安卓。片段不保存其状态

Question

我正在尝试创建自定义标签导航。单击每个选项卡时，它将替换屏幕中的片段。在 MainActivity.java 上，我正在为每个片段创建一个副本（我认为，通过这种方式，我可以在任何地方使用每个片段的一个副本）。

    public static WhereRootFragment whereRootFragment;
    public static FeedRootFragment feedRootFragment;

    public static WhereMainFragment whereMainFragment;
    public static FeedMainFragment feedMainFragment;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
.....
        whereRootFragment=new WhereRootFragment();
        feedRootFragment=new FeedRootFragment();

        whereMainFragment=new WhereMainFragment();
        feedMainFragment=new FeedMainFragment();

        ClickHandler.handleClick(MainActivity.this, llWhere);
}

当标签按钮被点击时，它用当前片段替换旧片段（handleClick方法）。

        switch (tag) {
        case TAG_WHERE:
            MainActivity.ivWhere.setImageDrawable(a.getResources().getDrawable(
                    R.drawable.where_black));
//          Fragment fragment_where_root=new WhereRootFragment();
            FragmentTransaction transaction_where = ((FragmentActivity) a).getSupportFragmentManager().beginTransaction();
            transaction_where.replace(R.id.root_frame, MainActivity.whereRootFragment);
            transaction_where.addToBackStack(null);
            transaction_where.commit();
            break;
        case TAG_FEED:
            MainActivity.ivFeed.setImageDrawable(a.getResources().getDrawable(
                    R.drawable.feed_black));
//          Fragment fragment_feed_root=new FeedRootFragment();
            FragmentTransaction transaction_feed = ((FragmentActivity) a).getSupportFragmentManager().beginTransaction();
            transaction_feed.replace(R.id.root_frame,  MainActivity.feedRootFragment);
            transaction_feed.addToBackStack(null);
            transaction_feed.commit();
            break;

WhereRootFragment：

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

    FragmentTransaction transaction = getFragmentManager()
            .beginTransaction();
    transaction.replace(R.id.where_root_frame, MainActivity.whereMainFragment);
    transaction.addToBackStack(null);
    transaction.commit();

    return view;
}

WhereMainFragment

@Override
public View onCreateView(LayoutInflater inflater,
        @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_where_main,
            container, false);
    return rootView;
}

结果：

[StartApp]->[1.Where:showing]->[2.Feed:showing]->[3.Where:不显示]->[4.Feed:显示保存状态]->[ 5.另一个片段：应该是空的]->[6.Feed：不显示]

问题：如何解决这个问题？如何让fragment保存状态？

Answer 1

WhereRootFragment 由属于活动的片段管理器管理。问题正在发生，因为相同的片段管理器被用于管理WhereMainFragment，它是WhereRootFragment 的子级。

WhereMainFragment 应该由属于WhereRootFragment 的片段管理器管理。要在片段中获取子片段管理器，请使用Fragment.getChildFragmentManager。

WhereRootFragment

public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    /* Inflate the layout for this fragment */
    View view = inflater.inflate(R.layout.fragment_where_root, container, false);

    // Use child fragment manager to manage WhereMainFragment
    FragmentTransaction transaction = getChildFragmentManager()
            .beginTransaction();
    transaction.replace(R.id.where_root_frame, MainActivity.whereMainFragment);
    transaction.addToBackStack(null);
    transaction.commit();

    return view;
}

Answer 2

有些链接可能对您有用：

-> 你可能必须使用FragmentTabHost | Android Developers

->Creating-and-Using-Fragments

->你可能想要这个Android Fragments

-> 最好的Android Beginner

Stackoverflow Answer says:

public final class MyFragment extends Fragment {
    private TextView vstup;
    private Bundle savedState = null;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.whatever, null);
        vstup = (TextView)v.findViewById(R.id.whatever);

        /* (...) */

        /* If the Fragment was destroyed inbetween (screen rotation), we need to recover the savedState first */
        /* However, if it was not, it stays in the instance from the last onDestroyView() and we don't want to overwrite it */
        if(savedInstanceState != null && savedState == null)
            savedState = savedInstanceState.getBundle(App.STAV);
        if(savedState != null)
            vstup.setText(savedState.getCharSequence(App.VSTUP));
        savedState = null;

        return v;
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        savedState = saveState(); /* vstup defined here for sure */
        vstup = null;
    }

    private Bundle saveState() { /* called either from onDestroyView() or onSaveInstanceState() */
        Bundle state = new Bundle();
        state.putCharSequence(App.VSTUP, vstup.getText());
        return state;
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        /* If onDestroyView() is called first, we can use the previously savedState but we can't call saveState() anymore */
        /* If onSaveInstanceState() is called first, we don't have savedState, so we need to call saveState() */
        /* => (?:) operator inevitable! */
        outState.putBundle(App.STAV, savedState != null ? savedState : saveState());
    }

    /* (...) */

}