android - 遍历Fragment返回栈时如何刷新ActionBar导航项？

Question

Android 4.1 ActionBar 提供了一种有用的导航模式，如列表或选项卡。我正在使用 SpinnerAdapter 从三个要显示在 View android.R.id.content 中的 fragment 中进行选择。onNavigationItemSelected() 监听器然后将每个 fragment 膨胀到 View ，并使用 FragmentTransaction.addToBackStack(null) 将其添加到返回堆栈。

这一切都像宣传的那样工作，但我不知道如何更新 ActionBar 以反射(reflect)当前的返回堆栈。使用 ActionBar.setSelectedNavigationItem(position) 可以工作，但也会触发一个新的 OnNavigationListener()，它还会创建另一个 FragmentTransaction(不是我想要的效果) .下面显示了代码以进行说明。对解决方案的任何帮助表示赞赏。

---

public class CalcActivity extends Activity {
private String[] mTag = {"calc", "timer", "usage"};
private ActionBar actionBar;

/** An array of strings to populate dropdown list */
String[] actions = new String[] {
    "Calculator",
    "Timer",
    "Usage"
};

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    this.actionBar = getActionBar();

    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_LIST);
    // may not have room for Title in actionbar
    actionBar.setDisplayOptions(0, ActionBar.DISPLAY_SHOW_TITLE);

    actionBar.setListNavigationCallbacks(
    // Specify a SpinnerAdapter to populate the dropdown list.
    new ArrayAdapter<String>(
        actionBar.getThemedContext(),
        android.R.layout.simple_list_item_1,
        android.R.id.text1,
        actions),
    // Provide a listener to be called when an item is selected.
    new NavigationListener()
    );      
}

public class NavigationListener implements ActionBar.OnNavigationListener {
    private Fragment mFragment;
    private int firstTime = 0;

    public boolean onNavigationItemSelected(int itemPos, long itemId) {
        mFragment = getFragmentManager().findFragmentByTag(mTag[itemPos]);

        if (mFragment == null) {
            switch (itemPos) {
            case 0:
                mFragment = new CalcFragment();
                break;
            case 1:
                mFragment = new TimerFragment();
                break;
            case 2:
                mFragment = new UsageFragment();
                break;
            default:
                return false;
            }               
            mFragment.setRetainInstance(true);
        }

        FragmentTransaction ft = getFragmentManager().beginTransaction();
        if (firstTime == 0) {
            firstTime++;
            ft.add(android.R.id.content, mFragment, mTag[itemPos]);
        } else {
            ft.replace(android.R.id.content, mFragment, mTag[itemPos]);
            ft.addToBackStack(null);
        }
        ft.commit();

        Toast.makeText(getBaseContext(), "You selected : " + 
                actions[itemPos], Toast.LENGTH_SHORT).show();

        return true;
    }       
}

public static class CalcFragment extends Fragment {             
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.fragment_calc, container, false);
    return v;
    }   
}

public static class TimerFragment extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.fragment_timer, container, false);
    return v;
    }   
}

public static class UsageFragment extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.fragment_usage, container, false);
    return v;
    }
}

Answer 1

你可以这样做:

1. 当您选择基于后退按钮的导航项时，创建一个 bool 值来跟踪:

   private boolean mListIsNavigatingBack = false;
   

2. 重写 onBackPressed，在重写中，检查 backstack 是否有项目，如果有请自行处理，如果没有则调用父类(super class):

   public void onBackPressed() {
       if(getFragmentManager().getBackStackEntryCount() > 0){
           mListIsNavigatingBack = true;
   
           //You need to get the previous index in the backstack through some means
           //possibly by storing it in a stack
           int previousNavigationItem = ???;
   
           getActionBar().setSelectedNavigationItem(previousNavigationItem);
       }
       else{
           super.onBackPressed();
       }
   }
   

3. 在 NavigationListener 内部，处理 mListIsNavigatingBack 状态，手动弹出返回堆栈并取消设置状态:

   if(mListIsNavigatingBack){
       if(fm.getBackStackEntryCount() > 0){
           fm.popBackStack();
       }
       mListIsNavigatingBack = false;
   }