ViewPager、FragmentStatePagerAdapter 和无限滚动

Question

我想要达到的目标：

带有 ViewPager 的 Activity 显示适配器中对象列表的片段 (FragmentStatePagerAdapter)。

最初，Activity 将 SQLite DB 中的 N 个（比如说 5 个）对象加载到适配器中。这些对象是随机选择的。

当用户到达列表末尾时，活动应从数据库中加载 M（假设 M 为 3）更多对象，将它们添加到适配器并调用 notifyDataSetChanged()。添加它们时，我会检查列表中是否已经存在新对象，如果存在，则会删除预先存在的对象，并将加载的对象添加到列表的尾部。

因此，我正在尝试实现类似 无限滚动 ViewPager （不是“循环” ViewPager，因为我希望不断获取新对象，而不是回到乞求列表）。

我有一些工作代码，并包含了我在下面遵循的模式的示例。但是，我不断收到此异常，不知道为什么：

IllegalStateException: Fragment MyObjectFragment{id...} is not currently in the FragmentManager
at android.app.FragmentManagerImpl.saveFragmentInstanceState(FragmentManager.java:553)
at android.support.v13.app.FragmentStatePagerAdapter.destroyItem(FragmentStatePagerAdapter.java:140)
at android.support.v4.ViewPager.populate(ViewPager.java:1002)
...

代码示例：

活动：

public class MyActivitty extends FragmentActivity {

    public MyPagerAdapter adapter;
    public ViewPager pager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.my_acyivity_layout);
        pager = (ViewPager) findViewById(R.id.viewPager);

        ArrayList<MyObject> myObjects = new ArrayList<MyObject>();

        // loadInitialObjectsFromDB(int N) goes to SQLite DB and loads the N first objects to show on the ViewPager...
        myObjects = loadInitialObjectsFromDB(5); 

        // Adapter will use the previously fetched objects
        adapter = new MyPagerAdapter(this, getFragmentManager(), myObjects); 

        pager.setAdapter(adapter);
        pager.setOffscreenPageLimit(2);

        // (...)
    }

    // (...)
}

PagerAdapter：

public class MyPagerAdapter extends FragmentStatePagerAdapter implements
    ViewPager.OnPageChangeListener {

    private MyActivity context;
    private ArrayList<MyObject> objectList;
    private int currentPosition = 0;

    // (...)

    public MyPagerAdapter(MyActivity context, FragmentManager fragmentManager, ArrayList<MyObject> objects) 
    {
            super(fragmentManager);
            this.context = context;
            this.objectList = objects;
    }

    @Override
    public Fragment getItem(int position) 
    {
            MyObject object = objectList.get(position);
            return MyObjectFragment.newInstance(position, object);
    }

    @Override
    public int getItemPosition(Object object){
            MyObjectFragment frag = (MyObjectFragment) object;
            MyObject object = frag.getMyObject();

            for(int i = 0; i < objectList.size(); i++)
            {
                    if(objectList.get(i).getId() == object.getId())
                    {
                            return i;
                    }
            }

            return PagerAdapter.POSITION_NONE;
    }

    @Override
    public int getCount()
    {       
            return objectList.size();
    }

    @Override
    public void onPageSelected(int position) 
    {
           currentPosition = position;
    }

    // (...)

}


@Override
public void onPageScrollStateChanged(int state) 
{   
    switch(state)
    {
    case ViewPager.SCROLL_STATE_DRAGGING:
            // (...)
        break;

    case ViewPager.SCROLL_STATE_IDLE:
         // if we are reaching the "end" of the list (while scrolling to the right), load more objects
            if(currentPosition <= position && position >= answerList.size()-1)
            {
                    // Function in MyActivity that fetches N more objects.
                    // and adds them to this adapter's ArrayList<MyObject> objectList
                    // it checks for duplicates so that if the fetched object was already in the back of the list, it is shown again
                    context.getMoreObjects(3); 
                    notifyDataSetChanged();
            }
        getMoreQuestions(currentPosition);



    case ViewPager.SCROLL_STATE_SETTLING:
        break;
    }

}

我的片段：

public class MyObjectFragment extends Fragment {

    // Object to represent
    private MyObject object;

    public static Fragment newInstance(MyActivity context, 
         int position, MyObject object) {

            MyObjectFragment frag = new MyObjectFragment();

            Bundle args = new Bundle();
            args.putParcelable("Object", object);
            frag.setArguments(args);

            return frag;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {

            this.currentInflater = inflater;

            final View rootView = inflater.inflate(
                    R.layout.fragment_my_object, container, false);

            // get object from Bundle, set UI, events, etc...

    }

    // (...)

}

知道为什么我会收到这个Exception 吗？似乎FragmentStatePagerAdapter 正在尝试销毁不再存在的项目，但我不明白为什么。

编辑 1：

如果我评论我的@Override getItemPosition(Object object)，我不会再收到异常。但是，我需要覆盖getItemPosition，因为在一个用例中，用户删除了当前显示的对象，导致它从适配器的数组中消失并强制getItemPosition返回POSITION_NONE，如果该项目不存在没有了。

编辑 2：

现在我知道只有当我从适配器的objectList 中删除项目时才会发生此异常。我有两种情况，从objectList 中删除MyObject 实例：

• 当getMoreObjects() 添加从数据库中获取一个已经在objectList 中的对象时，我将其删除并重新添加到列表的头部。我这样做是为了避免在 objectList 中有具有相同 ID 的对象，因为 getItemPosition() 使用它们的 ID 来了解它们是否存在以及它们的位置。

• 在返回之前，getMoreObjects() 从列表中删除前 N 个对象。我知道 FragmentStatePagerAdapter 已经通过只保留一些对象的内存片段来节省内存，但我仍然想避免我的objectList 增长太多。目前，我已将这行注释掉，因为它并不重要。

Answer 1

在this question 的帮助下解决了，它本身指向this issue。

FragmentStatePagerAdapter 将片段及其保存状态缓存在两个ArrayLists 中：mFragments 和mSavedState。但是当片段的顺序发生变化时（在我的情况下可能发生），没有重新排序mFragments 和mSavedState 的元素的机制。因此，适配器会向寻呼机提供错误的片段。

我已经修改了其中提供的代码，并将导入从 app.support.v4.Fragment 更改为 android.app.Fragment。

public abstract class MyFragmentStatePagerAdapter extends PagerAdapter {
    private static final String TAG = "FragmentStatePagerAdapter";
    private static final boolean DEBUG = true;

    private final FragmentManager mFragmentManager;
    private FragmentTransaction mCurTransaction = null;

    private long[] mItemIds = new long[] {};
    private ArrayList<Fragment.SavedState> mSavedState = new ArrayList<Fragment.SavedState>();
    private ArrayList<Fragment> mFragments = new ArrayList<Fragment>();
    private Fragment mCurrentPrimaryItem = null;

    public MyFragmentStatePagerAdapter(FragmentManager fm) {
        mFragmentManager = fm;
        mItemIds = new long[getCount()];
        for (int i = 0; i < mItemIds.length; i++) {
            mItemIds[i] = getItemId(i);
        }
    }

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

    /**
     * Return a unique identifier for the item at the given position.
     */
    public int getItemId(int position) {
        return position;
    }

    @Override
    public void notifyDataSetChanged() {
        long[] newItemIds = new long[getCount()];
        for (int i = 0; i < newItemIds.length; i++) {
            newItemIds[i] = getItemId(i);
        }

        if (!Arrays.equals(mItemIds, newItemIds)) {
            ArrayList<Fragment.SavedState> newSavedState = new ArrayList<Fragment.SavedState>();
            ArrayList<Fragment> newFragments = new ArrayList<Fragment>();

            for (int oldPosition = 0; oldPosition < mItemIds.length; oldPosition++) {
                int newPosition = POSITION_NONE;
                for (int i = 0; i < newItemIds.length; i++) {
                    if (mItemIds[oldPosition] == newItemIds[i]) {
                        newPosition = i;
                        break;
                    }
                }
                if (newPosition >= 0) {
                    if (oldPosition < mSavedState.size()) {
                        Fragment.SavedState savedState = mSavedState.get(oldPosition);
                        if (savedState != null) {
                            while (newSavedState.size() <= newPosition) {
                                newSavedState.add(null);
                            }
                            newSavedState.set(newPosition, savedState);
                        }
                    }
                    if (oldPosition < mFragments.size()) {
                        Fragment fragment = mFragments.get(oldPosition);
                        if (fragment != null) {
                            while (newFragments.size() <= newPosition) {
                                newFragments.add(null);
                            }
                            newFragments.set(newPosition, fragment);
                        }
                    }
                }
            }

            mItemIds = newItemIds;
            mSavedState = newSavedState;
            mFragments = newFragments;
        }

        super.notifyDataSetChanged();
    }

    @Override
    public void startUpdate(ViewGroup container) {
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        // If we already have this item instantiated, there is nothing
        // to do.  This can happen when we are restoring the entire pager
        // from its saved state, where the fragment manager has already
        // taken care of restoring the fragments we previously had instantiated.
        if (mFragments.size() > position) {
            Fragment f = mFragments.get(position);
            if (f != null) {
                return f;
            }
        }

        if (mCurTransaction == null) {
            mCurTransaction = mFragmentManager.beginTransaction();
        }

        Fragment fragment = getItem(position);
        if (DEBUG) Log.v(TAG, "Adding item #" + position + ": f=" + fragment);
        if (mSavedState.size() > position) {
            Fragment.SavedState fss = mSavedState.get(position);
            if (fss != null) {
                fragment.setInitialSavedState(fss);
            }
        }
        while (mFragments.size() <= position) {
            mFragments.add(null);
        }
        fragment.setMenuVisibility(false);
        mFragments.set(position, fragment);
        mCurTransaction.add(container.getId(), fragment);

        return fragment;
    }

    public void destroyItemState(int position) {
        mFragments.remove(position);
        mSavedState.remove(position);
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        Fragment fragment = (Fragment) object;

        if (mCurTransaction == null) {
            mCurTransaction = mFragmentManager.beginTransaction();
        }

        //position = getItemPosition(object);
        if (DEBUG) Log.v(TAG, "Removing item #" + position + ": f=" + object
                + " v=" + ((Fragment)object).getView());
        if (position >= 0) {
            while (mSavedState.size() <= position) {
                mSavedState.add(null);
            }
            mSavedState.set(position, mFragmentManager.saveFragmentInstanceState(fragment));
            if(position < mFragments.size()){
                mFragments.set(position, null);
            }
        }

        mCurTransaction.remove(fragment);
    }

    @Override
    public void setPrimaryItem(ViewGroup container, int position, Object object) {
        Fragment fragment = (Fragment)object;
        if (fragment != mCurrentPrimaryItem) {
            if (mCurrentPrimaryItem != null) {
                mCurrentPrimaryItem.setMenuVisibility(false);
            }
            if (fragment != null) {
                fragment.setMenuVisibility(true);
            }
            mCurrentPrimaryItem = fragment;
        }
    }

    @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() {
        Bundle state = new Bundle();
        if (mItemIds.length > 0) {
            state.putLongArray("itemids", mItemIds);
        }
        if (mSavedState.size() > 0) {
            Fragment.SavedState[] fss = new Fragment.SavedState[mSavedState.size()];
            mSavedState.toArray(fss);
            state.putParcelableArray("states", fss);
        }
        for (int i=0; i<mFragments.size(); i++) {
            Fragment f = mFragments.get(i);
            if (f != null) {
                String key = "f" + i;
                mFragmentManager.putFragment(state, key, f);
            }
        }
        return state;
    }

    @Override
    public void restoreState(Parcelable state, ClassLoader loader) {
        if (state != null) {
            Bundle bundle = (Bundle)state;
            bundle.setClassLoader(loader);
            mItemIds = bundle.getLongArray("itemids");
            if (mItemIds == null) {
                mItemIds = new long[] {};
            }
            Parcelable[] fss = bundle.getParcelableArray("states");
            mSavedState.clear();
            mFragments.clear();
            if (fss != null) {
                for (int i=0; i<fss.length; i++) {
                    mSavedState.add((Fragment.SavedState)fss[i]);
                }
            }
            Iterable<String> keys = bundle.keySet();
            for (String key: keys) {
                if (key.startsWith("f")) {
                    int index = Integer.parseInt(key.substring(1));
                    Fragment f = mFragmentManager.getFragment(bundle, key);
                    if (f != null) {
                        while (mFragments.size() <= index) {
                            mFragments.add(null);
                        }
                        f.setMenuVisibility(false);
                        mFragments.set(index, f);
                    } else {
                        Log.w(TAG, "Bad fragment at key " + key);
                    }
                }
            }
        }
    }
}

原始代码的功劳归用户@UgglyNoodle所有。

然后，我不使用FragmentStatePagerAdapter，而是使用上面的MyFragmentStatePagerAdapter，并与getItem()一致地覆盖getItemPosition()和getItemId()。