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

问题

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?

回答1:

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),

回答2:

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

回答3:

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 */

回答4:

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.

回答5:

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();
    }

回答6:

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.

回答7:

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

来源：https://stackoverflow.com/questions/5802141/is-this-the-right-way-to-clean-up-fragment-back-stack-when-leaving-a-deeply-nest

标签

android

android-fragments

fragment-backstack