具有ViewPager的活动，该活动显示适配器中对象列表的片段（FragmentStatePagerAdapter）。
最初，Activity将SQL（从5个）对象从SQLite DB加载到适配器中。这些对象的选择具有一定的随机性。
当用户到达列表末尾时，活动应从DB中加载M（让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存在新对象，如果已存在，则会删除预先存在的对象，并将已加载的对象添加到列表的尾部。
因此，我试图实现类似无限滚动ViewPager （不是“循环”ViewPager，因为我希望不断提取新对象，而不是回到乞讨列表）。
我有一些工作代码，并包含了我正在跟踪的模式的示例。但是，我不断得到这个例外，并且不知道为什么：
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)
...
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...
}
// (...)
}
我知道为什么会得到这个Exception？似乎FragmentStatePagerAdapter试图破坏不再存在的项目，但我不明白为什么。
编辑1：
如果我评论我的@Override getItemPosition(Object object)，我就不会再得到例外了。但是，我需要覆盖getItemPosition，因为有一个用例，用户在其中删除当前显示的Object，导致它从适配器的数组中消失，并强制getItemPosition返回POSITION_NONE该项目不再存在。
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MyObject删除objectList个实例：
当getMoreObjects()添加从objectList中的数据库中提取对象时，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中拥有具有相同Id的对象，因为getItemPosition()使用它们的ID来了解它们是否存在及其位置。
在返回getMoreObjects()之前，从列表中删除N个第一个对象。我知道FragmentStatePagerAdapter已经通过仅保留一些对象的内存片段来节省内存，但我仍然希望避免过多地增加objectList。现在，我对这一行进行了评论，因为它并不重要。
答案 0 :(得分：3)
在this question的帮助下解决，this issue本身指向@UgglyNoodle。
FragmentStatePagerAdapter将片段及其保存的状态缓存在两个ArrayLists：mFragments和mSavedState中。但是当片段的顺序发生变化时（在我的情况下可能发生），没有机制可以重新排序mFragments和mSavedState的元素。因此，适配器将向寻呼机提供错误的片段。
我已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);
}
}
}
}
}
}
原始代码的信用额转到用户{{3}}。
然后，我使用FragmentStatePagerAdapter而不是MyFragmentStatePagerAdapter使用上面的getItemPosition()并与getItemId()一致地覆盖getItem()和{{1}}。