#iOS | #Android ファショ通

Using LiveData & Flow in MVVM — Part I - ProAndroidDev 

Kotlin Flow の登場で盛り上がってきました。

どれにします? どの流れにします?

Repository

Result を返す。

	class WeatherForecastRepository @Inject constructor() {
	suspend fun fetchWeatherForecast(): Result<Int> {
	// Since you can only return one value from suspend function
	// you have to set data loading before calling fetchWeatherForecast
	// Fake api call
	delay(1000)
	// Return fake success data
	return Result.Success((0..20).random())
	}
	}

view raw WeatherForecastRepository.kt hosted with by GitHub

Flow<Result>を返す。

	class WeatherForecastRepository @Inject constructor() {
	/**
	* This methods is used to make one shot request to get
	* fake weather forecast data
	*/
	fun fetchWeatherForecast() = flow {
	emit(Result.Loading)
	// Fake api call
	delay(1000)
	// Send a random fake weather forecast data
	emit(Result.Success((0..20).random()))
	}
	/**
	* This method is used to get data stream of fake weather
	* forecast data in real time
	*/
	fun fetchWeatherForecastRealTime() = flow {
	emit(Result.Loading)
	// Fake data stream
	while (true) {
	delay(1000)
	// Send a random fake weather forecast data
	emit(Result.Success((0..20).random()))
	}
	}
	}

view raw WeatherForecastRepository.kt hosted with by GitHub

ViewModel

Result を受けて、LiveData<Result> を渡す。

	class WeatherForecastOneShotViewModel @Inject constructor(
	val weatherForecastRepository: WeatherForecastRepository
	) : ViewModel() {
	private var _weatherForecast = MutableLiveData<Result<Int>>()
	val weatherForecast: LiveData<Result<Int>>
	get() = _weatherForecast
	fun fetchWeatherForecast() {
	// Set value as loading
	_weatherForecast.value = Result.Loading
	viewModelScope.launch {
	// Fetch and update weather forecast LiveData
	_weatherForecast.value = weatherForecastRepository.fetchWeatherForecast()
	}
	}
	}

view raw WeatherForecastOneShotViewModel.kt hosted with by GitHub

Flow<Result> を受けて、LiveData<Result> を渡す。

	class WeatherForecastOneShotViewModel @Inject constructor(
	weatherForecastRepository: WeatherForecastRepository
	) : ViewModel() {
	private val _weatherForecast = weatherForecastRepository
	.fetchWeatherForecast()
	.asLiveData(viewModelScope.coroutineContext) // Use viewModel scope for auto cancellation
	val weatherForecast: LiveData<Result<Int>>
	get() = _weatherForecast
	}

view raw WeatherForecastOneShotViewModel.kt hosted with by GitHub

	class WeatherForecastDataStreamViewModel @Inject constructor(
	weatherForecastRepository: WeatherForecastRepository
	) : ViewModel() {
	private val _weatherForecast = weatherForecastRepository
	.fetchWeatherForecastRealTime()
	.map {
	// Do some heavy operation. This operation will be done in the
	// scope of this flow collected. In our case it is the scope
	// passed to asLiveData extension function
	// This operation will not block the UI
	delay(1000)
	it
	}
	.asLiveData(
	// Use Default dispatcher for CPU intensive work and
	// viewModel scope for auto cancellation when viewModel
	// is destroyed
	Dispatchers.Default + viewModelScope.coroutineContext
	)
	val weatherForecast: LiveData<Result<Int>>
	get() = _weatherForecast
	}

view raw WeatherForecastDataStreamViewModel.kt hosted with by GitHub

Fragment

LiveData<Result> を受け取る。

	class WeatherForecastDataStreamFragment : DaggerFragment() {
	...
	override fun onActivityCreated(savedInstanceState: Bundle?) {
	super.onActivityCreated(savedInstanceState)
	// Obtain viewModel
	viewModel = ViewModelProviders.of(
	this,
	viewModelFactory
	).get(WeatherForecastDataStreamViewModel::class.java)
	// Observe weather forecast data stream
	viewModel.weatherForecast.observe(viewLifecycleOwner, Observer {
	when (it) {
	Result.Loading -> {
	Toast.makeText(context, "Loading", Toast.LENGTH_SHORT).show()
	}
	is Result.Success -> {
	// Update weather data
	tvDegree.text = it.data.toString()
	}
	Result.Error -> {
	Toast.makeText(context, "Error", Toast.LENGTH_SHORT).show()
	}
	}
	})
	lifecycleScope.launch {
	while (true) {
	delay(1000)
	// Update text
	tvDegree.text = "Not blocking"
	}
	}
	}
	}

view raw WeatherForecastDataStreamFragment.kt hosted with by GitHub

Flow<Result> を受け取る。

override fun onActivityCreated(savedInstanceState: Bundle?) {
  super.onActivityCreated(savedInstanceState)

  viewModel = ViewModelProviders.of(
      this,
      viewModelFactory
  ).get(WeatherForecastDataStreamFlowViewModel::class.java)

  // Consume data when fragment is started
  lifecycleScope.launchWhenStarted {

    // Since collect is a suspend function it needs to be called
    // from a coroutine scope
    viewModel.weatherForecast.collect {
      when (it) {
        Result.Loading -> {
          Toast.makeText(context, "Loading", Toast.LENGTH_SHORT).show()
        }
        is Result.Success -> {
          tvDegree.text = it.data.toString()
        }
        Result.Error -> {
          Toast.makeText(context, "Error", Toast.LENGTH_SHORT).show()
        }
      }
    }
  }
}

WeatherForecastDataStreamFlowFragment #L47-L75

まとめ

とはいえ、今はまだ、完全に LiveData は捨てれんよの。

Kotlin で Result 
Kotlin Flow vs Android LiveData - Stack Overflow 
From RxJava 2 to Kotlin Flow: Threading - ProAndroidDev 

追記: ホットな Flow が登場したので以下。

【MVVM】 Kotlin Flow で使える5つの利用パターン