在backbone中定义的View类（见下面代码）非常简洁，给用户提供了无限扩展的可能！View主要做了几件事：

1、关联DOM 元素节点。View类为backbone MVC中的视觉层肯定会与DOM打交道！见构造函数中的_ensureElement方法，关联DOM有两种方式:如果实例化或扩展view类时设置了el属性就会关联页面上的DOM，如果没有，创建一个DOM元素默认为div！

2、事件代理。见方法 delegateEvents。这里的事件代理指的是事件不是直接注册在view对象上，而是对象属相$el上,如backbone依赖的是jQuery的$el为一个jQuery对象。

3、实例化VIew类时提供的一些便捷。见 var viewOptions = ['model', 'collection', 'el', 'id', 'attributes', 'className', 'tagName', 'events'];可以在实例化是设置这些属性，用于关联model、collection，events用于设置代理事件……  这样就不通过扩展View类来做这些事！

4、还有混入backbone的Events

// Backbone.View  // -------------  // Backbone Views are almost more convention than they are actual code. A View  // is simply a JavaScript object that represents a logical chunk of UI in the  // DOM. This might be a single item, an entire list, a sidebar or panel, or  // even the surrounding frame which wraps your whole app. Defining a chunk of  // UI as a **View** allows you to define your DOM events declaratively, without  // having to worry about render order ... and makes it easy for the view to  // react to specific changes in the state of your models.  // Creating a Backbone.View creates its initial element outside of the DOM,  // if an existing element is not provided...  //backbone view 类  用于创建backbone view 对象  var View = Backbone.View = function(options) {    this.cid = _.uniqueId('view');//创建cid    options || (options = {});    _.extend(this, _.pick(options, viewOptions));    this._ensureElement();    this.initialize.apply(this, arguments);    this.delegateEvents();  };  // Cached regex to split keys for `delegate`.  var delegateEventSplitter = /^(\S+)\s*(.*)$/;  // List of view options to be merged as properties.  //初始化view时可以设置这些属性！！！！！  var viewOptions = ['model', 'collection', 'el', 'id', 'attributes', 'className', 'tagName', 'events'];  // Set up all inheritable **Backbone.View** properties and methods.  _.extend(View.prototype, Events, {    // The default `tagName` of a View's element is `"div"`.    tagName: 'div',    // jQuery delegate for element lookup, scoped to DOM elements within the    // current view. This should be preferred to global lookups where possible.    $: function(selector) {      return this.$el.find(selector);    },    // Initialize is an empty function by default. Override it with your own    // initialization logic.    initialize: function(){},    // **render** is the core function that your view should override, in order    // to populate its element (`this.el`), with the appropriate HTML. The    // convention is for **render** to always return `this`.    render: function() {      return this;    },    // Remove this view by taking the element out of the DOM, and removing any    // applicable Backbone.Events listeners.    remove: function() {      this.$el.remove();      this.stopListening();      return this;    },    // Change the view's element (`this.el` property), including event    // re-delegation.    /**       设置view的el属性和$el属性！！！    */    setElement: function(element, delegate) {      if (this.$el) this.undelegateEvents();      this.$el = element instanceof Backbone.$ ? element : Backbone.$(element);//this.$el是jQuery对象，如果依赖的是jQuery的话！      this.el = this.$el[0];//一个DOM元素节点      if (delegate !== false) this.delegateEvents();      return this;    },    // Set callbacks, where `this.events` is a hash of    //    // *{"event selector": "callback"}*    //    //     {
        //       'mousedown .title':  'edit',    //       'click .button':     'save',    //       'click .open':       function(e) { ... }    //     }    //    // pairs. Callbacks will be bound to the view, with `this` set properly.    // Uses event delegation for efficiency.    // Omitting the selector binds the event to `this.el`.    // This only works for delegate-able events: not `focus`, `blur`, and    // not `change`, `submit`, and `reset` in Internet Explorer.    delegateEvents: function(events) {
   //注册代理事件，所谓代理就是不是直接注册在view  对象上  而是注册在$el 中！      if (!(events || (events = _.result(this, 'events')))) return this;//在扩展view类时可以定义event属性！！！       this.undelegateEvents();      for (var key in events) {        var method = events[key];        if (!_.isFunction(method)) method = this[events[key]];//如果method不是函数，获取对象中对应的方法！        if (!method) continue;//跳出循环！        var match = key.match(delegateEventSplitter);        var eventName = match[1], selector = match[2];        method = _.bind(method, this);//将method 绑定到this 如：method=this.method;        eventName += '.delegateEvents' + this.cid;        if (selector === '') {          this.$el.on(eventName, method);        } else {          this.$el.on(eventName, selector, method);        }      }      return this;    },    // Clears all callbacks previously bound to the view with `delegateEvents`.    // You usually don't need to use this, but may wish to if you have multiple    // Backbone views attached to the same DOM element.    undelegateEvents: function() {
   //remove 代理的事件！！！      this.$el.off('.delegateEvents' + this.cid);      return this;    },    // Ensure that the View has a DOM element to render into.    // If `this.el` is a string, pass it through `$()`, take the first    // matching element, and re-assign it to `el`. Otherwise, create    // an element from the `id`, `className` and `tagName` properties.    /***       确保  有DOM 元素将view 渲染进里面     ***/    _ensureElement: function() {       //设置或创建view的element      if (!this.el) {        var attrs = _.extend({}, _.result(this, 'attributes'));        if (this.id) attrs.id = _.result(this, 'id');        if (this.className) attrs['class'] = _.result(this, 'className');        var $el = Backbone.$('<' + _.result(this, 'tagName') + '>').attr(attrs);//创建DOM元素，并设置元素的相关属性！        this.setElement($el, false);      } else {        this.setElement(_.result(this, 'el'), false);      }    }  });

以上为backbone View的相关代码，自己写了点注释，不好请指正！