Android - Best Practices for ViewModel State in MVVM?

I am working on an Android App using the MVVM pattern along LiveData (possibly Transformations) and DataBinding between View and ViewModel. Since the app is "growing", now ViewModels contain lots of data, and most of the latter are kept as LiveData to have Views subscribe to them (of course, this data is needed for the UI, be it a Two-Way Binding as per EditTexts or a One-Way Binding). I heard (and googled) about keeping data that represents the UI state in the ViewModel. However, the results I found were just simple and generic. I would like to know if anyone has hints or could share some knowledge on best practices for this case. In simple words, What could be the best way to store the state of an UI (View) in a ViewModel considering LiveData and DataBinding available? Thanks in advance for any answer!

Solution

I struggled with the same problem at work and can share what is working for us. We're developing 100% in Kotlin so the following code samples will be as well.

UI state

To prevent the ViewModel from getting bloated with lots of LiveData properties, expose a single ViewState for views (Activity or Fragment) to observe. It may contain the data previously exposed by the multiple LiveData and any other info the view might need to display correctly:

data class LoginViewState (
    val user: String = "",
    val password: String = "",
    val checking: Boolean = false
)

Note, that I'm using a Data class with immutable properties for the state and deliberately don't use any Android resources. This is not something specific to MVVM, but an immutable view state prevents UI inconsistencies and threading problems.

Inside the ViewModel create a LiveData property to expose the state and initialize it:

class LoginViewModel : ViewModel() {
    private val _state = MutableLiveData<LoginViewState>()
    val state : LiveData<LoginViewState> get() = _state

    init {
        _state.value = LoginViewState()
    }
}

To then emit a new state, use the copy function provided by Kotlin's Data class from anywhere inside the ViewModel:

_state.value = _state.value!!.copy(checking = true)

In the view, observe the state as you would any other LiveData and update the layout accordingly. In the View layer you can translate the state's properties to actual view visibilities and use resources with full access to the Context:

viewModel.state.observe(this, Observer {
    it?.let {
        userTextView.text = it.user
        passwordTextView.text = it.password
        checkingImageView.setImageResource(
            if (it.checking) R.drawable.checking else R.drawable.waiting
        )
    }
})

Conflating multiple data sources

Since you probably previously exposed results and data from database or network calls in the ViewModel, you may use a MediatorLiveData to conflate these into the single state:

private val _state = MediatorLiveData<LoginViewState>()
val state : LiveData<LoginViewState> get() = _state

_state.addSource(databaseUserLiveData, { name ->
    _state.value = _state.value!!.copy(user = name)
})
...

Data binding

Since a unified, immutable ViewState essentially breaks the notification mechanism of the Data binding library, we're using a mutable BindingState that extends BaseObservable to selectively notify the layout of changes. It provides a refresh function that receives the corresponding ViewState:

Update: Removed the if statements checking for changed values since the Data binding library already takes care of only rendering actually changed values. Thanks to @CarsonHolzheimer

class LoginBindingState : BaseObservable() {
    @get:Bindable
    var user = ""
        private set(value) {
            field = value
            notifyPropertyChanged(BR.user)
        }

    @get:Bindable
    var password = ""
        private set(value) {
            field = value
            notifyPropertyChanged(BR.password)
        }

    @get:Bindable
    var checkingResId = R.drawable.waiting
        private set(value) {
            field = value
            notifyPropertyChanged(BR.checking)
        }

    fun refresh(state: AngryCatViewState) {
        user = state.user
        password = state.password
        checking = if (it.checking) R.drawable.checking else R.drawable.waiting
    }
}

Create a property in the observing view for the BindingState and call refresh from the Observer:

private val state = LoginBindingState()

...

viewModel.state.observe(this, Observer { it?.let { state.refresh(it) } })
binding.state = state

Then, use the state as any other variable in your layout:

<layout ...>

    <data>
        <variable name="state" type=".LoginBindingState"/>
    </data>

    ...

        <TextView
            ...
            android:text="@{state.user}"/>

        <TextView
            ...
            android:text="@{state.password}"/>

        <ImageView
            ...
            app:imageResource="@{state.checkingResId}"/>
    ...

</layout>

Advanced info

Some of the boilerplate would definitely benefit from extension functions and Delegated properties like updating the ViewState and notifying changes in the BindingState.

If you want more info on state and status handling with Architecture Components using a "clean" architecture you may checkout Eiffel on GitHub.

It's a library I created specifically for handling immutable view states and data binding with ViewModel and LiveData as well as glueing it together with Android system operations and business use cases. The documentation goes more in depth than what I'm able to provide here.