Use ViewModels (master)

[westnordost]

This is the master ticket for refactoring to include view models. View models should be used for Fragments and Activities that either have a state that transcends the view lifecycle or access the data layer.

Contributing

As this is a larger undertaking which can be implemented step by step - a Fragment at a time - this ticket shall serve as a master ticket. Whenever you would like to contribute, select a Fragment, post that you are now working on that one here and later post a PR when you are done.

How to

How to find Fragments and Activities that should have view models

Search for activities and fragments that...

1. have onSaveInstanceState - it means it saves its state manually. This state should be in the ViewModel
2. by inject() - it means that dependencies are injected which are likely part of the data layer. Data access should be managed by the ViewModel
3. NOT to be done for now: have something like parentFragment as? Listener ?: activity as? Listener - this pattern is used for communication between fragments. This can be replaced with shared view models, but when using Compose, probably only the top-level fragments will remain as fragments anyway. So, wait with that until using Compose

How to implement ViewModels

ViewModel documentation

If you are like me and learn best by example / by copying. Look in the code for classes named *ViewModel and *ViewModelImpl. For example EditStatisticsViewModelImpl.

This will also give you a good idea in which style ViewModels should be written and what code should go into the ViewModel and what should remain the the View (i.e. the Fragment).
The ViewModels job is to supply the view with the data it needs with a convenient interface and data structure, hold this state and also offer an interface to manipulate the data, if applicable.

Advantages

We want to refactor the code to use ViewModels for the following reasons:

• For the planned iOS version, we will migrate (step by step) to Compose Multiplatform as UI framework. This means that use of Android-specific Fragments and Activities must eliminated. A step towards this goal is to separate state and data access from these into platform independent code

• For keeping (view) state that transcends the view lifecycle it is more convenient, as it does not need to be saved and manually re-created

• It provides a better separation between view logic and view data. Quite necessary when using Compose as UI framework and at least highly recommended otherwise

• It will likely simplify communication between fragments

• It's quite standard to use this pattern and recommended by Android architectural design guidelines. The more standard things are used in StreetComplete, the easier it is for new contributors to join

(This ticket supsersedes #5070 which also included a lot of discussion.)

[westnordost]

I added view models for

• LoginFragment + OAuthFragment (merged them into one)
• AchievementsFragment
• LinksFragment

Now, the profile screen all has viewmodels.

[westnordost]

Added viewmodels for everything in the settings screen, the about screen and some controls on the main screen.

[westnordost]

and edit history, which spawned a crictical bug that has been fixed in v57.3