TestObserver

TestObserver.kt

class TestObserver<T> : Observer<T> {
    private val values = mutableListOf<T>()

    fun getValues(): List<T> = values

    override fun onChanged(value: T) {
        values.add(value)
    }
}

fun <T> LiveData<T>.test(): TestObserver<T> {
    val observer = TestObserver<T>()
    observeForever(observer)
    return observer
}

@Test
fun `test function`() {
    // given
    val observer = sut.getLiveData().test()

    // when
    sut.doSomething()

    // then
    Truth
      .assertThat(viewStateObserver.getValues().last())
      .isEqualTo(expected)
}