MediatorLiveData

ViewModel.kt

class ContentViewModel(application: Application) : AndroidViewModel(application) {
  ...
  fun processEvent(event: ContentViewEvent) {
        when (event) {
            is ContentViewEvent.PlayerLoad ->
                _playerViewState.value = PlayerViewState(getContentPlayer(
                        event.contentId, event.filePath, event.previewImageUrl))
            ...
        }
  }
  private fun getContentPlayer(contentId: String, filePath: String, imageUrl: String) =
            //'getContentUri' and `bitmapToByteArray' return LiveData.
            //LiveData is wrapped in an Event object to ensure data is only observed once, thus requiring 'peekEvent'
            //to return data.
            getContentUri(contentId, filePath).combineLiveData(bitmapToByteArray(imageUrl)) { a, b ->
                Event(ContentResult.ContentPlayer(a.peekEvent().uri, b.peekEvent().image,
                        getLiveDataErrors(a, b)
                ))
            }
  /**
     * Sets the value to the result of a function that is called when both `LiveData`s have data
     * or when they receive updates after that.
     */
    private fun <T, A, B> LiveData<A>.combineLiveData(other: LiveData<B>, onChange: (A, B) -> T) =
            MediatorLiveData<T>().also { result ->
                var source1emitted = false
                var source2emitted = false
                val mergeF = {
                    val source1Value = this.value
                    val source2Value = other.value
                    if (source1emitted && source2emitted)
                        result.value = onChange.invoke(source1Value!!, source2Value!!)
                }
                result.addSource(this) { source1emitted = true; mergeF.invoke() }
                result.addSource(other) { source2emitted = true; mergeF.invoke() }
            }
  private fun getLiveDataErrors(a: Event<ContentResult.ContentUri>, b: Event<ContentResult.ContentBitmap>) =
            a.peekEvent().errorMessage.apply { if (this.isNotEmpty()) this }.apply {
                b.peekEvent().errorMessage.also {
                    if (it.isNotEmpty()) this.plus(" " + it)
                }
            }
  ...
}