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android – Is this the right way to clean-up Fragment back stack when leaving a deeply nested stack?

Posted by: admin March 11, 2020 Leave a comment

Questions:

I’m using the Android Compatibility library to implement fragments and have extended the layout sample so that a fragment contains a button which fires off another fragment.

In the selection pane on the left I have 5 selectable items – A B C D E.

Each loads up a fragment (via FragmentTransaction:replace) in the details pane – a b c d e

Now I’ve extended fragment e to contain a button which loads up another fragment e1 also in the details pane. I’ve done this on fragment e‘s onClick method as follows:

FragmentTransaction ft = getActivity().getSupportFragmentManager().beginTransaction();
ft.replace(R.id.details_frag, newFrag);
ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
ft.addToBackStack(null);
ft.commit();

If I make the following selections:

E - e - e1 - D - E

Then fragment e is in the details pane. This is fine and what I want. However, if I hit the back button at this point it does nothing. I have to click it twice because e1 is still on the stack. Furthermore after clicking around I got a null pointer exception in onCreateView:

To ‘solve’ this problem I added the following whenever A B C D E is selected:

FragmentManager fm = getActivity().getSupportFragmentManager();
for(int i = 0; i < fm.getBackStackEntryCount(); ++i) {
    fm.popBackStack();
}

Just wondering whether this is the correct solution or whether I should be doing something different?

How to&Answers:

Well there are a few ways to go about this depending on the intended behavior, but this link should give you all the best solutions and not surprisingly is from Dianne Hackborn

http://groups.google.com/group/android-developers/browse_thread/thread/d2a5c203dad6ec42

Essentially you have the following options

  • Use a name for your initial back stack state and use
    FragmentManager.popBackStack(String name,
    FragmentManager.POP_BACK_STACK_INCLUSIVE)
    .
  • Use FragmentManager.getBackStackEntryCount()/getBackStackEntryAt().getId()
    to retrieve the ID of the first entry on the back stack, and
    FragmentManager.popBackStack(int id,
    FragmentManager.POP_BACK_STACK_INCLUSIVE)
    .
  • FragmentManager.popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE)
    is supposed to pop the entire back stack… I think the documentation for
    that is just wrong. (Actually I guess it just doesn’t cover the case where
    you pass in POP_BACK_STACK_INCLUSIVE),

Answer:

The other clean solution if you don’t want to pop all stack entries…

getSupportFragmentManager().popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);
getSupportFragmentManager().beginTransaction().replace(R.id.home_activity_container, fragmentInstance).addToBackStack(null).commit();

This will clean the stack first and then load a new fragment, so at any given point you’ll have only single fragment in stack

Answer:

Thanks to Joachim answer, I use the code to clear all back stack entry finally.

// In your FragmentActivity use getSupprotFragmentManager() to get the FragmentManager.

// Clear all back stack.
int backStackCount = getSupportFragmentManager().getBackStackEntryCount();
for (int i = 0; i < backStackCount; i++) {

    // Get the back stack fragment id.
    int backStackId = getSupportFragmentManager().getBackStackEntryAt(i).getId();

    getSupportFragmentManager().popBackStack(backStackId, 
        FragmentManager.POP_BACK_STACK_INCLUSIVE);

} /* end of for */

Answer:

I have researched a lot for cleaning Backstack, and finally see Transaction BackStack and its management. Here is the solution that worked best for me.

 // CLEAR BACK STACK.
    private void clearBackStack() {
        final FragmentManager fragmentManager = getSupportFragmentManager();
        while (fragmentManager.getBackStackEntryCount() != 0) {
            fragmentManager.popBackStackImmediate();
        }
    }

The above method loops over all the transactions in the backstack and removes them immediately one at a time.

Note: above code sometime not work and i face ANR because of this code,so please do not try this.

Update
below method remove all fregment of that “name” from backstack.

FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.popBackStack("name",FragmentManager.POP_BACK_STACK_INCLUSIVE);
  • name If non-null, this is the name of a previous back state
    to look for; if found, all states up to that state will be popped. The
  • POP_BACK_STACK_INCLUSIVE flag can be used to control whether the named state itself is popped. If null, only the top state is popped.

Answer:

I’m using a similar code as those that use the while loop but I call the entry count in every loop… so I suppose it’s somewhat slower

FragmentManager manager = getFragmentManager();
while (manager.getBackStackEntryCount() > 0){
        manager.popBackStackImmediate();
    }

Answer:

As written in How to pop fragment off backstack and by LarsH here, we can pop several fragments from top down to specifical tag (together with the tagged fragment) using this method:

fragmentManager?.popBackStack ("frag", FragmentManager.POP_BACK_STACK_INCLUSIVE);

Substitute “frag” with your fragment’s tag. Remember that first we should add the fragment to backstack with:

fragmentTransaction.addToBackStack("frag")

If we add fragments with addToBackStack(null), we won’t pop fragments that way.

Answer:

    // pop back stack all the way
    final FragmentManager fm = getSherlockActivity().getSupportFragmentManager();
    int entryCount = fm.getBackStackEntryCount(); 
    while (entryCount-- > 0) {
        fm.popBackStack();
    }