I found a case when architecture components ViewModel
isn't retained - in short it goes as follows:
- Activity is started and
ViewModel
instance is created - Activity is put to background
- Device screen is rotated
- Activity is put back to foreground
- ViewModel's
onCleared
method is called and new object is created
Is it normal behavior of Android that my ViewModel
instance is getting destroyed in this case? If so, is there any recommended solution of keeping its state?
One way I can think of is saving it once onCleared
is called, however, it would also persist the state whenever activity is actually finishing. Another way could be making use of onRestoreInstanceState
but it's fired on every screen rotation (not only if the app is in background).
Any silver bullet to handle such case?
AFAIK,
ViewModel
's only purpose is to survive and keep the data (i.e. "save the state") while its owner goes through different lifecycle events. So you don't have to "save the state" yourself.We can tell from this that it's "not normal behavior".
onCleared()
is only called after the activity is finished (and is not getting recreated again).Are you creating the
ViewModel
using theViewModelProvider
, or are you creating the instance using the constructor?In your activity, you should have something like:
By doing this, you should get the expected effect.
Yes @tomwyr, this was a bug from an android framework. Bug details
The fix is available in 28.0.0-alpha3 and AndroidX 1.0.0-alpha3
But if you don't want to update to above versions now itself, Then you can solve like this (I know this is a bad solution but I didn't see any other good way)
In your activity override onDestroy method and save all the required fields to local variables before calling super.onDestroy. Now call super.onDestroy then Initialize your ViewModel again and assign the required fields back to your new instance of ViewModel
about isFinishing
Below code is in Kotlin:
Change support library/compileSDK/targetSDK to 28.
I had similar issue with multi-window. When switching to split screen, my viewModel is recreated. Support library 28 fixed my problem. (My lifecycle version is 1.1.1)
For others that may not be helped by previous answers like me, the problem could be that you haven't set up your ViewModelProvider properly with a factory.
After digging around I solved my similiar problem by adding the following method to my Activities:
And then I did this in my Fragments:
I already had some abstract super classes for menu purposes so I hid the methods away there so I don't have to repeat it in every activity. That's why they are protected. I believe they could be private if you put them in every activity or fragment that you need them in.
To be as clear as possible I would then call the methods to assign my view model in onCreate() in my activity and it would look something like this
or in fragment
It seems to work fine so far. If someone have a better way please let us know!