Fragment backstack bug when replacing fragments in a transaction?

 

Here's the scenario that causes problems:

  • I start an Acitivity with a ViewGroup that'll hold the presented fragments.
  • I load Fragment A into this ViewGroup by calling .replace() in the transaction that I save onto the backstack.
  • I load Fragment B into the Viewgroup, replacing Fragment A. Again, I save the transaction.
  • I rotate the device twice.
  • On the second rotation, Fragment A (which is not visible at the moment) will throw a NullPointer exception.

    • This exception is thrown by Fragment A, because I'm saving some values of Views (EditTexts eg) in its onSaveInstanceState() method. It seems, that on the second rotation, the system doesn't instantiate these Views, so when I'm asking their values, I get a NullPointer exception. Can this be avoided somehow? Or is using .replace operations in a fragment transcation saved onto the backstack unadvised?

    I've had this but can't quite recollect the specifics of what I did to fix but generally speaking (and apologies for the brain dump) I do the following:

  • Call setRetainInstance(true) in onCreate to avoid fragment recreation

  • Save the state of edit controls etc. in onSaveInstanceState to be used if activity is killed and you get restored with a non-null bundle (you shouldn't get a non-null bundle on an orientation change with 1.)

  • Maintain edit control values in member variables (as the fragment is not going to be recreated) ensuring they get updated in an onDestroyView from the edit controls and then use them to restore the edit control values in onCreateView

  • Have a flag which I set to true in onCreateView and false in onDestroyView and ensure I don't touch UI controls when the view is not around.

    • BTW Using replace while adding the transaction to the back stack is perfectly OK.

    Hope there's something in there that helps. Peter.

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