backbone源码

Question

Backbone 版本:1.1.2 Model模型

var Model = Backbone.Model = function(attributes, options) {
    var attrs = attributes || {};
    options || (options = {});
    this.cid = _.uniqueId('c');
    this.attributes = {};
    if (options.collection) this.collection = options.collection;
    if (options.parse) attrs = this.parse(attrs, options) || {};
    attrs = _.defaults({}, attrs, _.result(this, 'defaults'));
    this.set(attrs, options);
    this.changed = {};                // 这个是干嘛用的？
    this.initialize.apply(this, arguments);
  };

set方法

      for (attr in attrs) {
        val = attrs[attr];
        if (!_.isEqual(current[attr], val)) changes.push(attr);
        if (!_.isEqual(prev[attr], val)) {   // 这里操作的意义在哪里
          this.changed[attr] = val;
        } else {
          delete this.changed[attr];
        }
        unset ? delete current[attr] : current[attr] = val;
      }

问题的核心就在于 changed这个属性，一直没太弄清楚是干嘛用的，set方法里面为什么要去判断 prev[attr]和val是否相等？