Demonstrates an Observer for LiveData to be used during unit testing.

ProfileRepository.kt

/**
 * This defines the contract for communicating with some data source to request profile information.
 */
interface ProfileRepository {
    fun fetchProfile(userId: String): Single<User>
}

ProfileViewState.kt

/**
 * Our profile view can be in one of three states: loading, successful data request, or an error requesting data.
 */
data class ProfileViewState(
    val loading: Boolean,
    val data: User?,
    val error: Throwable?
)

TestObserver.kt

/**
 * This TestObserver keeps track of all events emitted to a
 * LiveData so that our unit tests can validate all of the expected emissions
 * occurred.
 */
class TestObserver<T> : Observer<T> {
    var observedValues: MutableList<T> = mutableListOf()

    override fun onChanged(t: T) {
        observedValues.add(t)
    }
}

fun <T> LiveData<T>.testObserver() = TestObserver<T>().also {
    observeForever(it)
}