在SherlockFragmentActivity中，FragmentManager getBackStackEntryCount（）始终为零

Question

我有一个SherlockFragmentActivity，它负责添加标签并向这些标签添加监听器。我按照以下方式添加标签：

public class HomeActivity extends SherlockFragmentActivity  {
    protected void onCreate(Bundle savedInstanceState) {
        setTheme(R.style.Theme_Styled);
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_home);
        // Set up the action bar.
        final ActionBar actionBar = getSupportActionBar();

        // Specify that we will be displaying tabs in the action bar.
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        // First add some tabs and give them explaining names
        ActionBar.Tab homeTab = getSupportActionBar()
                               .newTab()
                               .setIcon(R.drawable.home_unselected)
                               .setTabListener(new TabListener<SherlockFragment>(this, "home", HomeFragment.class));

ActionBar.Tab profileTab = getSupportActionBar()
                               .newTab()
                               .setIcon(R.drawable.profile_unselected)
                               .setTabListener(new TabListener<SherlockFragment>(this, "profile", ProfileFragment.class));

    getSupportActionBar().addTab(homeTab);
    getSupportActionBar().addTab(profileTab);
}

    public static class TabListener<T extends SherlockFragment> implements ActionBar.TabListener
   {
      private SherlockFragment mFragment;
      private final SherlockFragmentActivity mActivity;
      private final String mTag;
      private final Class<?> mClass;

      /** Constructor used each time a new tab is created.
       * @param activity  The host Activity, used to instantiate the fragment
       * @param tag  The identifier tag for the fragment
       * @param clz  The fragment's Class, used to instantiate the fragment
       */
      public TabListener(Activity activity, String tag, Class<?> clz)
      {
         mActivity = (SherlockFragmentActivity) activity;
         mTag = tag;
         mClass = clz;
      }

    @Override
    public void onTabSelected(com.actionbarsherlock.app.ActionBar.Tab tab,
            android.support.v4.app.FragmentTransaction ft) {
        // TODO Auto-generated method stub
         // Check if the fragment has already been initialised
         SherlockFragment  preInitializedFragment = (SherlockFragment) mActivity.getSupportFragmentManager().findFragmentByTag(mTag);

         int resID = mActivity.getApplicationContext().getResources().getIdentifier(mTag + "_selected" , "drawable", mActivity.getPackageName());
         Drawable image = mActivity.getApplicationContext().getResources().getDrawable(resID);

         tab.setIcon(image);


         if (mFragment != null)
         {

            // If it exists, simply attach it in order to show it
            ft.attach(mFragment);

         }
         else if (preInitializedFragment != null)
         {
            mFragment = preInitializedFragment;
            ft.attach(mFragment);
         }
         else
         {
            // Not found, so instantiate and add it to the activity
            mFragment = (SherlockFragment) SherlockFragment.instantiate(mActivity, mClass.getName());
            ft.add(android.R.id.content, mFragment, mTag);
         }
    }


    @Override
    public void onTabUnselected(
            com.actionbarsherlock.app.ActionBar.Tab tab,
            android.support.v4.app.FragmentTransaction ft) {
        // TODO Auto-generated method stub
         if (mFragment != null) {
                // Detach the fragment, because another one is being attached
                ft.detach(mFragment);



                int resID = mActivity.getApplicationContext().getResources().getIdentifier(mTag + "_unselected" , "drawable", mActivity.getPackageName());
                 Drawable image = mActivity.getApplicationContext().getResources().getDrawable(resID);

                tab.setIcon(image);

         }
    }


    @Override
    public void onTabReselected(
            com.actionbarsherlock.app.ActionBar.Tab tab,
            android.support.v4.app.FragmentTransaction ft) {
        // TODO Auto-generated method stub

    }

   }
}

我想要每个选项卡，如果选中该选项卡，该选项卡的片段将被推送到堆栈。因此，当我按下后退按钮时，我将返回上一个片段而不是活动。

所以我的猜测是@Override我的HomeActivity中的onBackPressed如下：

@Override
public void onBackPressed() {

    // initialize variables
    FragmentManager fm = getSupportFragmentManager();
    FragmentTransaction ft = fm.beginTransaction();

    // check to see if stack is empty
    if (fm.getBackStackEntryCount() > 0) {          
        fm.popBackStack();
        ft.commit();    
    }
    else {

        super.onBackPressed();
    }
    return;
}

但事实证明fm.getBackStackEntryCount()总是具有0的值。所以我的片段永远不会被添加到堆栈中。他们为什么不呢？我错过了什么。

编辑：

尚未完全解决，但我现在有以下代码：

@Override
        public void onTabSelected(com.actionbarsherlock.app.ActionBar.Tab tab,
                android.support.v4.app.FragmentTransaction ft) {
            // TODO Auto-generated method stub
             // Check if the fragment has already been initialised
             SherlockFragment  preInitializedFragment = (SherlockFragment) mActivity.getSupportFragmentManager().findFragmentByTag(mTag);

             int resID = mActivity.getApplicationContext().getResources().getIdentifier(mTag + "_selected" , "drawable", mActivity.getPackageName());
             Drawable image = mActivity.getApplicationContext().getResources().getDrawable(resID);

             tab.setIcon(image);

             FragmentTransaction transaction = mActivity.getSupportFragmentManager().beginTransaction();

             if (mFragment != null)
             {

                // If it exists, simply attach it in order to show it
                transaction.add(android.R.id.content, mFragment);
                transaction.addToBackStack(null);
                transaction.commit();

             }
             else if (preInitializedFragment != null)
             {
                mFragment = preInitializedFragment;
                transaction.attach(mFragment);
                transaction.addToBackStack(null);
                transaction.commit();
             }
             else
             {
                // Not found, so instantiate and add it to the activity
                mFragment = (SherlockFragment) SherlockFragment.instantiate(mActivity, mClass.getName());
                transaction.add(android.R.id.content, mFragment);
                transaction.addToBackStack(null);
                transaction.commit();
             }
        }

但是现在，如果触发了popBackStack()方法，则片段的内容将变为空。此选项卡的图标也会在最后选择的选项卡上保持选中状态。