Observing repository LiveData from ViewModel and notify UI

Question

Google says that is not possible to observe a LiveData from a ViewModel:

[...] However ViewModel objects must never observe changes to lifecycle-aware observables, such as LiveData objects.[...]

And I would handle the repo.login() result in the ViewModel and notify the UI thought the two SingleLiveEvents, is it possible?

class Repository {
    // .... //
    fun login(user:String, password:String): LiveData<Status> { /* ... */ }
}

class LoginViewModel : ViewModel() {

    @Inject
    lateinit var repo: Repository

    private var auth = MutableLiveData<User>()
    private var showErrorEvent = SingleLiveEvent<String>()
    private var showSuccessEvent = SingleLiveEvent<String>()

    // Called from UI
    fun performLogin(user:User){
        repo.login(user.name, user.password)
        // We can't observe the result here 
        // and update error/success events
    }

    // Called from UI
    fun getErrorEvent() = showErrorEvent

    // Called from UI
    fun getSuccessEvent() = showSuccessEvent

}

Answer 1

I think that the official guideline is to combine multiple livedatas in the ViewModel using a transformation. However usually I prefer to use livedatas only in the communication between the ViewModel and the View, in the repositories I use RxJava or coroutines. So in your example I'd modify the login method in the repository to return a Single instead of a livedata and subscribe to it in the ViewModel. You can then cancel the subscription when the ViewModel is destroyed. You can find an example of this architecture in this repository.