android.support.v4.app.Fragment.setUserVisibleHint 应用程序恢复时的空指针

Question

我在片段代码中的应用程序恢复时遇到崩溃。我自己从未见过这种崩溃，但我通过 TestFlight 收到了用户反馈的崩溃报告。 我想我缺少一些东西，因为代码在大多数机器上都可以正常工作。任何帮助将不胜感激。

这是调用堆栈。

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)

首先我在activity的OnResume函数中设置了view pager。

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();
}

所有片段都被保留，除非它是具有小内存堆的设备。目前有 6 个碎片，当内存中只有 4 个时发生崩溃。所以我知道片段的数量不是问题。我确实尝试将 View Pagers 设置保留为默认值，唯一的区别是速度，因为应用程序需要在用户滑动时加载片段。当应用暂停时，所有的 Fragment 都会被销毁。

@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;
}

这是我的视图寻呼机代码。

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

终于！我现在能够可靠地重现此错误！

要重新创建错误，请关闭活动/应用程序，然后快速重新打开带有片段的页面。您可能需要尝试几次，因为在我的测试中，我必须在大约 30 毫秒内重新打开应用程序。对于不同速度的设备，此时间可能会更慢或更快。

问题是我只显式地创建了一次片段（使用new），并保留了对该实例的引用，以便我可以重用它。解决此问题的一个简单方法是始终返回 Fragment 的 new 实例 FragmentPagerAdapter.getItem(...)，如下所示。

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();
        }
    }
}

对于 OP 的具体情况，使用 List<Fragment> 保存引用可能与上述问题相同。

ps - 根本问题可能与 Fragment 生命周期有关，并在它被销毁时尝试再次使用它。

pps - 重新创建错误的另一种方法是在足够多的选项卡之间快速切换，以便片段想要被销毁以从缓存中释放一些内存，然后快速返回它。默认情况下，FragmentPagerAdapter 只缓存一个 Fragment 到“左”和“右”。因此，根据您的缓存限制，您必须至少拥有三个选项卡才能以这种方式重新创建错误。

ppps - 此解决方案修复了 android.app.Fragment.setUserVisibleHint(Fragment.java:997) 的 NullPointerException，并且也适用于 android.support.v4.app.Fragment.setUserVisibleHint。

Answer 2

我认为提供的信息不足以回答您的问题。我想查看您的代码中导致空指针的确切行。

无论如何，查看您的代码，我能猜到的唯一可能导致您出现问题的罪魁祸首是您传递 null 的 getItem()：

    @Override
public Fragment getItem(int aPosition)
{
    if (mFragments.size() > aPosition)
        return mFragments.get(aPosition);

    return null;
}

您应该能够只返回 mFragments.get(aPosition)，因为您已经覆盖了 getCount()。