ViewPager和动态片段：re-instaciation提供NPE

Question

我在Android中遇到了ViewPager和ViewPagerAdapter的问题。我使用一个带有两个静态片段的viewpager（一个使用textEdit，第二个使用listview）。他们工作得很好。但我对第三个片段有一个问题，它是动态的。它使用相机，必须在一个场景下被实例化、销毁、重新实例化。因此，ViewPagerAdapter可以包含2或3个框架。当我重新安装第三个片段时，问题出现了，我在OnCreateView（）之后得到了一个NPE（主布局视图在这个方法之后是null，但在方法内部不是null）。

主要活动的代码如下：

    mViewPager = (ViewPager) findViewById(R.id.viewpager);
    setupViewPager();
    mTabLayout = (TabLayout) findViewById(R.id.tabs);
    mTabLayout.setupWithViewPager(mViewPager);

private void setupViewPager() {
    mViewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());
    mTabFragmentDocument = new TabFragmentDocument();
    mTabFragmentDocument.setApp(this);
    mTabFragmentText = new TabFragmentText();
    mTabFragmentText.setApp(this);

    mViewPagerAdapter.addFragment(mTabFragmentText, AbstractDefiner.TEXT);
    mViewPagerAdapter.addFragment(mTabFragmentDocument, AbstractDefiner.DOCUMENT);
    mViewPager.setAdapter(mViewPagerAdapter);
}

                mTabFragment = new TabFragment();
                mTabFragment .setApp(this);
                mViewPagerAdapter.addFragment(mTabFragment, "THIRD");
                mViewPagerAdapter.notifyDataSetChanged();
                new Handler().postDelayed(new Runnable() {
                    @Override
                    public void run() {
                        mViewPager.setCurrentItem(2);
                        mViewPagerAdapter.notifyDataSetChanged();
                        mTabFragment .setParams(tmp[1], tmp[2], tmp[3], tmp[4]);
                        mTabFragment .setupView();
                        mTabFragment .startWork();
                    }
                }, 1000);

                mViewPager.setCurrentItem(0);
                mViewPager.removeViewAt(2);
                mTabFragment .onDestroy();
                mViewPagerAdapter.remove(2);
                mViewPagerAdapter.notifyDataSetChanged();
                mTabFragment = null;

然后，适配器代码：

static class ViewPagerAdapter extends FragmentPagerAdapter {
    private final List<Fragment> mFragmentList = new ArrayList<>();
    private final List<String> mFragmentTitleList = new ArrayList<>();

    public ViewPagerAdapter(FragmentManager manager) {
        super(manager);
    }

    @Override
    public Fragment getItem(int position) {
        return mFragmentList.get(position);
    }

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

    public void addFragment(Fragment fragment, String title) {
        mFragmentList.add(fragment);
        mFragmentTitleList.add(title);
    }

    public void remove(int index) {
        mFragmentList.remove(index);
        mFragmentTitleList.remove(index);
    }

    @Override
    public int getItemPosition(Object object) {
        return PagerAdapter.POSITION_NONE;
    }


    @Override
    public CharSequence getPageTitle(int position) {
        return mFragmentTitleList.get(position);
    }
}

最后，第三个片段的代码：

public class TabFragment extends Fragment implements SurfaceHolder.Callback,
    GLSurfaceView.Renderer {

private static final String LOGCAT = "WEB_RTC_VISIO";

private HomeActivity mApp;
private String p1;
private String p2;
private String p3;
private String p4;

private VideoSource mLocalVideoSource;
private VideoRenderer.Callbacks mLocalRenderer;
private VideoRenderer.Callbacks mRemoteRenderer;
private GLSurfaceView mVideoView;
private SurfaceView mDrawView;
private SurfaceHolder mDrawHolder;
private ImageView mCursor;
private String mBgBytesString;
private ImageView mImgView;

private View mV;

public TabFragment() {
    // Required empty public constructor
}

public void setApp(HomeActivity app) {
    mApp = app;
}

public void setParams(String p1, String p2, String p3,
                      String p4) {
    this.p1= p1;
    this.p2= p2;
    this.p4= p4;
    this.p3= p3;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    mV = inflater.inflate(R.layout.third_layout, container, false);
    return mV;
}

@Override
public void onDetach() {
    super.onDetach();

}

public void setupView() {
    RequestUserPermission requestUserPermission = new RequestUserPermission(mApp);
    requestUserPermission.verifyStoragePermissions();
    mImgView = (ImageView) mV.findViewById(R.id.img_display); // NPE HERE
    TextView mTest1= (TextView) mV.findViewById(R.id.test1);
    mRequestLabelTextView.setText("test 1");
    TextView mTest2= (TextView) mV.findViewById(R.id.test2);
    mEquipmentSerialTextView.setText("test 2");
    // View that displays the view from the camera
    mVideoView = (GLSurfaceView) mV.findViewById(R.id.gl_surface);
    // View that displays the cursor and drawing associated
    mDrawView = (SurfaceView) mV.findViewById(R.id.draw_surface);
    mDrawHolder = mDrawView.getHolder();
    mDrawHolder.setFormat(PixelFormat.TRANSPARENT);
    mDrawHolder.addCallback(this);
    // Image of the cursor
    mCursor = (ImageView) mV.findViewById(R.id.mouseCursor);
    // Some more inits
}
public void startWork() {
//SOME WORK
}
}

所以，第一个实例是可以的，但在第二个实例中，我在获得ImageView时得到了NPE...有人能帮我理解这个问题吗？

提前感谢！

Answer 1

尝试使用FragmentStatePagerAdapter而不是FragmentPagerAdapter，它将解决许多问题。希望这对你也有用。