Fragment addToBackStack() and popBackStackImmediat

问题:

I am currently building an application for Android (14 <= SDK <= 21) by using one ActionBarActivity and more Fragments, such as ListFragment and MapFragment, which are swapped within a single FrameLayout view.

The ActionBarActivity automatically replace/commit fragment A. Then, when the user tap a button, the hosting Activity replace/commit a new different fragment B. My goal is to let the user go back on fragment A as soon as she presses the back button.

Some code now.

MainActivity

public class MainActivity extends ActionBarActivity implements StopFragment.OnFragmentInteractionListener,
    StopItemFragment.OnFragmentInteractionListener {
...

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

        fragmentManager = getFragmentManager();
        fragmentManager.enableDebugLogging(true);
        ...
        if (fragmentManager.findFragmentById(R.id.content_frame) == null) {
            StopItemFragment list = StopItemFragment.newInstance(null); //A - extends ListFragment
            fragmentManager.beginTransaction()
                .replace(R.id.content_frame, list)
                .addToBackStack(null)
                .commit();
        }
        ...

        @Override
        public void onFragmentInteraction(String id) {
        selectItem(Integer.parseInt(id));
        }


       private void selectItem(int position) {
       StopFragment fragment = StopFragment.newInstance(null, null); //B - extends Fragment
       ...
       fragmentManager.beginTransaction()
            .replace(R.id.content_frame, fragment)
            .commit();

       ...
       }
}

Problem

Even if addToBackStack() is called, when I am on fragment B, I am not able to go back to fragment A. MainActivity is directly closed. Yet I tried to manage the back stack by myself with no luck. I can see that the fragment is on the stack but if I call popBackStackImmediate(), fragment A is popped out and the fragment transaction is not performed. (first back press nothing happen, second activity closed)

I attach also the FragmentManager logcat:
http://pastebin.com/hFLHprL8

回答1:

For those, who are still looking for solution.

In the main Activity class (which is hosting the fragments)just override onBackPressed().

@Override
public void onBackPressed() {
    if (getFragmentManager().getBackStackEntryCount() > 0 ){
        getFragmentManager().popBackStack();
    } else {
        super.onBackPressed();
    }
}

There is no onBackPressed() method in fragment, and this method is just for the activity. So,when we press the back key, the default behaviour of activity is shown, which is

you will either go to previous activity(if there is any) or the app will exit.

Now we need to override this method to tell the activity that when we press the back key, if there are any fragments in back stack, pop them out (and this is when the addToBackStack() comes into picture). Otherwise follow the default behaviour.

回答2:

Try this (Note add not replace for fragmentA, and addToBackStack() for fragmentB)

StopItemFragment list = StopItemFragment.newInstance(null); //A - extends ListFragment
        fragmentManager.beginTransaction()
            .add(R.id.content_frame, list)
            .commit();

and

StopFragment fragment = StopFragment.newInstance(null, null); //B - extends Fragment
   ...
   fragmentManager.beginTransaction()
        .replace(R.id.content_frame, fragment)
        .addToBackStack("FragmentB")
        .commit();

回答3:

I had to call beginTransaction() and commit() of FragmentManager manually.
Solved by overriding onBackPressed():

@Override
public void onBackPressed() {
    ...
    if (fragmentManager.getBackStackEntryCount() > 1){
        fragmentManager.popBackStackImmediate();
        fragmentManager.beginTransaction().commit();
    } else {
        //handle finish
        finish(); // Closes app
    }
}

回答4:

If you are Struggling with addToBackStack() & popBackStack() then simply use

FragmentTransaction ft =getSupportFragmentManager().beginTransaction();
ft.replace(R.id.content_frame, new HomeFragment(), "Home");
ft.commit();`

In your Activity In OnBackPressed() find out fargment by tag and then do your stuff

Fragment home = getSupportFragmentManager().findFragmentByTag("Home");

if (home instanceof HomeFragment && home.isVisible()) {
    // do you stuff
}

For more Information https://github.com/DattaHujare/NavigationDrawer I never use addToBackStack() for handling fragment.

回答5:

the reason why the popBackStack() and popBackStackImmediatly() is due to the fact that you didnt add that fragment (that you want to pop) to the backStack. In order to do this you have to make a call to addToBackStack() at the moment of making the transaction to add/replace your fragment.