Android:使用不断变化的 fragment 改变菜单项的可见性

Question

每当我更改 Activity 中的 fragment 时，我都想更改 fragment Activity (abs) 菜单项的可见性。这些 fragment 是 SherlockListFragments。

我想显示/隐藏的菜单项是我在创建菜单时创建的微调器:

public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    MenuInflater inflater = getSupportMenuInflater();

    IcsSpinner herbSortSpinner = new IcsSpinner(this, null, R.attr.actionDropDownStyle);
    SpinnerAdapter herbSortSpinnerAdapter = ArrayAdapter.createFromResource(this, R.array.herb_sort_elements, android.R.layout.simple_spinner_dropdown_item);
    herbSortSpinner.setAdapter(herbSortSpinnerAdapter);
    herbSortSpinner.setOnItemSelectedListener(this);
    herbSortSpinner.setId(HERB_SPINNER_ID);
    menu.add(R.string.menu_sort).setActionView(herbSortSpinner).setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM);

    IcsSpinner noteSortSpinner = new IcsSpinner(this,  null, R.attr.actionDropDownStyle);
    SpinnerAdapter noteSortSpinnerAdapter = ArrayAdapter.createFromResource(this, R.array.note_sort_elements, android.R.layout.simple_spinner_dropdown_item);
    noteSortSpinner.setAdapter(noteSortSpinnerAdapter);
    noteSortSpinner.setOnItemSelectedListener(this);
    noteSortSpinner.setId(NOTE_SPINNER_ID);
    menu.add(R.string.menu_sort).setActionView(noteSortSpinner).setVisible(false).setEnabled(false).setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM);

    inflater.inflate(R.menu.activity_lexicon, menu);
    menuHolder.setMenu(menu);
    return true;
}

我切换 fragment 的逻辑是:

public boolean onNavigationItemSelected(int position, long itemId) {

    FragmentManager fragmentManager = getSupportFragmentManager();
    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
    switch(position) {
    case 0:
        List<Herb> herbList = findHerbsByIntent(getHerbLocale());
        HerbListFragment herbListFragment = new HerbListFragment();
        herbListFragment.setListAdapter(new HerbListAdapter(this, getHerbLocale(), herbList));
        fragmentTransaction.replace(R.id.fragment_container, herbListFragment, HERB_LIST_FRAGMENT_TAG);
        //menuHolder.showHerbsSpinner();
        break;
    case 1:
        SymptomListFragment symptomListFragment = new SymptomListFragment();
        symptomListFragment.setListAdapter(new SymptomListAdapter(this, getDefaultSymptomLocale()));
        fragmentTransaction.replace(R.id.fragment_container, symptomListFragment, SYMPTOM_LIST_FRAGMENT_TAG);
        //menuHolder.showHerbsSpinner();
        break;
    case 2:
        NoteRepository noteRepository = new NoteRepository(this);
        List<Note> notes = noteRepository.getNoticables(ReferedType.HERB);
        NoteListFragment noteListFragment = new NoteListFragment();
        noteListFragment.setListAdapter(new NoteListAdapter(this, getHerbLocale(), notes));
        fragmentTransaction.replace(R.id.fragment_container, noteListFragment, NOTE_LIST_FRAGMENT_TAG);
        //menuHolder.showNotesSpinner();
        break;
    case 3:
        FavoriteRepository favoriteRepository = new FavoriteRepository(this);
        Set<Integer> favoriteHerbs = favoriteRepository.getFavorables(ReferedType.HERB);
        List<Herb> favoriteHerbList = herbRepository.getHerbsByIds(favoriteHerbs, getHerbLocale());
        FavoriteHerbListFragment favoriteHerbListFragment = new FavoriteHerbListFragment();
        favoriteHerbListFragment.setListAdapter(new HerbListAdapter(this, getHerbLocale(), favoriteHerbList));
        fragmentTransaction.replace(R.id.fragment_container, favoriteHerbListFragment, HERB_LIST_FRAGMENT_TAG);
        //menuHolder.showHerbsSpinner();
        break;
    }
    fragmentTransaction.commit();

    return true;
}

我的第一个想法是将菜单对象保存在 holder 类中，并在我切换 fragment 时在那里操作它(在上面的每个 case 语句中)。

static class MenuHolder {

    private Menu mMenu;

    void setMenu(Menu menu) {
        mMenu = menu;
    }

    void showNotesSpinner() {
        if (mMenu != null) {
            mMenu.findItem(HERB_SPINNER_ID).setVisible(false).setEnabled(false);
            mMenu.findItem(NOTE_SPINNER_ID).setVisible(true).setEnabled(true);
        }
    }

    void showHerbsSpinner() {
        if (mMenu != null) {
            mMenu.findItem(NOTE_SPINNER_ID).setVisible(false).setEnabled(false);            
            mMenu.findItem(HERB_SPINNER_ID).setVisible(true).setEnabled(true);
        }
    }
}

我的问题是没有具有给定 ID 的菜单项，它们是 Activity 本地常量。我在这里得到了一个 NPE。有人知道我该如何解决吗？有没有更好的方法来更改切换 fragment 的菜单？

最好的问候卡斯滕

Answer 1

Is there a better way to change the menu on switching fragments?

可能是 :)。

• 在您的 fragment onCreate 中，添加:

  setHasOptionsMenu (true);

文档:

Report that this fragment would like to participate in populating the options menu by receiving a call to onCreateOptionsMenu(Menu, MenuInflater) and related methods.

• 覆盖 onPrepareOptionMenu fragment 类中的方法。

Prepare the Screen's standard options menu to be displayed. This is called right before the menu is shown, every time it is shown. You can use this method to efficiently enable/disable items or otherwise dynamically modify the contents.

然后，在这个方法中，尝试通过 ID 找到您的菜单项，并使它们可见/不可见，启用/禁用，如下所示:

@Override
public void onPrepareOptionsMenu(Menu menu) {    
    menu.findItem(HERB_SPINNER_ID).setVisible(false).setEnabled(false);
}

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