[android] Fragments onResume from back stack

I'm using the compatibility package to use Fragments with Android 2.2. When using fragments, and adding transitions between them to the backstack, I'd like to achieve the same behavior of onResume of an activity, i.e., whenever a fragment is brought to "foreground" (visible to the user) after poping out of the backstack, I'd like some kind of callback to be activated within the fragment (to perform certain changes on a shared UI resource, for instance).

I saw that there is no built in callback within the fragment framework. is there s a good practice in order to achieve this?

The answer is


I've changed the suggested solution a little bit. Works better for me like that:

private OnBackStackChangedListener getListener() {
    OnBackStackChangedListener result = new OnBackStackChangedListener() {
        public void onBackStackChanged() {
            FragmentManager manager = getSupportFragmentManager();
            if (manager != null) {
                int backStackEntryCount = manager.getBackStackEntryCount();
                if (backStackEntryCount == 0) {
                    finish();
                }
                Fragment fragment = manager.getFragments()
                                           .get(backStackEntryCount - 1);
                fragment.onResume();
            }
        }
    };
    return result;
}

My workaround is to get the current title of the actionbar in the Fragment before setting it to the new title. This way, once the Fragment is popped, I can change back to that title.

@Override
public void onResume() {
    super.onResume();
    // Get/Backup current title
    mTitle = ((ActionBarActivity) getActivity()).getSupportActionBar()
            .getTitle();
    // Set new title
    ((ActionBarActivity) getActivity()).getSupportActionBar()
        .setTitle(R.string.this_fragment_title);
}

@Override
public void onDestroy() {
    // Set title back
    ((ActionBarActivity) getActivity()).getSupportActionBar()
        .setTitle(mTitle);

    super.onDestroy();
}

A little improved and wrapped into a manager solution.

Things to keep in mind. FragmentManager is not a singleton, it manages only Fragments within Activity, so in every activity it will be new. Also, this solution so far doesn't take ViewPager into account that calls setUserVisibleHint() method helping to control visiblity of Fragments.

Feel free to use following classes when dealing with this issue (uses Dagger2 injection). Call in Activity:

//inject FragmentBackstackStateManager instance to myFragmentBackstackStateManager
FragmentManager fragmentManager = getSupportFragmentManager(); 
myFragmentBackstackStateManager.apply(fragmentManager);

FragmentBackstackStateManager.java:

@Singleton
public class FragmentBackstackStateManager {

    private FragmentManager fragmentManager;

    @Inject
    public FragmentBackstackStateManager() {
    }

    private BackstackCallback backstackCallbackImpl = new BackstackCallback() {
        @Override
        public void onFragmentPushed(Fragment parentFragment) {
            parentFragment.onPause();
        }

        @Override
        public void onFragmentPopped(Fragment parentFragment) {
            parentFragment.onResume();
        }
    };

    public FragmentBackstackChangeListenerImpl getListener() {
        return new FragmentBackstackChangeListenerImpl(fragmentManager, backstackCallbackImpl);
    }

    public void apply(FragmentManager fragmentManager) {
        this.fragmentManager = fragmentManager;
        fragmentManager.addOnBackStackChangedListener(getListener());
    }
}

FragmentBackstackChangeListenerImpl.java:

public class FragmentBackstackChangeListenerImpl implements FragmentManager.OnBackStackChangedListener {

    private int lastBackStackEntryCount = 0;
    private final FragmentManager fragmentManager;
    private final BackstackCallback backstackChangeListener;

    public FragmentBackstackChangeListenerImpl(FragmentManager fragmentManager, BackstackCallback backstackChangeListener) {
        this.fragmentManager = fragmentManager;
        this.backstackChangeListener = backstackChangeListener;
        lastBackStackEntryCount = fragmentManager.getBackStackEntryCount();
    }

    private boolean wasPushed(int backStackEntryCount) {
        return lastBackStackEntryCount < backStackEntryCount;
    }

    private boolean wasPopped(int backStackEntryCount) {
        return lastBackStackEntryCount > backStackEntryCount;
    }

    private boolean haveFragments() {
        List<Fragment> fragmentList = fragmentManager.getFragments();
        return fragmentList != null && !fragmentList.isEmpty();
    }


    /**
     * If we push a fragment to backstack then parent would be the one before => size - 2
     * If we pop a fragment from backstack logically it should be the last fragment in the list, but in Android popping a fragment just makes list entry null keeping list size intact, thus it's also size - 2
     *
     * @return fragment that is parent to the one that is pushed to or popped from back stack
     */
    private Fragment getParentFragment() {
        List<Fragment> fragmentList = fragmentManager.getFragments();
        return fragmentList.get(Math.max(0, fragmentList.size() - 2));
    }

    @Override
    public void onBackStackChanged() {
        int currentBackStackEntryCount = fragmentManager.getBackStackEntryCount();
        if (haveFragments()) {
            Fragment parentFragment = getParentFragment();

            //will be null if was just popped and was last in the stack
            if (parentFragment != null) {
                if (wasPushed(currentBackStackEntryCount)) {
                    backstackChangeListener.onFragmentPushed(parentFragment);
                } else if (wasPopped(currentBackStackEntryCount)) {
                    backstackChangeListener.onFragmentPopped(parentFragment);
                }
            }
        }

        lastBackStackEntryCount = currentBackStackEntryCount;
    }
}

BackstackCallback.java:

public interface BackstackCallback {
    void onFragmentPushed(Fragment parentFragment);

    void onFragmentPopped(Fragment parentFragment);
}

I have used enum FragmentTags to define all my fragment classes.

TAG_FOR_FRAGMENT_A(A.class),
TAG_FOR_FRAGMENT_B(B.class),
TAG_FOR_FRAGMENT_C(C.class)

pass FragmentTags.TAG_FOR_FRAGMENT_A.name() as fragment tag.

and now on

@Override
public void onBackPressed(){
   FragmentManager fragmentManager = getFragmentManager();
   Fragment current
   = fragmentManager.findFragmentById(R.id.fragment_container);
    FragmentTags fragmentTag = FragmentTags.valueOf(current.getTag());

  switch(fragmentTag){
    case TAG_FOR_FRAGMENT_A:
        finish();
        break;
   case TAG_FOR_FRAGMENT_B:
        fragmentManager.popBackStack();
        break;
   case default: 
   break;
}

In my activity onCreate()

getSupportFragmentManager().addOnBackStackChangedListener(getListener());

Use this method to catch specific Fragment and call onResume()

private FragmentManager.OnBackStackChangedListener getListener()
    {
        FragmentManager.OnBackStackChangedListener result = new FragmentManager.OnBackStackChangedListener()
        {
            public void onBackStackChanged()
            {
                Fragment currentFragment = getSupportFragmentManager().findFragmentById(R.id.fragment_container);
                if (currentFragment instanceof YOURFRAGMENT) {
                    currentFragment.onResume();
                }
            }
        };

        return result;
    }

onResume() for the fragment works fine...

public class listBook extends Fragment {

    private String listbook_last_subtitle;
...

    @Override
       public void onCreate(Bundle savedInstanceState) {

        String thisFragSubtitle = (String) getActivity().getActionBar().getSubtitle();
        listbook_last_subtitle = thisFragSubtitle;
       }
...

    @Override
        public void onResume(){
            super.onResume();
            getActivity().getActionBar().setSubtitle(listbook_last_subtitle);
        }
...

If a fragment is put on backstack, Android simply destroys its view. The fragment instance itself is not killed. A simple way to start should to to listen to the onViewCreated event, an put you "onResume()" logic there.

boolean fragmentAlreadyLoaded = false;
    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onViewCreated(view, savedInstanceState);

        if (savedInstanceState == null && !fragmentAlreadyLoaded) {
            fragmentAlreadyLoaded = true;

            // Code placed here will be executed once
        }

        //Code placed here will be executed even when the fragment comes from backstack
    }

The following section at Android Developers describes a communication mechanism Creating event callbacks to the activity. To quote a line from it:

A good way to do that is to define a callback interface inside the fragment and require that the host activity implement it. When the activity receives a callback through the interface, it can share the information with other fragments in the layout as necessary.

Edit: The fragment has an onStart(...) which is invoked when the fragment is visible to the user. Similarly an onResume(...) when visible and actively running. These are tied to their activity counterparts. In short: use onResume()


public abstract class RootFragment extends Fragment implements OnBackPressListener {

 @Override
 public boolean onBackPressed() {
  return new BackPressImpl(this).onBackPressed();
 }

 public abstract void OnRefreshUI();

}


public class BackPressImpl implements OnBackPressListener {

 private Fragment parentFragment;

 public BackPressImpl(Fragment parentFragment) {
  this.parentFragment = parentFragment;
 }

 @Override
 public boolean onBackPressed() {
  ((RootFragment) parentFragment).OnRefreshUI();
 }
}

and final extent your Frament from RootFragment to see effect


After a popStackBack() you can use the following callback : onHiddenChanged(boolean hidden) within your fragment


This is the correct answer you can call onResume() providing the fragment is attached to the activity. Alternatively you can use onAttach and onDetach


Examples related to android

Under what circumstances can I call findViewById with an Options Menu / Action Bar item? How to implement a simple scenario the OO way My eclipse won't open, i download the bundle pack it keeps saying error log getting " (1) no such column: _id10 " error java doesn't run if structure inside of onclick listener Cannot retrieve string(s) from preferences (settings) strange error in my Animation Drawable how to put image in a bundle and pass it to another activity FragmentActivity to Fragment A failure occurred while executing com.android.build.gradle.internal.tasks

Examples related to android-fragments

FragmentActivity to Fragment How to start Fragment from an Activity How to use data-binding with Fragment In android how to set navigation drawer header image and name programmatically in class file? Android Fragment onAttach() deprecated How to convert any Object to String? Activity, AppCompatActivity, FragmentActivity, and ActionBarActivity: When to Use Which? Difference and uses of onCreate(), onCreateView() and onActivityCreated() in fragments java.lang.IllegalStateException: Fragment not attached to Activity java.lang.NullPointerException: Attempt to invoke virtual method 'int android.view.View.getImportantForAccessibility()' on a null object reference

Examples related to compatibility

Which TensorFlow and CUDA version combinations are compatible? IE11 Document mode defaults to IE7. How to reset? Internet Explorer 11 disable "display intranet sites in compatibility view" via meta tag not working Possible to restore a backup of SQL Server 2014 on SQL Server 2012? How to set IE11 Document mode to edge as default? Is it possible to run a .NET 4.5 app on XP? I want to get Year, Month, Day, etc from Java Date to compare with Gregorian Calendar date in Java. Is this possible? Fragments onResume from back stack Uninitialized constant ActiveSupport::Dependencies::Mutex (NameError) How to run multiple Python versions on Windows

Examples related to android-3.0-honeycomb

Why fragments, and when to use fragments instead of activities? How to implement the Android ActionBar back button? How to hide action bar before activity is created, and then show it again? Android replace the current fragment with another fragment getActionBar() returns null Fragments onResume from back stack How to set a Fragment tag by code? Android: How to change the ActionBar "Home" Icon to be something other than the app icon? ActionBar text color Unable to resolve host "<insert URL here>" No address associated with hostname

Examples related to back-stack

Android - save/restore fragment state How to resume Fragment from BackStack if exists Android: Remove all the previous activities from the back stack Problems with Android Fragment back stack How can I maintain fragment state when added to the back stack? Fragment onResume() & onPause() is not called on backstack Programmatically go back to the previous fragment in the backstack get the latest fragment in backstack How to prevent going back to the previous activity? Fragments onResume from back stack