// Set a PagerAdapter to supply views for this pager.
 ViewPager viewPager = (ViewPager) findViewById(R.id.my_viewpager_id);
 viewPager.setAdapter(mMyFragmentPagerAdapter);
 
 private FragmentPagerAdapter mMyFragmentPagerAdapter = new FragmentPagerAdapter(getSupportFragmentManager()) {
  @Override
  public int getCount() {
   return 2; // Return the number of views available.
  }
 
  @Override
  public Fragment getItem(int position) {
   return new MyFragment(); // Return the Fragment associated with a specified position.
  }
 
  // Called when the host view is attempting to determine if an item's position has changed.
  @Override
  public int getItemPosition(Object object) {
   if (object instanceof MyFragment) {
    ((MyFragment)object).updateView();
   }
   return super.getItemPosition(object);
  }
 };
 
 private class MyFragment extends Fragment {
  @Override
  public void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   // do something such as init data
  }
 
  @Override
  public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
   View view = inflater.inflate(R.layout.fragment_my, container, false);
   // init view in the fragment
   return view;
  }
 
  public void updateView() {
   // do something to update the fragment
  }
 }

// 刚开始处在page0
D/Adapter (25946): getItem(0)
D/Fragment0(25946): newInstance(2015-09-10) // 注释：newInstance()调用了Fragment的构造器方法，下同。
D/Adapter (25946): getItem(1)
D/Fragment1(25946): newInstance(Hello World, I'm li2.)
D/Fragment0(25946): onAttach()
D/Fragment0(25946): onCreate()
D/Fragment0(25946): onCreateView()
D/Fragment1(25946): onAttach()
D/Fragment1(25946): onCreate()
D/Fragment1(25946): onCreateView()

// 第1次滑到page1
D/Adapter (25946): onPageSelected(1)
D/Adapter (25946): getItem(2)
D/Fragment2(25946): newInstance(true)
D/Fragment2(25946): onAttach()
D/Fragment2(25946): onCreate()
D/Fragment2(25946): onCreateView()

// 第N次（N不等于1）向右滑动选中page2
D/Adapter (25946): onPageSelected(2)
D/Adapter (25946): destroyItem(0) // 销毁page0的视图
D/Fragment0(25946): onDestroyView()
D/Fragment3(25946): onCreateView() // page3的Fragment实例仍保存在FragmentManager中，所以只需创建它的视图
FragmentStatePagerAdapter说：上上页的实例和视图都被俺销毁啦：
// 第N次（N不等于1）向右滑选中page2
D/Adapter (27880): onPageSelected(2)
D/Adapter (27880): destroyItem(0) // 销毁page0的实例和视图
D/Adapter (27880): getItem(3) // 创建page3的Fragment
D/Fragment3(27880): newInstance()
D/Fragment0(27880): onDestroyView()
D/Fragment0(27880): onDestroy()
D/Fragment0(27880): onDetach()
D/Fragment3(27880): onAttach()
D/Fragment3(27880): onCreate()
D/Fragment3(27880): onCreateView()
Fragment getItem(int position)

// Return the Fragment associated with a specified position.
public abstract Fragment getItem(int position);

D/Adapter (25946): getItem(1)
D/Fragment1(25946): newInstance(Hello World, I'm li2.) // newInstance()调用了Fragment的构造器方法；
D/Fragment1(25946): onAttach()
D/Fragment1(25946): onCreate()
D/Fragment1(25946): onCreateView()

void destroyItem(ViewGroup container, int position, Object object)
// Remove a page for the given position. 
public void FragmentPagerAdapter.destroyItem(ViewGroup container, int position, Object object) {
  mCurTransaction.detach((Fragment)object);
}

public void FragmentStatePagerAdapter.destroyItem(ViewGroup container, int position, Object object) {
  mSavedState.set(position, mFragmentManager.saveFragmentInstanceState(fragment));
  mFragments.set(position, null);
  mCurTransaction.remove(fragment);
}

// 对于FragmentPagerAdapter
D/Adapter (25946): onPageSelected(2)
D/Adapter (25946): destroyItem(0)
D/Fragment0(25946): onDestroyView() // 销毁视图

// 对于FragmentStatePagerAdapter
D/Adapter (27880): onPageSelected(2)
D/Adapter (27880): destroyItem(0)
D/Fragment0(27880): onDestroyView() // 销毁视图
D/Fragment0(27880): onDestroy() // 销毁实例
D/Fragment0(27880): onDetach()
FragmentPagerAdapter和FragmentStatePagerAdapter对比总结

// Called when the host view is attempting to determine if an item's position has changed. Returns POSITION_UNCHANGED if the position of the given item has not changed or POSITION_NONE if the item is no longer present in the adapter.
public int getItemPosition(Object object) {
  return POSITION_UNCHANGED;
}

@Override
// To update fragment in ViewPager, we should override getItemPosition() method,
// in this method, we call the fragment's public updating method.
public int getItemPosition(Object object) {
  Log.d(TAG, "getItemPosition(" + object.getClass().getSimpleName() + ")");
  if (object instanceof Page0Fragment) {
    ((Page0Fragment) object).updateDate(mDate);
  } else if (object instanceof Page1Fragment) {
    ((Page1Fragment) object).updateContent(mContent);
  } else if (object instanceof Page2Fragment) {
    ((Page2Fragment) object).updateCheckedStatus(mChecked);
  } else if (...) {
  }
  return super.getItemPosition(object);
};

// 更新界面时方法的调用情况
// 当前页为0时
D/Adapter (21517): notifyDataSetChanged(+0)
D/Adapter (21517): getItemPosition(Page0Fragment)
D/Fragment0(21517): updateDate(2015-09-12)
D/Adapter (21517): getItemPosition(Page1Fragment)
D/Fragment1(21517): updateContent(Hello World, I am li2.)

// 当前页为1时
D/Adapter (21517): notifyDataSetChanged(+1)
D/Adapter (21517): getItemPosition(Page0Fragment)
D/Fragment0(21517): updateDate(2015-09-13)
D/Adapter (21517): getItemPosition(Page1Fragment)
D/Fragment1(21517): updateContent(Hello World, I am li2.)
D/Adapter (21517): getItemPosition(Page2Fragment)
D/Fragment2(21517): updateCheckedStatus(true)

@Override  

  public int getItemPosition(Object object) { 

  return PagerAdapter.POSITION_NONE;  

 }