LiveDataExtensions.kt

/**
 * @return [LiveData] with value produced from combining [source1] and [source2] using [combiner]
 *
 * This function produces new value only if [source1] and [source2] values are not null
 */
fun <T1, T2, R> combineNonNull(
    source1: LiveData<T1>,
    source2: LiveData<T2>,
    combiner: (T1, T2) -> R
): LiveData<R> {
    val mediator = MediatorLiveData<R>()

    val combinerFunction = {
        val source1Value = source1.value
        val source2Value = source2.value
        if (source1Value != null && source2Value != null) {
            mediator.value = combiner(source1Value, source2Value)
        }
    }

    mediator.addSource(source1) { combinerFunction() }
    mediator.addSource(source2) { combinerFunction() }
    return mediator
}

LiveDataExtensionsTest.kt

class LiveDataExtensionKtTest {

    @get:Rule
    val instantTaskExecutorRule = InstantTaskExecutorRule()

    @Test
    fun `verify combine successfully combines values from sources`() {
        val liveData1 = mutableLiveDataOf<Int>()
        val liveData2 = mutableLiveDataOf<Int>()

        val resultLiveData = combineNonNull(liveData1, liveData2) {
                source1, source2 -> source1 + source2
        }

        liveData1.value = 2
        liveData2.value = 2

        val result = resultLiveData.await()
        assertThat(result).isEqualTo(4)
    }

    @Test
    fun `verify combine result is null when one of the sources value is null`() {
        val liveData1 = mutableLiveDataOf<Int>()
        val liveData2 = mutableLiveDataOf<Int>()

        val resultLiveData = combineNonNull(liveData1, liveData2) {
                source1, source2 -> source1 + source2
        }

        liveData1.value = 2
        // liveData2 didn't emit any values so its value is null

        val result = resultLiveData.await()
        assertThat(result).isNull()
    }
}

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