Using Kotlin Flow in MVVM

LiveData was never designed to be a full-fledged reactive stream. Thread management with LiveData is very hard and making data transformation with transformation operators is very limited. Moreover, testing LiveData becomes very tricky and is likely to open for race conditions if you are using it with coroutines.

Luckily, Kotlin Flow provides asynchronously computed values with the power of Coroutines. Making data transformations becomes very easy with Flow intermediate operators. Furthermore, StateFlow provides caching the current state just like LiveData. Since there is a back-pressure in Flow, unit testing is much simpler with handy Flow terminal operators.

Kotlin Flow is a very powerful reactive stream with its robust operators. Using Flow instead of LiveData makes it easier to handle data transformations and background processes. Finally, testing Flow is much safer and painless compared to LiveData.

Transcript

1. Using Kotlin Flow in MVVM S. Fatih Giris Android Lead @ DNB Bank fgiris fatih_grs
2. Sample https://github.com/fgiris/LiveDataWithFlowSample
3. Agenda • LiveData in MVVM • LiveData + Flow • Only Flow • Testing
4. LiveData in MVVM • LiveData ✓Lifecycle aware ✓No memory leak ✓Cache ✓…
5. LiveData in MVVM • LiveData - Backpressure ❌ Producer Observer Ref: https://github.com/Froussios/Intro-To-RxJava/blob/master/Part%204%20-%20Concurrency/4.%20Backpressure.md
6. LiveData in MVVM • LiveData - Backpressure ❌ - Data transformation 🥵
7. LiveData in MVVM • LiveData - Backpressure ❌ - Data transformation ❌ - Requires lifecycle owner to observe (Activity, Fragment …) 🤔
8. LiveData in MVVM • Weather forecast app • One-shot request • Data stream
9. LiveData in MVVM • ViewModel: LiveData • Repository: Suspend Ref: https://developer.android.com/jetpack/guide
10. LiveData in MVVM WeatherForecastRepository.kt
11. LiveData in MVVM WeatherForecastRepository.kt
12. LiveData in MVVM WeatherForecastViewModel.kt
13. LiveData in MVVM WeatherForecastViewModel.kt
14. LiveData in MVVM WeatherForecastViewModel.kt
15. LiveData in MVVM WeatherForecastViewModel.kt
16. LiveData in MVVM WeatherForecastViewModel.kt
17. Return multiple values?
19. LiveData in MVVM Open Twitter app
20. LiveData in MVVM Open Twitter app Get the tweets from cache
21. LiveData in MVVM Open Twitter app Get the tweets from cache Fetch new tweets from server Update UI
22. LiveData in MVVM Open Twitter app Get the tweets from cache Fetch new tweets from server Update UI
23. LiveData in MVVM WeatherForecastRepository.kt
24. LiveData in MVVM WeatherForecastRepository.kt
25. Flow 🤩
26. Flow • Reactive stream which can return asynchronously computed values
27. Flow Pros • Structured concurrency • Cold stream • Efficient data transformation • Easy testing
28. LiveData + Flow • ViewModel: LiveData • Repository: Flow Ref: https://developer.android.com/jetpack/guide
29. LiveData + Flow WeatherForecastRepository.kt
30. LiveData + Flow WeatherForecastRepository.kt
31. LiveData + Flow WeatherForecastOneShotViewModel.kt
32. LiveData + Flow WeatherForecastOneShotViewModel.kt Flow
33. LiveData + Flow WeatherForecastOneShotViewModel.kt Flow
34. LiveData + Flow WeatherForecastOneShotFragment.kt
35. LiveData + Flow WeatherForecastRepository.kt
36. LiveData + Flow WeatherForecastRepository.kt Data per second
37. LiveData + Flow WeatherForecastDataStreamViewModel.kt
39. Flow Intermediate Flow Operators • map • filter • transform • onEach • …
40. LiveData + Flow map operator
41. LiveData + Flow map operator
42. LiveData + Flow map operator
43. LiveData + Flow onStart operator
44. LiveData + Flow filter operator
45. LiveData + Flow onEach operator
46. LiveData + Flow flowOn operator
47. LiveData + Flow catch operator
48. LiveData + Flow asLiveData extension
49. Why not only Flow?
50. LiveData 👋
51. Flow • Cold stream • ≠ LiveData • Configuration changes 🙄
52. Flow WeatherForecastDataStreamFlowViewModel.kt 😭
53. Flow WeatherForecastDataStreamFlowViewModel.kt No asLiveData
54. Flow WeatherForecastDataStreamFlowFragment.kt
55. Flow WeatherForecastDataStreamFlowFragment.kt Terminal Operator 👈
56. Flow WeatherForecastDataStreamFlowFragment.kt
58. Testing🚦
59. Testing • LiveData • LiveData & Coroutines might result some race condition • Need to observe again when Transformations used
60. Testing • Flow • Sequential • No race condition • Test like a list (toList )
61. Testing WeatherForecastRepository.kt
62. Testing WeatherForecastRepositoryTest.kt
63. Testing WeatherForecastRepositoryTest.kt
64. Testing WeatherForecastRepositoryTest.kt
65. Testing WeatherForecastRepositoryTest.kt
66. Testing WeatherForecastRepositoryTest.kt
67. Testing WeatherForecastRepositoryTest.kt
68. Testing WeatherForecastRepositoryTest.kt
69. Testing Dispatchers.Main -> Android Main Looper -> Android OS
70. Testing Solution: TestCoroutineDispatcher
71. Testing MainCoroutineRule.kt
72. Testing MainCoroutineRule.kt
73. Testing MainCoroutineRule.kt
74. TL;DR • Flow & LiveData similar • One shot request • Repository: Suspend or Flow • ViewModel: LiveData or Flow (with repeatOnLifecycle in the View layer) • Data stream • Repository: Flow • ViewModel: LiveData or Flow (with repeatOnLifecycle in the View layer) • LiveData Tests 😥 - Flow Tests 😎
75. https://medium.com/@girisfth Missed something?
76. QUESTIONS @fatih_grs
77. Stay safe!
78. References • https://proandroiddev.com/using-livedata-flow-in-mvvm-part-i- a98fe06077a0 • https://proandroiddev.com/using-livedata-flow-in-mvvm-part- ii-252ec15cc93a • https://github.com/fgiris/LiveDataWithFlowSample • https://developer.android.com/topic/libraries/architecture/livedata • https://proandroiddev.com/when-not-to-use-livedata-6a1245b054a6 • https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/ kotlinx.coroutines.flow/index.html
79. References • https://medium.com/@elizarov/kotlin-flows-and- coroutines-256260fb3bdb • https://github.com/Kotlin/kotlinx.coroutines/pull/1354 • https://codelabs.developers.google.com/codelabs/ advanced-kotlin-coroutines/ • https://medium.com/androiddevelopers/unit-testing-livedata- and-other-common-observability-problems-bb477262eb04 • https://medium.com/swlh/kotlin-coroutines-in-android-unit- test-28ff280fc0d5