Kotlin: Read Only access of Immutable Type to an Internal Variable of a Mutable Type

Question

While learning ViewModels in Android, a problem has arisen that feels like Kotlin was meant to solve. In the code below, we can see that MutableLiveData values are being use to edit values and indicators. However, we do not want these mutable values to be exposed to anything else, specifically members of an Android lifecycle. We DO want Android Lifecycle members to have access to read values but not set them. Therefore, the 3 exposed functions, displayed below, are of the LiveData<> immutable type.

Is there an easier or more concise way to expose read only values that can be edited internally? This seems like what Kotlin was made to avoid: boilerplate verbosity.

class HomeListViewModel: ViewModel(){
    //Private mutable data
    private val repositories = MutableLiveData<List<Repo>>()
    private val repoLoadError = MutableLiveData<Boolean>()
    private val loading = MutableLiveData<Boolean>()


    //Exposed uneditable LIveData
    fun getRepositories():LiveData<List<Repo>> = repositories
    fun getLoadError(): LiveData<Boolean> = repoLoadError
    fun getLoadingStatuses(): LiveData<Boolean> = loading

    init{...//Do some stuff to MutableLiveData<>

    }
}

A non-Android scenario that might be similar is:

class ImmutableAccessExample{

    private val theThingToBeEditedInternally = mutableListOf<String>()

    fun theThingToBeAccessedPublicly(): List<String> = theThingToBeEditedInternally

    init {
        theThingToBeEditedInternally.add(0, "something")
    }

}

Answer 1

I don't know if it is possible to avoid the verbosity. But, I've seen that before and it is usually declared as a property.

private val _repositories = MutableLiveData<List<Repo>>()
val repositories : LiveData<List<Repo>> 
    get() = _repositories

This is the convention, see the doc here in Names for backing properties

If a class has two properties which are conceptually the same but one is part of a public API and another is an implementation detail, use an underscore as the prefix for the name of the private property:

Answer 2

Following the idea of this post:

class HomeListViewModel: ViewModel(){
    val repositories: LiveData<List<Repo>> = MutableLiveData()

    init {
        repositories as MutableLiveData
        ...//Do some stuff to repositories
    }
}