Android：Fragment+ViewPager+FragmentPagerAdapter

1）ViewPager 官方的建议：建议ViewPager和Fragment一起使用，便于管理每一页的生命周期
ViewPager is most often used in conjunction with {@link android.app.Fragment},which is a convenient way to supply and manage the lifecycle of each page.
There are standard adapters implemented for using fragments with the ViewPager,which cover the most common use cases.

2）FragmentPagerAdapter

   FragmentPagerAdapter：
   getCount()
   Fragment getItem(position)