ViewPager使用FragmentPagerAdapter加载多个fragment，实现对已加载的list做插入、删除及排序操作

ViewPager使用FragmentPagerAdapter加载多个fragment，实现对已加载的list做插入、删除及排序操作。

例子1：

List<Fragment> list = new ArrayList<>();
list.add(fragment1);
list.add(fragment2);
list.add(fragment3);

adapter = new MyListAdapter(list,fragmentManager);
viewpager.setAdapter(adapter);

//删除fragment2
list.remove(fragment2);
adapter.notifyDataSetChanged();

执行上面代码是没任何改变的，因为在调用adapter.notifyDataSetChanged()时adapter会通过getItemPosition(Object object)
来判断是否删除fragment：

通过重写getItemPosition：

@Override
public int getItemPosition(Object object) {
//每次都刷新都调用instantiateItem 和destroyItem方法
return POSITION_NONE;
}

再次执行例子1，viewpager是有改变的但是不是期望的效果，继续看源码：

而getItemId(position)方法默认返回的是

到这里可以发现instantiateItem()新增fragment是通过position创建一个name然后在
fragmentManager中查询是否存在改name的fragment，存在就调用attch方法重新加载(这
里与destroyItem方法的detach对于的，attch会重新创建View)。

所以例子1删除fragment2时，list的size为2，调用notifyDataSetChanged，
adapter会删除所有的fragemnt调用instantiateItem重新添加，在创健fragment的name时
默认返回position，所以剩下两个的name是以position为0、1的tag，viewpager显示的
是fragment1、fragment2，而不是fragment1、fragment3。

public abstract class MyFragmentPagerAdapter extends PagerAdapter{
private final FragmentManager mFragmentManager;
private FragmentTransaction mCurTransaction = null;
private Fragment mCurrentPrimaryItem = null;

public MyFragmentPagerAdapter(FragmentManager mFragmentManager){
this.mFragmentManager =mFragmentManager;
}

public abstract Fragment getItem(int position);

/**作为fragment的标识,每个fragment都有单独的标识，不能直接用position，否则在排序和删除会出现fragment错乱*/
public abstract long getItemId(int position);
@Override
public void startUpdate(ViewGroup container) {
}

@Override
public Object instantiateItem(ViewGroup container, int position) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
final long itemId = getItemId(position);

String name = makeFragmentName(container.getId(), itemId);
Fragment fragment = mFragmentManager.findFragmentByTag(name);
if (fragment != null) {
/** mCurTransaction.attach(fragment);
* 用show、hide代替attach、detach，这样fragment不会reCreateView
*/
mCurTransaction.show(fragment);
} else {
fragment = getItem(position);
mCurTransaction.add(container.getId(), fragment,
makeFragmentName(container.getId(), itemId));
}
if (fragment != mCurrentPrimaryItem) {
fragment.setMenuVisibility(false);
fragment.setUserVisibleHint(false);
}

return fragment;
}

@Override
public void destroyItem(ViewGroup container, int position, Object object) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
/** mCurTransaction.detach((Fragment)object);*/
mCurTransaction.hide((Fragment)object);
}

@Override
public void setPrimaryItem(ViewGroup container, int position, Object object) {
Fragment fragment = (Fragment)object;
if (fragment != mCurrentPrimaryItem) {
if (mCurrentPrimaryItem != null) {
mCurrentPrimaryItem.setMenuVisibility(false);
mCurrentPrimaryItem.setUserVisibleHint(false);
}
if (fragment != null) {
fragment.setMenuVisibility(true);
fragment.setUserVisibleHint(true);
}
mCurrentPrimaryItem = fragment;
}
}
@Override
public int getItemPosition(Object object) {
//每次都刷新都调用instantiateItem 和destroyItem方法
return POSITION_NONE;
}

@Override
public void finishUpdate(ViewGroup container) {
if (mCurTransaction != null) {
mCurTransaction.commitAllowingStateLoss();
mCurTransaction = null;
mFragmentManager.executePendingTransactions();
}
}

@Override
public boolean isViewFromObject(View view, Object object) {
return ((Fragment)object).getView() == view;
}

@Override
public Parcelable saveState() {
return null;
}

@Override
public void restoreState(Parcelable state, ClassLoader loader) {
}

private static String makeFragmentName(int viewId, long id) {
return "android:switcher:" + viewId + ":" + id;
}
}

用法和FragmentPagerAdapter一样,将getItemId改为抽象方法，返回每个fragment的标识。