Android: MVVM is it possible to display a message (toast/snackbar etc.) from the ViewModel

Question

I want to know what is the best approach to display some sort of message in the view from the ViewModel. My ViewModel is making a POST call and "onResult" I want to pop up a message to the user containing a certain message.

This is my ViewModel:

public class RegisterViewModel extends ViewModel implements Observable {
.
.   
.
public void registerUser(PostUserRegDao postUserRegDao) {

    repository.executeRegistration(postUserRegDao).enqueue(new Callback<RegistratedUserDTO>() {
        @Override
        public void onResponse(Call<RegistratedUserDTO> call, Response<RegistratedUserDTO> response) {
            RegistratedUserDTO registratedUserDTO = response.body();
            /// here I want to set the message and send it to the Activity

            if (registratedUserDTO.getRegisterUserResultDTO().getError() != null) {

            }
        }

    });
}

And my Activity:

public class RegisterActivity extends BaseActivity {   

@Override
protected int layoutRes() {
    return R.layout.activity_register;
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    AndroidInjection.inject(this);
    super.onCreate(savedInstanceState);

    ActivityRegisterBinding binding = DataBindingUtil.setContentView(this, layoutRes());
    binding.setViewModel(mRegisterViewModel);       
}

What would the best approach be in this case?

Saxen

You can use regular callback interface for this. – Abram

Answer 1

Display Toast/snackbar message in view (Activity/Fragment) from viewmodel using LiveData.

Step:

• Add LiveData into your viewmodel
• View just observe LiveData and update view related task

For example:

In Viewmodel:

var status = MutableLiveData<Boolean?>()
//In your network successfull response
status.value = true

In your Activity or fragment:

yourViewModelObject.status.observe(this, Observer { status ->
    status?.let {
        //Reset status value at first to prevent multitriggering
        //and to be available to trigger action again
        yourViewModelObject.status.value = null
        //Display Toast or snackbar
    }
})

Genethlialogy

just a note for those not familiar with Kotlin syntax, that safe call operator ?. will run whether the status is true or false. doc – Spermiogenesis

Not the ideal pattern - LiveData emmits the last value once the view (activity/fragment) is resumed from background, which means that the Toast or Snackbar will be shown again. I myself am looking for a good design using SingleLiveEvent. – Splendent

@Splendent No, it will not. You reset status value just when showing Toast, so the last value will be null - how would it show a Toast again? There is null check for this case. You can also use Flow instead of LiveData to avoid this. – Dressingdown

@Mikołaj I seem to have missed that - but I think this is still not the most ideal pattern - resetting status to null is a side-effect in this case. The Event<T> wrapper with LiveData ( ` LiveData<Event<T>>` instead of LiveData<T> suggested by Google seems like the best approach with clear responsibility segregation. – Splendent

Answer 2

We can use a SingleLiveEvent class as a solution. But it is a LiveData that will only send an update once. In my personal experience, using an Event Wrapper class with MutableLiveData is the best solution.

Here is a simple code sample.

Step 1 : Create an Event class (this is a boilerplate code you can reuse for any android project).

open class Event<out T>(private val content: T) {

    var hasBeenHandled = false
        private set // Allow external read but not write

    /**
     * Returns the content and prevents its use again.
     */
    fun getContentIfNotHandled(): T? {
        return if (hasBeenHandled) {
            null
        } else {
            hasBeenHandled = true
            content
        }
    }

    /**
     * Returns the content, even if it's already been handled.
     */
    fun peekContent(): T = content
}

Step 2 : At the top of your View Model class, define a MutableLiveData with wrapper (I used a String here, but you can use your required data type), and a corresponding live data for encapsulation.

private val statusMessage = MutableLiveData<Event<String>>()

val message : LiveData<Event<String>>
  get() = statusMessage

Step 3 : You can update the status message within the functions of the ViewModel like this:

statusMessage.value = Event("User Updated Successfully")

Step 4 :

Write code to observe the live data from the View (activity or fragment)

 yourViewModel.message.observe(this, Observer {
     it.getContentIfNotHandled()?.let {
         Toast.makeText(this, it, Toast.LENGTH_LONG).show()
     }
 })

Decima

Brilliant solution. This takes into account when screen rotation and other lifecycle changes. – Rosmunda

The solution is good but if I have to show Toast on different screens then I'll have to add code to every viewModel and fragment. Is there any way to reduce this process? – Polecat

What if instead View (activity or fragment) I'm using compose function? – Longhorn