MVVM模式通过ViewModel实现view和model的低耦合

http://nerds.airbnb.com/airbnb-tablet/

Each of these representations would differ from tablet to phone, so instead
of having branching logic in place everywhere these objects were used, we decided to ask the model how it preferred to be displayed. We built a view-model protocol that allows us to ask any model object for its “default representation view controller.” The
model returns a fully allocated device-specific view controller to be displayed. At first, these view-model objects simply returned the phone view controller, but when we eventually started building the tablet version we simply had to change a single line
of code for the tablet view controllers to be displayed app-wide.

MVVM模式和MVC模式一样，主要目的是分离视图（View）和模型（Model），有几大优点

1. 低耦合。视图（View）可以独立于Model变化和修改，一个ViewModel可以绑定到不同的"View"上，当View变化的时候Model可以不变，当Model变化的时候View也可以不变。

2. 可重用性。你可以把一些视图逻辑放在一个ViewModel里面，让很多view重用这段视图逻辑。

3. 独立开发。开发人员可以专注于业务逻辑和数据的开发（ViewModel），设计人员可以专注于页面设计，使用Expression Blend可以很容易设计界面并生成xaml代码。

4. 可测试。界面素来是比较难于测试的，而现在测试可以针对ViewModel来写。