1、连接视图与数据、数据封装；
2、视图构造与配置；
3、数据更新监听；
4、视图重用机制。
* An Adapter object acts as a bridge between an {@link AdapterView} and the
* underlying data for that view. The Adapter provides access to the data items.
* The Adapter is also responsible for making a {@link android.view.View} for
* each item in the data set.
/**
* Extended {@link Adapter} that is the bridge between a {@link ListView}
* and the data that backs the list. Frequently that data comes from a Cursor,
* but that is not
* required. The ListView can display any data provided that it is wrapped in a
* ListAdapter.
*/