Android 4.2: back stack behaviour with nested fragments

Question

With Android 4.2, the support library got support for nested fragments see here. I\'ve played around with it and found an interesting behaviour / bug regarding back stack an

Answer 1

After observing some solutions presented here, I found that to allow flexibility & control for the parent fragment as to when popping the stack or when the back action should be ignored by it, I rather use such implementation:

Defining a "ParentFragment" interface:

interface ParentFragment {
/**
 * Fragments that host child fragments and want to control their BackStack behaviour when the back button is pressed should implement this
 *
 * @return true if back press was handled, false otherwise
 */
fun onBackPressed(): Boolean

}

Overriding the "onBackPressed" in the parent activity (or in the BaseActivity):

override fun onBackPressed() {
    val fm: FragmentManager = supportFragmentManager
    for (frag in fm.fragments) {
        if (frag.isVisible && frag is ParentFragment && frag.onBackPressed()) {
            return
        }
    }
    super.onBackPressed()
}

And then allow the parent fragment to handle as it wishes, for example:

override fun onBackPressed(): Boolean {
    val childFragment = childFragmentManager.findFragmentByTag(SomeChildFragment::class.java.simpleName)
    if (childFragment != null && childFragment.isVisible) {
        // Only for that case, pop the BackStack (perhaps when other child fragments are visible don't)
        childFragmentManager.popBackStack()
        return true
    }
    return false
}

This allows to avoid thinking that there is some legitimate child fragment to remove when using a view pager for example (and back stack entry count > 0).

Answer 2

This solution may be better version of @Sean answer:

@Override
public void onBackPressed() {
    // if there is a fragment and the back stack of this fragment is not empty,
    // then emulate 'onBackPressed' behaviour, because in default, it is not working
    FragmentManager fm = getSupportFragmentManager();
    for (Fragment frag : fm.getFragments()) {
        if (frag.isVisible()) {
            FragmentManager childFm = frag.getChildFragmentManager();
            if (childFm.getBackStackEntryCount() > 0) {
                childFm.popBackStack();
                return;
            }
        }
    }
    super.onBackPressed();
}

Again, I prepared this solution based on @Sean answer above.

As @AZ13 said, this solution is only feasible in one level child fragments situations. In multiple level fragments case, works become a little complex, so I recommend that try this solution only the feasible case I have said. =)

Note: Since getFragments method is now a private method, this solution will not work. You can check comments for a link which suggests a solution about this situation.

Answer 3

This code will navigate the tree of fragment managers and return the last one that was added that has any fragments it can pop off the stack:

private FragmentManager getLastFragmentManagerWithBack(FragmentManager fm)
{
  FragmentManager fmLast = fm;

  List<Fragment> fragments = fm.getFragments();

  for (Fragment f : fragments)
  {
    if ((f.getChildFragmentManager() != null) && (f.getChildFragmentManager().getBackStackEntryCount() > 0))
    {
      fmLast = f.getFragmentManager();
      FragmentManager fmChild = getLastFragmentManagerWithBack(f.getChildFragmentManager());

      if (fmChild != fmLast)
        fmLast = fmChild;
    }
  }

  return fmLast;
}

Call the method:

@Override
public void onBackPressed()
{
  FragmentManager fm = getLastFragmentManagerWithBack(getSupportFragmentManager());

  if (fm.getBackStackEntryCount() > 0)
  {
    fm.popBackStack();
    return;
  }

  super.onBackPressed();
}

Answer 4

With this answer it will handle recursive back checking and give each fragment the chance to override the default behaviour. This means you can have a fragment that hosts a ViewPager do something special like scroll to the page that as a back-stack, or scroll to the home page and then on the next back press exit.

Add this to your Activity that extends AppCompatActivity.

@Override
public void onBackPressed()
{
    if(!BaseFragment.handleBackPressed(getSupportFragmentManager())){
        super.onBackPressed();
    }
}

Add this to your BaseFragment or the class you can have all your fragments inherit from.

public static boolean handleBackPressed(FragmentManager fm)
{
    if(fm.getFragments() != null){
        for(Fragment frag : fm.getFragments()){
            if(frag != null && frag.isVisible() && frag instanceof BaseFragment){
                if(((BaseFragment)frag).onBackPressed()){
                    return true;
                }
            }
        }
    }
    return false;
}

protected boolean onBackPressed()
{
    FragmentManager fm = getChildFragmentManager();
    if(handleBackPressed(fm)){
        return true;
    }
    else if(getUserVisibleHint() && fm.getBackStackEntryCount() > 0){
        fm.popBackStack();
        return true;
    }
    return false;
}

Answer 5

The reason is that your Activity derives from FragmentActivity, which handles the BACK key press (see line 173 of FragmentActivity.

In our application, I'm using a ViewPager (with fragments) and each fragment can have nested fragments. The way I've handled this is by:

• defining an interface OnBackKeyPressedListener with a single method void onBackKeyPressed()
• implemented this interface in "top" fragments that ViewPager shows
• overriding onKeyDown and detecting BACK press, and calling onBackKeyPressed in a currently active fragment in the view pager.

Also note, that I'm using getChildFragmentManager() in the fragments to properly nest fragments. You can see a discussion and an explanation in this android-developers post.