Android Reorder Fragment Backstack

Question

I have a number of pages/fragments listed in my navigation drawer, the user is likely to switch between these frequently and I want them in the backstack so that they can navigate back, but I only want one instance of each fragment in the backstack so that the user doesn't not have to press back an insane number of times to exit the app. I can't figure out how to effectively 'reorder' the backstack' without pages getting removed.

Currently when I change page I was using this code to change the fragment and make sure it's only in the back stack once

  if (mFragMgr == null) {
      mFragMgr = getSupportFragmentManager();
  }

  String backStateName = fragmentDescriptor.name();
  boolean fragmentPopped = mFragMgr.popBackStackImmediate(backStateName, 0);
  if (!fragmentPopped){
      mFragMgr.beginTransaction()
      .remove((Fragment) mFragment)
      .replace(R.id.content_frame, (Fragment) mFragment)
      .addToBackStack(backStateName)
      .commit();
  }

I use this code in onBackPressed

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

This works but it means it removes pages I don't want removed. Example:

When my user visits 6 pages in the order A > B > C > D > E > C because I'm doing a remove I expected the following stack:

                            [E]    [C]
                     [D]    [D]    [E]
              [C]    [C]    [C]    [D]
       [B]    [B]    [B]    [B]    [B]
[A] -> [A] -> [A] -> [A] -> [A] -> [A]

But what I actually get is the following - it pops everything up to the element that matches the name, this is regardless of whether I include the ".remove((Fragment) mFragment)" or not - ( I've already realised now that remove isn't affecting the backstack, so no need to point that out ):

                            [E]
                     [D]    [D]
              [C]    [C]    [C]    [C]
       [B]    [B]    [B]    [B]    [B]
[A] -> [A] -> [A] -> [A] -> [A] -> [A]

If I don't use a name and instead use null when adding to the backstack I get the following:

                                   [C]
                            [E]    [E]   
                     [D]    [D]    [D]    
              [C]    [C]    [C]    [C]   
       [B]    [B]    [B]    [B]    [B]  
[A] -> [A] -> [A] -> [A] -> [A] -> [A] 

How can I get the behaviour I expect? Is it possible at all or am I going to need to record the changes myself and skip the backstack altogether?

Answer 1

There is no API to do this, but becuse I was wondering the same thing last week,
I took it as an exercise an implemented it myself.

My method allows you to remove a Fragment from anywhere in the backstack and this is achieved by using refrection to modify the internal variables that store the backstack information in FragmentManagerImpl and BackStackRecord .

There are few important variables:

• mBackStack - stores the BackStackRecord s that hold the Fragment info, the previous ones, animations, etc
• mActive - all added Fragment s
• mAvailBackStackIndices - stores the indices that can be used by the newly inserted records, ie indices of null values in mActive
• and other

Tested with 150+ Fragment s and couldn't notice any leaks, but only monitored the heap in DDMS and did not perform any other memory analysis.
So because someting may be broken, explore the code, re-test it, see if anyone provides a better answer, and rethink if you really need to do this in your project.

I uploaded the code as a gist on GitHub , hope it helps.

Answer 2

When you want to reuse the fragment before it, just first hide all the fragments and show the desired fragment at a time. By doing this you can reuse your fragments without removing them and without reloading the fragments. But be sure before to implement it you have to add the fragment to backstack.