NullPointerException android.support.v4.app.FragmentManagerImpl.saveFragmentBasicState

fragment 加 viewpager做tab的时候容易出这个空指针异常，看源码：

if (f.mSavedViewState != null) {    if (result == null) {        result = new Bundle();    }    result.putSparseParcelableArray(            FragmentManagerImpl.VIEW_STATE_TAG, f.mSavedViewState);}if (!f.mUserVisibleHint) {    if (result == null) {        result = new Bundle();    }    // Only add this if it's not the default value    result.putBoolean(FragmentManagerImpl.USER_VISIBLE_HINT_TAG, f.mUserVisibleHint);}

不保存状态会进入第二个if抛空指针异常。

在fragment中加入如下代码，bug可解：

@Override public void onSaveInstanceState(Bundle outState) {//first saving my state, so the bundle wont be empty.outState.putString("KEY",  "WBUG_VALUE");super.onSaveInstanceState(outState);}

或者：

class MyFragment extends Fragment {    @Override    public void onSaveInstanceState(Bundle outState) {        super.onSaveInstanceState(outState);        setUserVisibleHint(true);    }

或者使用FragmentStatePagerAdapter替代 FragmentPageAdapter。