android.support.v4.app.Fragment.setUserVisibleHint null pointer on app resuming

Question

I am getting a crash on the resume of the app in the fragments code. I have never seen this crash myself but I have received crash reports back from users via TestFlight. I guess there is something that I am missing as the code works fine on most machines. Any help would be greatly appreciated.

Here is the call stack.

java.lang.NullPointerException
android.support.v4.app.Fragment.setUserVisibleHint in Fragment.java on Line 819
android.support.v4.app.FragmentPagerAdapter.setPrimaryItem in FragmentPagerAdapter.java on Line 130
android.support.v4.view.ViewPager.populate in ViewPager.java on Line 1066
android.support.v4.view.ViewPager.populate in ViewPager.java on Line 914
android.support.v4.view.ViewPager.onMeasure in ViewPager.java on Line 1436
android.view.View.measure in View.java on Line 15323
android.view.ViewGroup.measureChildWithMargins in ViewGroup.java on Line 4924
android.widget.LinearLayout.measureChildBeforeLayout in LinearLayout.java on Line 1421
android.widget.LinearLayout.measureVertical in LinearLayout.java on Line 698
android.widget.LinearLayout.onMeasure in LinearLayout.java on Line 579
android.view.View.measure in View.java on Line 15323
android.view.ViewGroup.measureChildWithMargins in ViewGroup.java on Line 4924
android.widget.FrameLayout.onMeasure in FrameLayout.java on Line 315
android.view.View.measure in View.java on Line 15323
android.support.v4.widget.DrawerLayout.onMeasure in DrawerLayout.java on Line 639
android.view.View.measure in View.java on Line 15323
android.view.ViewGroup.measureChildWithMargins in ViewGroup.java on Line 4924
android.widget.FrameLayout.onMeasure in FrameLayout.java on Line 315
android.view.View.measure in View.java on Line 15323
android.view.ViewGroup.measureChildWithMargins in ViewGroup.java on Line 4924
android.widget.LinearLayout.measureChildBeforeLayout in LinearLayout.java on Line 1421
android.widget.LinearLayout.measureVertical in LinearLayout.java on Line 698
android.widget.LinearLayout.onMeasure in LinearLayout.java on Line 579
android.view.View.measure in View.java on Line 15323
android.view.ViewGroup.measureChildWithMargins in ViewGroup.java on Line 4924
android.widget.FrameLayout.onMeasure in FrameLayout.java on Line 315
com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure in PhoneWindow.java on Line 2155
android.view.View.measure in View.java on Line 15323
android.view.ViewRootImpl.performMeasure in ViewRootImpl.java on Line 1854
android.view.ViewRootImpl.measureHierarchy in ViewRootImpl.java on Line 1102
android.view.ViewRootImpl.performTraversals in ViewRootImpl.java on Line 1275
android.view.ViewRootImpl.doTraversal in ViewRootImpl.java on Line 1000
android.view.ViewRootImpl$TraversalRunnable.run in ViewRootImpl.java on Line 4218
android.view.Choreographer$CallbackRecord.run in Choreographer.java on Line 725
android.view.Choreographer.doCallbacks in Choreographer.java on Line 555
android.view.Choreographer.doFrame in Choreographer.java on Line 525
android.view.Choreographer$FrameDisplayEventReceiver.run in Choreographer.java on Line 711
android.os.Handler.handleCallback in Handler.java on Line 615
android.os.Handler.dispatchMessage in Handler.java on Line 92
android.os.Looper.loop in Looper.java on Line 137
android.app.ActivityThread.main in ActivityThread.java on Line 4744
java.lang.reflect.Method.invokeNative(Native Method)
java.lang.reflect.Method.invoke in Method.java on Line 511
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run in ZygoteInit.java on Line 786
com.android.internal.os.ZygoteInit.main in ZygoteInit.java on Line 553
dalvik.system.NativeStart.main(Native Method)

First I set up the view pager in the OnResume function of the activity.

private void initialiseViewPager()
{
    mLoginFragment = new WeakReference<LoginFragment>(new LoginFragment());
    Bundle loginBundle = new Bundle();
    loginBundle.putInt("SpinnerIndex", HDMSLiveSession.getInstance().getSpinnerPosition());
    loginBundle.putString("UserName", HDMSLiveSession.getInstance().getUsername());
    loginBundle.putString("Password", HDMSLiveSession.getInstance().getPassword());
    loginBundle.putInt("Mode", HDMSLiveSession.getInstance().getConnectionMode().ordinal());
    loginBundle.putBoolean("LoggedIn", HDMSLiveSession.getInstance().isLoggedIn());
    loginBundle.putBoolean("Connected", HDMSLiveSession.getInstance().isConnected());
    loginBundle.putString("LoginResult", HDMSLiveSession.getInstance().getLoginResult());
    loginBundle.putString("System", HDMSLiveSession.getInstance().getSystem());
    loginBundle.putInt("code", HDMSLiveSession.getInstance().getAccessCode());
    loginBundle.putLong("bytesSent", mCurrentSB);
    loginBundle.putLong("bytesReceived", mCurrentRB);
    loginBundle.putLong("nbytesSent", mNCurrentSB);
    loginBundle.putLong("nbytesReceived", mNCurrentRB);
    loginBundle.putInt("appid", mApp.getApplicationInfo().uid);
    loginBundle.putString(mWebSocketAddressPreference, mConnect.getWebSocketURL());
    loginBundle.putString(mAPIAddressPreference, mLogin.getLiveServerURL());
    loginBundle.putBoolean(mAutoLoginPreference, mAutoLoginEnabled);
    loginBundle.putBoolean(mAutoConnectPreference, mLogin.isAutoConnectEnabled());
    loginBundle.putInt(mAutoReconnectTimePreference, mConnect.getAutoReconnectTime());
    loginBundle.putInt(mMaxAutoReconnectionAttemptsPreference, mConnect.getMaxAutoReconnectionAttempts());
    loginBundle.putInt(mPingResponseTimePreference, mConnect.getPingResponseTime());
    loginBundle.putInt(mAutoPingTimePreference, mConnect.getAutoPingTime());
    loginBundle.putInt(mCurrentPingPreference, mConnect.getCurrentPing());
    loginBundle.putInt(mAutoReconnectAttemptsPreference, mConnect.getAutoReconnectAttempts());
    loginBundle.putInt(mAutoReconnectTotalAttemptsPreference, mConnect.getAutoReconnectTotalAttempts());
    loginBundle.putBoolean(mPlayListMessagePreference, messageSubscriptionContains(mPlayListMessage));
    loginBundle.putBoolean(mAutoPageSwapPreference, mAutoPageSwap);
    loginBundle.putBoolean(mWifiCheckedPreference, mWifiChecked);
    loginBundle.putBoolean(mAutoWebCheckedPreference, mConnect.isAutoWebChecked());
    loginBundle.putBoolean(mGatewayCheckedPreference, mGatewayChecked);
    loginBundle.putBoolean(mDHS1CheckedPreference, mDNS1Checked);
    loginBundle.putBoolean(mHDMSLiveCheckedPreference, mHDMSLiveChecked);
    loginBundle.putBoolean(mGoogleCheckedPreference, mGoogleChecked);
    loginBundle.putBoolean(mHDMSCheckedPreference, mHDMSChecked);
    loginBundle.putBoolean(mParrotCheckedPreference, mParrotChecked);
    loginBundle.putBoolean(mLocalIPCheckedPreference, mLocalIPChecked);
    loginBundle.putString(mLocalIPPreference, mLocalIP);
    loginBundle.putInt(mMaxImagesFromWebPreference, mMaxImagesFromWeb);
    loginBundle.putInt(mMaxPingAttemptsPreference, mConnect.getMaxPingAttempts());
    loginBundle.putInt(mFailedPingsPreference, mConnect.getFailedPings());
    loginBundle.putBoolean("AutoLogin", mLogin.isAutoLogin());
    loginBundle.putBoolean("wasLoggedIn", HDMSLiveSession.getInstance().wasLoggedIn());
    mLoginFragment.get().setArguments(loginBundle);

    mBAUFragment = new WeakReference<BAUFragment>(new BAUFragment());
    Bundle bauBundle = new Bundle();
    bauBundle.putBoolean("jump", mJumpToCurrent);
    bauBundle.putInt("place", mBAUPosition);
    bauBundle.putBoolean(mBAUExpandedPreference, mBAUExpanded);
    mBAUFragment.get().setArguments(bauBundle);
    mPlayerFragment = new WeakReference<PlayerFragment>(new PlayerFragment());

    mListFragment = new WeakReference<ListFragment>(new ListFragment());
    Bundle listBundle = new Bundle();
    listBundle.putInt(mListModePreference, mListMode);
    mListFragment.get().setArguments(listBundle);

    mSearchFragment = new WeakReference<SearchFragment>(new SearchFragment());
    Bundle searchBundle = new Bundle();
    searchBundle.putInt(mSearchModePreference, mSearchMode);
    searchBundle.putString("searchQueryA", mSearchTextA);
    searchBundle.putString("searchQueryS", mSearchTextS);
    searchBundle.putString("titleLast", mTitleLast);
    searchBundle.putString("artistLast", mArtistLast);
    searchBundle.putString("listLast", mListLast);
    searchBundle.putString("yearLast", mYearLast);
    searchBundle.putString("genreLast", mGenreLast);
    mSearchFragment.get().setArguments(searchBundle);

    mVideoFragment = new WeakReference<VideoFragment>(new VideoFragment());
    Bundle videoBundle = new Bundle();
    videoBundle.putInt(mVideoOutputPreference, mVideoOutput);
    mVideoFragment.get().setArguments(videoBundle);

    List<Fragment> fragments = new Vector<Fragment>();
    fragments.add(mVideoFragment.get());
    fragments.add(mPlayerFragment.get());
    fragments.add(mBAUFragment.get());
    fragments.add(mListFragment.get());
    fragments.add(mSearchFragment.get());
    fragments.add(mLoginFragment.get());
    mPagerAdapter  = new ViewPagerAdapter(getSupportFragmentManager(), fragments);

    mViewPager = (ViewPager)findViewById(R.id.contentViewPager);
    mViewPager.setAdapter(mPagerAdapter);
    mViewPager.setOnPageChangeListener(this);
    mViewPager.setVisibility(View.VISIBLE);

    ActivityManager am = (ActivityManager) getSystemService(ACTIVITY_SERVICE);
    if (am.getMemoryClass() >= 32)
        mViewPager.setOffscreenPageLimit(Fragments.Max.ordinal());

    mLastFragment = -1;
    UpdateDisplay();
}

All fragments are retained unless it is a device with a small memory heap. Currently there are 6 fragments, the crash was occuring when there where only 4 in memory. So I know that the number of fragments isn't the problem. I did try leaving the View Pagers settings at the default, the only difference is speed as the app needs to load fragments in when the user swipes. All the fragments are destroyed when the app is paused.

@Override
protected void onSaveInstanceState(Bundle outState)
{
    FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
    if (mLoginFragment.get() != null)
        ft.remove(mLoginFragment.get());
    if (mPlayerFragment.get() != null)
        ft.remove(mPlayerFragment.get());
    if (mBAUFragment.get() != null)
        ft.remove(mBAUFragment.get());
    if (mListFragment.get() != null)
        ft.remove(mListFragment.get());
    if (mSearchFragment.get() != null)
        ft.remove(mSearchFragment.get());
    if (mVideoFragment.get() != null)
        ft.remove(mVideoFragment.get());
    ft.commit();

    mLoginFragment = new WeakReference<LoginFragment>(null);
    mPlayerFragment = new WeakReference<PlayerFragment>(null);
    mBAUFragment = new WeakReference<BAUFragment>(null);
    mListFragment = new WeakReference<ListFragment>(null);
    mSearchFragment = new WeakReference<SearchFragment>(null);
    mVideoFragment = new WeakReference<VideoFragment>(null);
    mPagerAdapter = null;
    mViewPager = null;

    mFragmentsLoaded = 0;
    mLastFragment = -1;
}

Here is my view pager code.

package com.hdms.manager.Fragments;

/**
 * Created by bradj on 8/10/13.
 *
 */
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.app.FragmentTransaction;
import android.view.View;

import java.util.List;

public class ViewPagerAdapter extends FragmentPagerAdapter
{
    private final List<Fragment> mFragments;
    FragmentManager mFragmentManager;

    public ViewPagerAdapter(FragmentManager aFragmentManager, List<Fragment> aFragments)
    {
        super(aFragmentManager);

        mFragmentManager = aFragmentManager;
        mFragments = aFragments;
    }

    @Override
    public Fragment getItem(int aPosition)
    {
        return mFragments.get(aPosition);
    }

    @Override
    public long getItemId(int aPosition)
    {
        return aPosition;
    }

    @Override
    public void destroyItem(android.view.ViewGroup aContainer, int aPosition, java.lang.Object aObject)
    {
        if (aPosition <= getCount() && aObject != null)
        {
            FragmentTransaction trans = mFragmentManager.beginTransaction();
            trans.remove((Fragment) aObject);
            trans.commit();
        }
    }

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

Answer 1

Finally! I'm now able to reliably recreate this error!

To recreate error, close activity/app, and quickly reopen page with fragment. You may have to try a few times because in my tests I had to reopen the app within about 30ms. This time may be slower or faster for different speed devices.

The problem was that I only explicitly created the Fragment (using new) once, and kept a reference to that instance so that I could reuse it. One simple fix to this problem is to always return a new instance of the Fragment the FragmentPagerAdapter.getItem(...), as shown below.

public class ViewPagerAdapter extends FragmentPagerAdapter {
    ...

    @Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0: return mMyFragment; // Error. Has the edge-case crash.
            case 1: return new MyFragment(); // Works.
            default: return new MyDefaultFragment();
        }
    }
}

For the OP's specific case, using the List<Fragment> to hold references is likely the same problem case as above.

ps - The root problem likely has something to do with the Fragment lifecycle and trying to use it again while it's being destroyed.

pps - Another way to recreate error is to quickly switch between enough tabs so that the Fragment wants to be destroyed to free some memory from cache, then quickly go back to it. By default, the FragmentPagerAdapter only caches one Fragment to the "left" and "right". So, depending on your cache limit, you will have to have at least three tabs to recreate the error this way.

ppps - This solution fixes the NullPointerException for android.app.Fragment.setUserVisibleHint(Fragment.java:997) and should also work for android.support.v4.app.Fragment.setUserVisibleHint.