ViewPager使用FragmentPagerAdapter加載多個fragment，實現對已加載的list做插入、刪除及排序操作

ViewPager使用FragmentPagerAdapter加載多個fragment，實現對已加載的list做插入、刪除及排序操作。

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);

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

執行上面代碼是沒任何改變的，因爲在調用adapter.notifyDataSetChanged()時adapter會通過getItemPosition(Object object)
來判斷是否刪除fragment：

通過重寫getItemPosition：

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

再次執行例子1，viewpager是有改變的但是不是期望的效果，繼續看源碼：

而getItemId(position)方法默認返回的是

到這裏可以發現instantiateItem()新增fragment是通過position創建一個name然後在
fragmentManager中查詢是否存在改name的fragment，存在就調用attch方法重新加載(這
裏與destroyItem方法的detach對於的，attch會重新創建View)。

所以例子1刪除fragment2時，list的size爲2，調用notifyDataSetChanged，
adapter會刪除所有的fragemnt調用instantiateItem重新添加，在創健fragment的name時
默認返回position，所以剩下兩個的name是以position爲0、1的tag，viewpager顯示的
是fragment1、fragment2，而不是fragment1、fragment3。

接下來MyFragmentPagerAdapter：

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);

    /**作爲fragment的標識,每個fragment都有單獨的標識，不能直接用position，否則在排序和刪除會出現fragment錯亂*/
    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，這樣fragment不會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) {
        //每次都刷新都調用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一樣,將getItemId改爲抽象方法，返回每個fragment的標識。