421_FragmentNoLongerExists错误

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FragmentNoLongerExists错误

根本原因在getFragment方法,
所以从setAdapter方法入手,

看setAdapter源码

public void setAdapter(PagerAdapter adapter) {
if (mAdapter != null) {
mAdapter.setViewPagerObserver(null);
mAdapter.startUpdate(this);
for (int i = 0; i < mItems.size(); i++) {
final ItemInfo ii = mItems.get(i);
mAdapter.destroyItem(this, ii.position, ii.object);
}
mAdapter.finishUpdate(this);
mItems.clear();
removeNonDecorViews();
mCurItem = 0;
scrollTo(0, 0);
}

final PagerAdapter oldAdapter = mAdapter;
mAdapter = adapter;
mExpectedAdapterCount = 0;

if (mAdapter != null) {
if (mObserver == null) {
mObserver = new PagerObserver();
}
mAdapter.setViewPagerObserver(mObserver);
mPopulatePending = false;
final boolean wasFirstLayout = mFirstLayout;
mFirstLayout = true;
mExpectedAdapterCount = mAdapter.getCount();
if (mRestoredCurItem >= 0) {
mAdapter.restoreState(mRestoredAdapterState, mRestoredClassLoader);
setCurrentItemInternal(mRestoredCurItem, false, true);
mRestoredCurItem = -1;
mRestoredAdapterState = null;
mRestoredClassLoader = null;
} else if (!wasFirstLayout) {
populate();
} else {
requestLayout();
}
}

if (mAdapterChangeListener != null && oldAdapter != adapter) {
mAdapterChangeListener.onAdapterChanged(oldAdapter, adapter);
}
}

问题出在

mAdapter.restoreState(mRestoredAdapterState, mRestoredClassLoader);

看看

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

再看看FragmentStatePagerAdapter的restoreState

@Override
public void restoreState(Parcelable state, ClassLoader loader) {
if (state != null) {
Bundle bundle = (Bundle)state;
bundle.setClassLoader(loader);
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);
}
}
}
}
}

看看getFragment方法

@Override
public Fragment getFragment(Bundle bundle, String key) {
int index = bundle.getInt(key, -1);
if (index == -1) {
return null;
}
if (index >= mActive.size()) {
throwException(new IllegalStateException("Fragment no longer exists for key "
+ key + ": index " + index));
}
Fragment f = mActive.get(index);
if (f == null) {
throwException(new IllegalStateException("Fragment no longer exists for key "
+ key + ": index " + index));
}
return f;
}

从bundle中可以取出来，但是有两种情况会抛出此异常
1.没有存活的Fragment
2.获取到的Fragemnt为空

已经设置过mAdapter才会触发saveState()，接着来看下FragmentStatePagerAdapter.saveState()的源码

@Override
public Parcelable saveState() {
Bundle state = null;
if (mSavedState.size() > 0) {
state = new Bundle();
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 && f.isAdded()) {
if (state == null) {
state = new Bundle();
}
String key = "f" + i;
mFragmentManager.putFragment(state, key, f);
}
}
return state;
}

由此找到了Bundle的根源，网上解决此BUG的一种解法就是覆写此函数返回为空，这样FragmentManager.getFragment函数中就不满足第一个判断条件，不会执行后续代码也不会抛出异常了

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