为什么我在RecyclerView上获得空引用

Question

我正在尝试在片段中使用RecyclerView。对于第一个选项卡，它显示得很好，但是当我滑动到第二个选项卡然后又回到第一个选项卡时，出现以下错误：

java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“ void
android.support.v7.widget.RecyclerView $ LayoutManager.stopSmoothScroller（）”

有谁知道为什么我会收到此错误？在片段切换之前，似乎有一个电话丢失了，但我无法弄清楚。

片段的寻呼机适配器：

public class PagerAdapter extends FragmentPagerAdapter {

    private final String[] TITLES = {"Header 0", "Header 1" };

    public PagerAdapter(FragmentManager fm){
        super(fm);
    }

    @Override
    public CharSequence getPageTitle(int position){
        return TITLES[position];
    }

    @Override
    public int getCount() {
        return TITLES.length;
    }

    @Override
    public Fragment getItem(int position){
        switch(position){
            case 0:
                return new FragmentOne();
            case 1:
                return new FragmentTwo();
            default:
                return new FragmentOne();
        }
    }
}

FragmentOne和FragmentTwo使用我创建的样板类：

public class FragmentOne extends Base {
    ... code for displaying content in the RecyclerView. Just contains my custom Adapter for the RecyclerView ...
}

基础：

public abstract class Base extends Fragment {

    public View mView;
    public RecyclerView mDataView;
    public ProgressBar mProgressBar;
    public Context mContext;
    private RecyclerView.LayoutManager mLayoutManager;

    public Base() { }

    @Override
    public View onCreateView(LayoutInflater _i, ViewGroup _vg, Bundle savedInstanceBundle){
        mView = _i.inflate(R.layout.f_base, null);
        mDataView = (RecyclerView) mView.findViewById(R.id.data);
        mProgressBar = (ProgressBar)mView.findViewById(R.id.loading);
        mLayoutManager = new LinearLayoutManager(mContext);
        mDataView.setLayoutManager(mLayoutManager);
        mContext = this.getActivity();
        return mView;
    }

    @Override
    public void onStart() {
        super.onStart();
        init();
        doWork();
    }

    public abstract void init();

    public abstract void doWork();

}

编辑

错误堆栈跟踪：

11-02 12:49:44.171    3708-3708/com.nitrox.digitune E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.nitrox.digitune, PID: 3708
    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView$LayoutManager.stopSmoothScroller()' on a null object reference
            at android.support.v7.widget.RecyclerView.stopScrollersInternal(RecyclerView.java:1159)
            at android.support.v7.widget.RecyclerView.stopScroll(RecyclerView.java:1151)
            at android.support.v7.widget.RecyclerView.onDetachedFromWindow(RecyclerView.java:1356)
            at android.view.View.dispatchDetachedFromWindow(View.java:13441)
            at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2837)
            at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2834)
            at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2834)
            at android.view.ViewGroup.removeViewInternal(ViewGroup.java:4163)
            at android.view.ViewGroup.removeViewInternal(ViewGroup.java:4136)
            at android.view.ViewGroup.removeView(ViewGroup.java:4068)
            at android.support.v4.view.ViewPager.removeView(ViewPager.java:1326)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1045)
            at android.support.v4.app.FragmentManagerImpl.detachFragment(FragmentManager.java:1280)
            at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:724)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1489)
            at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:486)
            at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:1073)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:919)
            at android.support.v4.view.ViewPager$3.run(ViewPager.java:249)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
            at android.view.Choreographer.doCallbacks(Choreographer.java:580)
            at android.view.Choreographer.doFrame(Choreographer.java:549)
            at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
            at android.os.Handler.handleCallback(Handler.java:739)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

Answer 1

基本上，在回收者完成之前要先处理LayoutManager。

从Android来源：

@Override
protected void onDetachedFromWindow() {
    super.onDetachedFromWindow();
    if (mItemAnimator != null) {
        mItemAnimator.endAnimations();
    }
    mFirstLayoutComplete = false;
    stopScroll();
    mIsAttached = false;
    if (mLayout != null) {
        mLayout.onDetachedFromWindow(this, mRecycler);
    }
    removeCallbacks(mItemAnimatorRunner);
}

问题出在stopScroll尝试调用mLayout.stopSmoothScroller（）;时。而不检查mLayout是否为null。

我为正在开发的应用程序添加了一个非常骇人的补丁，但我不建议将其用于长期解决方案，因为这是一个很大的难题。如果像我一样，您的截止日期很紧，那么最好只是捕获空指针异常并忽略它。

我的修补程序只是创建一个扩展RecyclerView的自定义视图：

import android.content.Context;
import android.support.v7.widget.RecyclerView;
import android.util.AttributeSet;

public class HotFixRecyclerView extends RecyclerView
{
    public HotFixRecyclerView( Context context )
    {
        super( context );
    }

    public HotFixRecyclerView( Context context, AttributeSet attrs )
    {
        super( context, attrs );
    }

    public HotFixRecyclerView( Context context, AttributeSet attrs, int defStyle )
    {
        super( context, attrs, defStyle );
    }

    @Override
    public void stopScroll()
    {
        try
        {
            super.stopScroll();
        }
        catch( NullPointerException exception )
        {
            /**
             *  The mLayout has been disposed of before the 
             *  RecyclerView and this stops the application 
             *  from crashing.
             */
        }
    }
}

然后将所有引用从RecyclerView更改为HotFixRecyclerView。如果您确实使用过，请在Android修复此问题后将其删除，因为这有点hack。

• 如果您使用recylerview inn XML，请不要忘记相应地更改XML文件以使用com.your.package.HotFixRecyclerView而不是android.support.v7.widget.RecyclerView