块Backbone.Events在Backbone中占有十分重要的位置，其他模块Model，Collection，View所有事件模块都依赖它。通过继承Events的方法来实现事件的管理，可以说，它是Backbone的核心组成部分。
此外，事件模块的所有方法都挂在了全局的Backbone上，如果你的代码中需要用到自定义事件(实现观察者模式)，可以直接使用它。
所以很有必要一起来研究下Backbone.Events的源码，一来学习Backbone事件模块优秀的写法和思想，二来可以更好的灵活使用Backbone事件模块。
// Backbone.Events // --------------- // A module that can be mixed in to *any object* in order to provide it with // custom events. You may bind with `on` or remove with `off` callback // functions to an event; `trigger`-ing an event fires all callbacks in // succession. // // var object = {}; // _.extend(object, Backbone.Events); // object.on('expand', function(){ alert('expanded'); }); // object.trigger('expand'); // var Events = Backbone.Events = { // Bind an event to a `callback` function. Passing `"all"` will bind // the callback to all events fired. on: function(name, callback, context) { //两种情况： //1. 一次添加多个事件时，通过eventsApi一个一个添加，所以eventsApi返回false，那么直接return //2. 回调函数没定义，没有意义，直接return if (!eventsApi(this, 'on', name, [callback, context]) || !callback) return this; //因此这里往下事件是一个一个添加的，即name是一个事件名（如：click | custom） //初始化私有变量，用于存储事件的信息 this._events || (this._events = {}); var events = this._events[name] || (this._events[name] = []); events.push({callback: callback, context: context, ctx: context || this}); return this; }, // Bind an event to only be triggered a single time. After the first time // the callback is invoked, it will be removed. once: function(name, callback, context) { if (!eventsApi(this, 'once', name, [callback, context]) || !callback) return this; var self = this; //将callback进行包装，返回的新函数newCallback内部会调用calllback //不同的是，newCallback只会调用callback一次，之后只会返回callback执行的结果 //也就是说once实质上并没有去除掉事件监听函数，而是控制了callback只会执行一次 var once = _.once(function() { self.off(name, once); callback.apply(this, arguments); }); //保留原callback，用于off操作 once._callback = callback; //实质调用.on()方法注册事件 return this.on(name, once, context); }, // Remove one or many callbacks. If `context` is null, removes all // callbacks with that function. If `callback` is null, removes all // callbacks for the event. If `name` is null, removes all bound // callbacks for all events. off: function(name, callback, context) { var retain, ev, events, names, i, l, j, k; //两种情况： //1. 根本没注册过事件，何谈删除事件，直接return //2. 像上述所说支持多事件删除 if (!this._events || !eventsApi(this, 'off', name, [callback, context])) return this; //如果是obj.off()这样调用，那么删除该对象obj上所有的事件监听 //也就是是将_events清空了 if (!name && !callback && !context) { this._events = void 0; return this; } //如果name为空，像obj.off(undefined, cb1, ctx1) //那么name就为所有注册过的事件（即_.keys(this._events)） names = name ? [name] : _.keys(this._events); //根据name遍历events for (i = 0, l = names.length; i < l; i++) { name = names[i]; if (events = this._events[name]) { this._events[name] = retain = []; //如果callback或者context有一个有值 //那么接下来将它们作为条件进行接下来事件的off操作 //实质其实是先清空_events，将不满足条件删除条件的事件监听器重新填入_events中 if (callback || context) { for (j = 0, k = events.length; j < k; j++) { ev = events[j]; //这里对指定了callback或者context的情况，做了条件判断 //这里的_callback是因为.once方法会对原callback进行包装，这里的evn.callback就是包装后的，原callback保存在_callback中 if ((callback && callback !== ev.callback && callback !== ev.callback._callback) || (context && context !== ev.context)) { retain.push(ev); } } } //发现该事件的事件监听器被删光了，那么做了清理工作，删除_events对应的key if (!retain.length) delete this._events[name]; } } return this; }, // Trigger one or many events, firing all bound callbacks. Callbacks are // passed the same arguments as `trigger` is, apart from the event name // (unless you're listening on `"all"`, which will cause your callback to // receive the true name of the event as the first argument). trigger: function(name) { if (!this._events) return this; //分离出传给callback的参数 var args = slice.call(arguments, 1); //同样支持多事件同时trigger if (!eventsApi(this, 'trigger', name, args)) return this; var events = this._events[name]; var allEvents = this._events.all; if (events) triggerEvents(events, args); if (allEvents) triggerEvents(allEvents, arguments); return this; }, // Tell this object to stop listening to either specific events ... or // to every object it's currently listening to. stopListening: function(obj, name, callback) { var listeningTo = this._listeningTo; if (!listeningTo) return this; var remove = !name && !callback; //这里是兼容(obj, {click: cb1, change: cb2})这种形式 //保证第三个参数是作为context传入，这里是this if (!callback && typeof name === 'object') callback = this; //如果有指定obj，那么解除对象只针对obj //如果没有指定，则是解除监听的所有对象的事件绑定 if (obj) (listeningTo = {})[obj._listenId] = obj; for (var id in listeningTo) { obj = listeningTo[id]; obj.off(name, callback, this); //两种情况下做清理工作 //1. 已经表明清除对obj的的所有事件监听（即name和callback都为空） //2. obj对象自身都没有被绑定事件了，哪来的事件让你监听呢？ if (remove || _.isEmpty(obj._events)) delete this._listeningTo[id]; } return this; } }; // Regular expression used to split event strings. var eventSplitter = /\s+/; // Implement fancy features of the Events API such as multiple event // names `"change blur"` and jQuery-style event maps `{change: action}` // in terms of the existing API. var eventsApi = function(obj, action, name, rest) { if (!name) return true; // Handle event maps. //支持映射关系 //如：(obj, 'on', {'click': function x () {}, 'blur': function xx () {}}, context) if (typeof name === 'object') { for (var key in name) { //反复调用action(on | off | once), 每次添加一个事件监听，从而达到添加多个。 obj[action].apply(obj, [key, name[key]].concat(rest)); } return false; } // Handle space separated event names. //支持空格分割事件（即多事件共享同一个函数） //如：(obj, 'on', 'click blur', function () {}, context) if (eventSplitter.test(name)) { var names = name.split(eventSplitter); for (var i = 0, l = names.length; i < l; i++) { obj[action].apply(obj, [names[i]].concat(rest)); } return false; } return true; }; // A difficult-to-believe, but optimized internal dispatch function for // triggering events. Tries to keep the usual cases speedy (most internal // Backbone events have 3 arguments). //这里做了个优化，就是如果arg参数在3个之类的话，用call进行调用 //因为call要比apply的效率高(http://jsperf.com/function-versus-function-call-versus-function-apply) var triggerEvents = function(events, args) { var ev, i = -1, l = events.length, a1 = args[0], a2 = args[1], a3 = args[2]; switch (args.length) { case 0: while (++i < l) (ev = events[i]).callback.call(ev.ctx); return; case 1: while (++i < l) (ev = events[i]).callback.call(ev.ctx, a1); return; case 2: while (++i < l) (ev = events[i]).callback.call(ev.ctx, a1, a2); return; case 3: while (++i < l) (ev = events[i]).callback.call(ev.ctx, a1, a2, a3); return; default: while (++i < l) (ev = events[i]).callback.apply(ev.ctx, args); return; } }; var listenMethods = {listenTo: 'on', listenToOnce: 'once'}; // Inversion-of-control versions of `on` and `once`. Tell *this* object to // listen to an event in another object ... keeping track of what it's // listening to. //添加listenTo和listernToOnce方法 //实质是: //1. 给需要监听的对象obj赋予一个_listenId的随机id //2. 再给监听者（调用对象）添加一个map就是listeningTo属性，添加上述的id和obj //3. 给obj绑定被监听的事件，将this指向调用者 //这里实质就是调用obj的on或者once方法来添加事件监听， //那么单独列出这样的一个方法的好处在于方便监听者，可以随时监听和解除监听，上述的1，2两不操作是为了以后解除监听做准备 _.each(listenMethods, function(implementation, method) { Events[method] = function(obj, name, callback) { var listeningTo = this._listeningTo || (this._listeningTo = {}); var id = obj._listenId || (obj._listenId = _.uniqueId('l')); listeningTo[id] = obj; if (!callback && typeof name === 'object') callback = this; obj[implementation](name, callback, this); return this; }; }); // Aliases for backwards compatibility. Events.bind = Events.on; Events.unbind = Events.off; // Allow the `Backbone` object to serve as a global event bus, for folks who // want global "pubsub" in a convenient place. _.extend(Backbone, Events);
