Android: How to change the FragmentManager Backstack when going back (insert a new fragment)?

Question

I'm trying to add a view to the fragment stack, without any visible indication that it happened, so it appears when going back.

Forexample,

3 Fragments, A, B and C

A -> C

Then when going BACK from C, I want B to appear (i.e. we 'skipped' over it)

C -> B -> A

My issue is these all need to be backstack + animation compatible.

I can show B by hijacking the back event, and doing a fragment transaction to replace C with B (and invert the animations so it 'enters' as if the C popped), but then going back again reverses that so I get stuck in a C -> B -> C -> B loop.

I try calling PopBackstack first before doing the transaction, but then C just 'vanishes' as B enters - but then going back from B -> A works fine.

• Alternatively, is it better to somehow add B + C simultaneously when navigating from A -> C and if so, how best to do that so the backstack is fine too? Baring in mind, B needs to not be visible so visually its animating from A -> C.
• Otherwise, any ideas on injecting a fragment into the stack when going back this way?

And no, Compose isn't an option.

Here's example pseudoish code of what I've been trying. I'm not trying to show B + C at the same time such that it appears you navigate A -> C.

Activity

<androidx.coordinatorlayout.widget.CoordinatorLayout
 xmlns:android="http://schemas.android.com/apk/res/android"
 android:id="@+id/content_root"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:fitsSystemWindows="true">
 <FrameLayout
 android:id="@+id/content_frame"
 android:layout_width="match_parent"
 android:layout_height="match_parent" />
</androidx.coordinatorlayout.widget.CoordinatorLayout>

So first I'm showing fragment A which is shown with the following rough transaction

var fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction()
 .setCustomAnimations(slide_left_in, slide_left_out, slide_right_out, slide_right_in)
 .replace("@id/content_frame", fragmentA)
 .addToBackStack("A")
 .commitAllowingStateLoss();

Its not 100% on the above as I'm deep in various other views by this point but basically, FragmentA is currently the active fragment in content_frame and is working fine.

Now, I want to navigate to C but with B 'hidden' in between. So as suggested by /u/ianhanniballake I'm doing them as two transactions (so they are both on the backstack). This however is not working currently...

fragmentManager.beginTransaction()
 .setCustomAnimations(slide_left_in, slide_left_out, slide_right_out, slide_right_in)
 .replace("@id/content_frame", fragmentB)
 .addToBackStack("B")
 .commitAllowingStateLoss();
// If I don't do this, the second transaction appears to never execute so just FragmentB appears
fragmentManager.ExecutePendingTransactions();
fragmentManager.beginTransaction()
 .setCustomAnimations(slide_left_in, slide_left_out, slide_right_out, slide_right_in)
 .replace("@id/content_frame", fragmentC)
 .addToBackStack("C")
 .commitAllowingStateLoss();

What I essentially see is FragmentA gets destroyed and vanishes before its animated out, so I then see Fragment C slide over 'nothing', but then going back works fine seeing FragmentB then FragmentA.

EDIT: Ok, so changing FragmentB to an 'Add' instead mostly works. But this does mean the 'pop' of FragmentB doesn't animate FragmentA, its just static (as it was added I guess, not replaced). Not sure if theres anything I can do to resolve that? If I do Replace, FragmentA vanishes due to the second transaction, if I do Add it works, but visually doesn't animate (slide) correctly when popping back.

• ianhanniballake: Adding B + C at the same time is exactly what you should be doing and fully supported (without ever showing B). Show your code and what you've attempted.
• Iain Stanford: What would the fragment transactions be to add them.both? Would I do two separate Replace transactions committing one at a time, or an Add and then Replace? Or can I do two replaced in the one transaction? Will the visually show A -> C?
• ianhanniballake: If they need to be two separate entries in the back stack and addToBackStack is a property on a FragmentTransaction, then it needs to be two separate FragmentTransactions. Again, if you show what you've tried, we can take a look and offer an answer that does what you want.
• Iain Stanford: I'll try adding code, its going to be fairly cut down but hopefully shows what I've been trying to do...
• Iain Stanford: I've added code and I'm very close now. I'm doing an Add transaction showing fragment B, then immediately doing a Replace transaction for Fragment C. This all looks good going A -> C. This all looks good going back C -> B. However going back B -> A, B animates out but A is static when it appears. If I do a Replace showing fragment B. Going from A -> C fragment A just vanishes instantly. This looks good going back C -> B. This looks good going back B -> A. (sorry, hate SO comment formatting with no new lines)

Answer 1

Why not jump to B directly and remove fragment C, then when you remove B it will back to fragment A.

• Iain Stanford: Not sure I follow how that would work. Do you mean do a Replace of B on A and then an Add for C? Would I not have the same issue but with B remaining static on pop enter?