在popBackStack之后,ViewPager中的片段未恢复

Question

问题

从另一个片段返回后,片段不会重新附加到其宿主ViewPager.

情况

一个托管片段的活动,其布局包含ViewPager(PageListFragment在下面的示例中).ViewPager由FragmentStateViewPagerAdapter填充.托管内部托管的单个片段(PageFragment在下面的示例中)可以打开包含一组新页面的子页面列表.

行为

只要没有按下后退按钮,一切正常.一旦用户关闭其中一个子页面列表,就会重新创建先前的列表,但不会显示先前显示的页面.在父页面列表中的其他页面上滑动仍然有效.

码

可以在github上找到示例应用程序:

活动

public class MainActivity extends FragmentActivity {

private static final String CURRENT_FRAGMENT = MainActivity.class.getCanonicalName() + ".CURRENT_FRAGMENT";

public static final String ARG_PARENTS = "Parents";

public void goInto(String mHostingLevel, String mPosition) {
    Fragment hostingFragment = newHostingFragment(mHostingLevel, mPosition);
    addFragment(hostingFragment);
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    addBaseFragment();
}

private void addBaseFragment() {
    Fragment hostingFragment = newHostingFragment("", "");
    addFragment(hostingFragment);
}

private Fragment newHostingFragment(String mHostingLevel, String oldPosition) {
    Fragment hostingFragment = new PageListFragment();
    Bundle args = new Bundle();
    args.putString(ARG_PARENTS, mHostingLevel + oldPosition +" > ");
    hostingFragment.setArguments(args);
    return hostingFragment;
}

private void addFragment(Fragment hostingFragment) {
    FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
    transaction.replace(R.id.fragmentSpace, hostingFragment, CURRENT_FRAGMENT);
    transaction.addToBackStack(null);
    transaction.commit();
}

}

PageListFragment

public class PageListFragment extends Fragment {

private String mParentString;

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

@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
    return inflater.inflate(R.layout.fragment_hosting, container, false);
}

@Override
public void onResume() {
    mParentString = getArguments().getString(MainActivity.ARG_PARENTS);
    ViewPager viewPager = (ViewPager) getView().findViewById(R.id.viewPager);
    viewPager.setAdapter(new SimpleFragmentStatePagerAdapter(getFragmentManager(),mParentString));
    super.onResume();
}

private static class SimpleFragmentStatePagerAdapter extends FragmentStatePagerAdapter {

    private String mHostingLevel;

    public SimpleFragmentStatePagerAdapter(FragmentManager fm, String hostingLevel) {
        super(fm);
        this.mHostingLevel = hostingLevel;
    }

    @Override
    public android.support.v4.app.Fragment getItem(int position) {
        PageFragment pageFragment = new PageFragment();
        Bundle args = new Bundle();
        args.putString(MainActivity.ARG_PARENTS, mHostingLevel);
        args.putInt(PageFragment.ARG_POSITION, position);
        pageFragment.setArguments(args);
        return pageFragment;
    }

    @Override
    public int getCount() {
        return 5;
    }
}
}

PageFragment

public class PageFragment extends Fragment {

public static final String ARG_POSITION = "Position";

private String mHostingLevel;
private int mPosition;

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

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View contentView = inflater.inflate(R.layout.fragment_page, container, false);
    setupTextView(contentView);
    setupButton(contentView);
    return contentView;
}

private void setupTextView(View contentView) {
    mPosition = getArguments().getInt(ARG_POSITION);
    mHostingLevel = getArguments().getString(MainActivity.ARG_PARENTS);
    TextView text = (TextView) contentView.findViewById(R.id.textView);
    text.setText("Parent Fragments " + mHostingLevel + " \n\nCurrent Fragment "+ mPosition);
}

private void setupButton(View contentView) {
    Button button = (Button) contentView.findViewById(R.id.button);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            openNewLevel();
        }
    });
}

protected void openNewLevel() {
    MainActivity activity = (MainActivity) getActivity();
    activity.goInto(mHostingLevel, Integer.toString(mPosition));
}

}

Answer 1

经过长时间的调查,结果证明片段管理器存在问题.

当使用像上面那样的构造时,片段事务将片段重新附加到页面列表时会被静默丢弃.它基本上是导致问题的同一个问题

java.lang.IllegalStateException: Recursive entry to executePendingTransactions 

当试图改变FragmentPager内的片段时.

对于此错误的问题,同样的解决方案也适用于此处.构造FragmentStatePagerAdapter时,提供正确的子片段管理器.

代替

    viewPager.setAdapter(new SimpleFragmentStatePagerAdapter(getFragmentManager(),mParentString));

做

    viewPager.setAdapter(new SimpleFragmentStatePagerAdapter(getChildFragmentManager(),mParentString));

另见:github

Answer 2

保罗没有提到的是,如果你使用getChildFragmentManager,那么你将遭遇"背压空白屏幕"问题.