Rotating with Fragments fails with saved states

[Skycoder42]

Rotating with Fragments that have a saved state will fail. Consider the following: I have a simple MvxCachingFragmentCompatActivity With a single MvxFragment inside. The Fragments viewmodel will save and restore it's state to an MvxBundle.

If I now rotate, the following happens:

• Rotation begin:
  • Fragments viewmodel (vm1) saves the state (state1) to the bundle
  • The Viewmodel is put into the MvxMultipleViewModelCache (vm1_state1)
  • The fragment gets destroyed
• Rotation end:
  • A new Fragment gets created
  • The Fragment restores it's viewmodel from the MvxMultipleViewModelCache (vm1_state1)
• MvxCachingFragmentCompatActivity.OnPostCreate gets called
  • A new viewmodel (vm2) is created and the state (state1) gets restored from the bundle
  • The new viewmodel (vm2_state1) is put into the MvxMultipleViewModelCache

Now as you can see, there are now 2 viewmodels at the same time, with only one beeing used. The real problem however occurs, if you rotate a second time, with a different state:

• The Fragments viewmodel has a different state (vm1_state2)
• Rotation begin:
  • Fragments viewmodel (vm1) saves the state (state2) to the bundle
  • The Viewmodel is not put into the MvxMultipleViewModelCache, because there is already one inside (vm2_state1)
  • The fragment gets destroyed
• Rotation end:
  • A new Fragment gets created
  • The Fragment restores it's viewmodel from the MvxMultipleViewModelCache (vm2_state1) !!!
• MvxCachingFragmentCompatActivity.OnPostCreate gets called
  • A new viewmodel (vm3) is created and the state (state2) gets restored from the bundle
  • The new viewmodel (vm3_state2) is put into the MvxMultipleViewModelCache

As you can see, after the second rotation the old state is reused.

There are basically two ways to fix this. I assume the error here is the wrong order: Debugging showed that the fragments get created before the state is restored. This is the origin of this error. Thus, in this pull-request, all I did was to move the code from OnPostCreate to OnCreate, because this one gets called before fragments are created.

However, you can also change the behavior of the MvxMultipleViewModelCache to overwrite existing viewmodels in the cache. This would save the problem too.

Note: If you need a working example, just leave a comment. But there is no complicated setup needed. Just a Fragment with a viewmodel and a changing saved state.

[martijn00]

I think there was a reason this moved to OnPostCreated, but i can't remember what it was. Can you see in the history if there where any comments about it in the commit?

[Skycoder42]

As far as the history goes, this was always in OnPostCreate

[martijn00]

Lets try and merge this. I see you still call OnPostCreate from the OnCreate method, so that needs to be fixed.