I am trying to use the new Architecture Components in Android, and so far it has worked pretty well. However, I have stumbled into an architectural problem.
I have a MainActivity that hosts a bunch of fragments, A, B, and C. Until now, every time I need my ViewModel (VM) I fetch it in the context of MainActivity (like this: MyViewModel vm = ViewModelProviders.of(getActivity()).get(MyViewModel.class);). Now consider this:
C lets the user select some value, and writes it to the VM.A uses fragment C to have the user select the value, and then A reads the value directly from VM and shows it in its UI.B uses the same approach as fragment A.The problem is, that since the VM is always in the context of the MainActivity, if fragment A has been used before B, the value will still be available, and B will show some old data.
The most obvious solution that I see is to create the VM in the contexts of fragments A and B respectively. But then I cannot figure out how to let fragment C access those VMs.
I could also create the VM in the context of fragment C, but that would require fragments A and B to create an instance of C, which I don't think is a nice solution.
A third solution would be keep the current approach and clear the data in the VM when appropriate, but that's also messy, I think.
What is the nicest way to go about this?
You can have a mapping in VM and store the values for A and B under different keys.
So, when A starts C it passes its A_key as an argument. When user chooses a value in C, it is being stored in VM's map using A_key as a key. When A checks whether a value is available, it only checks the value stored for A_key.
The same for B and B_key.