[Note]ViewPager & ViewPagerAdapter & ViewPageStateAdapter

-ViewPager

• allows u to flip left & right to show data
• often conjunction with fragment, which is convenience to manage lifecycle of page
• there are two standard adapter  'ViewPageAdapter' & 'ViewPageStateAdapter' implemented using fragment, which cover the most common use cases

-ViewPageAdapter

• this adapter is best for use when there are static fragments to be paged through, such as a set of tabs

-ViewPageStateAdapter

• this adapter is best for use when there are a large number of pages, working like the listview