Problems with Android Fragment back stack
android, android-fragments, back-stack
Solution
Explanation: on what's going on here?
If we keep in mind that `.replace()` is equal with `.remove().add()` that we know by the documentation:
Replace an existing fragment that was added to a container. This is essentially the same as calling `remove(Fragment)` for all currently added fragments that were added with the same `containerViewId` and then `add(int, Fragment, String)` with the same arguments given here.
then what's happening is like this (I'm adding numbers to the frag to make it more clear):
// transaction.replace(R.id.detailFragment, frag1);
Transaction.remove(null).add(frag1) // frag1 on view
// transaction.replace(R.id.detailFragment, frag2).addToBackStack(null);
Transaction.remove(frag1).add(frag2).addToBackStack(null) // frag2 on view
// transaction.replace(R.id.detailFragment, frag3);
Transaction.remove(frag2).add(frag3) // frag3 on view
(here all misleading stuff starts to happen)
Remember that `.addToBackStack()` is saving only transaction not the fragment as itself! So now we have `frag3` on the layout:
< press back button >
// System pops the back stack and find the following saved back entry to be reversed:
// [Transaction.remove(frag1).add(frag2)]
// so the system makes that transaction backward!!!
// tries to remove frag2 (is not there, so it ignores) and re-add(frag1)
// make notice that system doesn't realise that there's a frag3 and does nothing with it
// so it still there attached to view
Transaction.remove(null).add(frag1) //frag1, frag3 on view (OVERLAPPING)
// transaction.replace(R.id.detailFragment, frag2).addToBackStack(null);
Transaction.remove(frag3).add(frag2).addToBackStack(null) //frag2 on view
< press back button >
// system makes saved transaction backward
Transaction.remove(frag2).add(frag3) //frag3 on view
< press back button >
// no more entries in BackStack
< app exits >
Possible solution
Consider implementing `FragmentManager.BackStackChangedListener` to watch for changes in the back stack and apply your logic in `onBackStackChanged()` methode:
- Trace a count of transaction;
- Check particular transaction by name `FragmentTransaction.addToBackStack(String name);`
- Etc.
Problem
I've got a massive problem with the way the android fragment backstack seems to work and would be most grateful for any help that is offered. Imagine you have 3 Fragments `[1] [2] [3]` I want the user to be able to navigate `[1] > [2] > [3]` but on the way back (pressing back button) `[3] > [1]`. As I would have imagined this would be accomplished by not calling `addToBackStack(..)` when creating the transaction that brings fragment `[2]` into the fragment holder defined in XML. The reality of this seems as though that if I dont want `[2]` to appear again when user presses back button on `[3]`, I must not call `addToBackStack` in the transaction that shows fragment `[3]`. This seems completely counter-intuitive (perhaps coming from the iOS world). Anyway if i do it this way, when I go from `[1] > [2]` and press back I arrive back at `[1]` as expected. If I go `[1] > [2] > [3]` and then press back I jump back to `[1]` (as expected). Now the strange behavior happens when I try and jump to `[2]` again from `[1]`. First of all `[3]` is briefly displayed before `[2]` comes into view. If I press back at this point `[3]` is displayed, and if I press back once again the app exits. Can anyone help me to understand whats going on here? And here is the layout xml file for my main activity: ``` <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:orientation="vertical" > <fragment android:id="@+id/headerFragment" android:layout_width="match_parent" android:layout_height="wrap_content" class="com.fragment_test.FragmentControls" > <!-- Preview: layout=@layout/details --> </fragment> <FrameLayout android:id="@+id/detailFragment" android:layout_width="match_parent" android:layout_height="fill_parent" /> ``` Update This is the code I'm using to build by nav heirarchy ``` Fragment frag; FragmentTransaction transaction; //Create The first fragment [1], add it to the view, BUT Dont add the transaction to the backstack frag = new Fragment1(); transaction = getSupportFragmentManager().beginTransaction(); transaction.replace(R.id.detailFragment, frag); transaction.commit(); //Create the second [2] fragment, add it to the view and add the transaction that replaces the first fragment to the backstack frag = new Fragment2(); transaction = getSupportFragmentManager().beginTransaction(); transaction.replace(R.id.detailFragment, frag); transaction.addToBackStack(null); transaction.commit(); //Create third fragment, Dont add this transaction to the backstack, because we dont want to go back to [2] frag = new Fragment3(); transaction = getSupportFragmentManager().beginTransaction(); transaction.replace(R.id.detailFragment, frag); transaction.commit(); //END OF SETUP CODE------------------------- //NOW: //Press back once and then issue the following code: frag = new Fragment2(); transaction = getSupportFragmentManager().beginTransaction(); transaction.replace(R.id.detailFragment, frag); transaction.addToBackStack(null); transaction.commit(); //Now press back again and you end up at fragment [3] not [1] ``` Many thanks