Compose Beyond the UI: Architecting Reactive State Machines at Scale

Jetpack Compose has redefined how we build UIs... but most apps still fall back on ViewModels and scattered logic that don’t scale well. As complexity grows, this leads to inconsistent screens, hard-to-reproduce bugs, and duplicated code. In this talk, we’ll look at how to move beyond MVVM/MVI into a reactive state machine model that fits Compose’s declarative style. You’ll learn how to design clean state contracts, coordinate transitions across navigation stacks and dialogs, and handle side effects like navigation or snackbars in a predictable way. We’ll also cover async patterns, modularizing domain state, debugging techniques for tracking state evolution, and testing UIs with simulated states and time-based transitions. By the end, you’ll leave with practical techniques to make Compose apps more scalable, debuggable, and easier to maintain.

Transcript

1. Patterns for predictable, testable, scalable UI state Compose Beyond the UI: Architecting Reactive State Machines at Scale Adit Lal GDE Android
2. Wh a t bre a ks a t sc a le Why it bre a ks? Build ment a l models How to f ix it I d e n t i f y P a t t e rns So l u t ions Compose Beyond the UI: Architecting Reactive State Machines at Scale Patterns for predictable, testable, scalable UI state
3. - Common bugs th a t seem r a ndom - Symptoms vs root c a uses - The "it works on my m a chine" problem I d e n t i f y P e So t
4. - St a te t a xonomy (ephemer a l → screen → dom a in) - Boole a n explosion → st a te m a chines - T a ngled tr a nsitions → pure functions y P a t t e rns So t
5. - St a - Boole a - T a y P e So l u t ions - Se a led interf a ces for impossible st a tes - Tr a nsitionResult for e ff ects a s outputs - Testing without mocks
6. Three Bugs Everyone Has These Aren't Compose Bugs
7. Three Bugs Everyone Has
8. Three Bugs Everyone Has - Sn a ckb a r shows a g a in a fter rot a tion
9. Three Bugs Everyone Has - Sn a ckb a r shows a g a in a fter rot a tion - Button Fires twice on f a st t a p
10. Three Bugs Everyone Has - Sn a ckb a r shows a g a in a fter rot a tion - Button Fires twice on f a st t a p - Screen shows st a le d a t a a fter b a ck n a vig a tion
11. Three Bugs Everyone Has - Sn a - Button Fires twice on f a - Screen shows st a These Aren't Compose Bugs These are State bugs
12. Three Bugs Everyone Has - Sn a - Button Fires twice on f a - Screen shows st a These Aren't Compose Bugs These are State bugs The Three Layers of State
13. The Three Layers of State
14. The Three Layers of State Ephemeral remember { }
15. The Three Layers of State Ephemeral remember { } Screen ViewModel
16. The Three Layers of State Ephemeral remember { } Screen ViewModel Domain Repository
17. Ephemeral remember { } Screen ViewModel Domain Repository Form Screen The Three Layers of State Where does e a ch st a te live? Wrong Layer == Bugs
18. Ephemeral remember { } Screen ViewModel Domain Repository Form Screen The Three Layers of State Where does e a ch st a te live? TextField focus state Keyboard visible/hidden Scroll position Wrong Layer == Bugs
19. Ephemeral remember { } Screen ViewModel Domain Repository Form Screen The Three Layers of State Where does e a ch st a te live? TextField focus state Keyboard visible/hidden Scroll position Form values (name, etc) Validation errors isLoading, isSaving Wrong Layer == Bugs
20. Ephemeral remember { } Screen ViewModel Domain Repository Form Screen The Three Layers of State Where does e a ch st a te live? TextField focus state Keyboard visible/hidden Scroll position Form values (name, etc) Validation errors isLoading, isSaving User pro fi le (from API) Saved pro fi le (in DB) Wrong Layer == Bugs
21. Wrong Layer == Bugs Ephemeral remember { } Screen ViewModel Domain Repository Form Screen The Three Layers of State The Rule
22. Wrong Layer == Bugs Ephemeral remember { } Screen ViewModel Domain Repository Form Screen The Three Layers of State Screen state here Form resets on rotation The Rule
23. Wrong Layer == Bugs Ephemeral remember { } Screen ViewModel Domain Repository Form Screen The Three Layers of State Screen state here Form resets on rotation Ephemeral state in ViewModel Keyboard survives navigation The Rule
24. Wrong Layer == Bugs Ephemeral remember { } Screen ViewModel Domain Repository Form Screen The Three Layers of State Screen state here Form resets on rotation Ephemeral state in ViewModel Keyboard survives navigation Screen logic trapped here Stale data a ft er back nav The Rule
25. The Three Layers of State The Rule W h e n Shou l d t h e s t at e di e ? State Machines
26. The Three Layers of State The Rule - Dies with recomposition → remember { } W h e n Shou l d t h e s t at e di e ? State Machines
27. The Three Layers of State The Rule - Dies with recomposition → remember { } - Dies with n a vig a tion → ViewModel W h e n Shou l d t h e s t at e di e ? State Machines
28. The Three Layers of State The Rule - Dies with recomposition → remember { } - Dies with n a vig a tion → ViewModel - Never dies → Repository W h e n Shou l d t h e s t at e di e ? State Machines
29. State Machines ViewModel State booleans 🫣
30. State Machines The Boole a n Explosion Problem ViewModel State booleans 🫣
31. State Machines The Boole a n Explosion Problem data class Sc r eenState( val isLoading: Boolean = false, val isSaving: Boolean = false, val isE r r o r : Boolean = false, val isSuccess: Boolean = false, ) ViewModel State booleans 🫣
32. State Machines The Boole a n Explosion Problem data class Sc r eenState( val isLoading: Boolean = false, val isSaving: Boolean = false, val isE r r o r : Boolean = false, val isSuccess: Boolean = false, ) 2⁴ = 16 possible combinations ViewModel State booleans 🫣
33. State Machines The Boole a n Explosion Problem data class Sc r eenState( val isLoading: Boolean = false, val isSaving: Boolean = false, val isE r r o r : Boolean = false, val isSuccess: Boolean = false, ) 2⁴ = 16 possible combinations On l y 4 a r e v al id ViewModel State booleans 🫣
34. State Machines The Boole a n Explosion Problem data class Sc r eenState( val isLoading: Boolean = false, val isSaving: Boolean = false, val isE r r o r : Boolean = false, val isSuccess: Boolean = false, ) 2⁴ = 16 possible combinations On l y 4 a r e v al id
35. State Machines The Boole a n Explosion Problem data class Sc r eenState( val isLoading: Boolean = false, val isSaving: Boolean = false, val isE r r o r : Boolean = false, val isSuccess: Boolean = false, ) 2⁴ = 16 possible combinations On l y 4 a r e v al id ViewModel
36. State Machines The Boole a n Explosion Problem data class Sc r eenState( val isLoading: Boolean = false, val isSaving: Boolean = false, val isE r r o r : Boolean = false, val isSuccess: Boolean = false, ) 2⁴ = 16 possible combinations On l y 4 a r e v al id ViewModel State booleans 🫣
37. State Machines The Boole a n Explosion Problem data class Sc r eenState( val isLoading: Boolean = false, val isSaving: Boolean = false, val isE r r o r : Boolean = false, val isSuccess: Boolean = false, ) 2⁴ = 16 possible combinations On l y 4 a r e v al id ViewModel State booleans 🫣 E v e r y a c t ion c a n t ouch e v e r y fl a g
38. State Machines Se a led Interf a ce Solution M a ke IMPOSSIBLE STATES impossible
39. sealed inte r face Sc r eenState { data object Loading : Sc r eenState data class Editing(val fo r m: Fo r m) : Sc r eenState data class Saving(val fo r m: Fo r m) : Sc r eenState data class Success(val msg: St r ing) : Sc r eenState data class E r r o r (val r eason: St r ing) : Sc r eenState } State Machines Se a led Interf a ce Solution M a ke IMPOSSIBLE STATES impossible
40. sealed inte r face Sc r eenState { data object Loading : Sc r eenState data class Editing(val fo r m: Fo r m) : Sc r eenState data class Saving(val fo r m: Fo r m) : Sc r eenState data class Success(val msg: St r ing) : Sc r eenState data class E r r o r (val r eason: St r ing) : Sc r eenState } State Machines Se a led Interf a ce Solution M a ke IMPOSSIBLE STATES impossible 5 s t at e s. On ly 5. N o combin at ions.
41. UniDirectional Pipe UI Events ViewModel Tr a nsition St a te (se a led) UI Render Observes State Machines
42. UniDirectional Pipe State Machines
43. UniDirectional Pipe State Machines onLoad
44. UniDirectional Pipe State Machines Lo a ding onLoad
45. UniDirectional Pipe State Machines Lo a ding onLoad d a t a lo a ded
46. UniDirectional Pipe State Machines Lo a ding onLoad d a t a lo a ded Editing
47. UniDirectional Pipe State Machines Lo a ding onLoad d a t a lo a ded Editing Error
48. UniDirectional Pipe State Machines Lo a ding onLoad d a t a lo a ded Editing Error Error Retry
49. UniDirectional Pipe State Machines Lo a ding onLoad d a t a lo a ded Editing Error S a ving Error Retry
50. UniDirectional Pipe State Machines Lo a ding onLoad d a t a lo a ded Editing Error S a ving Success Error Retry Success
51. UniDirectional Pipe State Machines Lo a ding onLoad d a t a lo a ded Editing Error S a ving Success Error Retry Success E a ch a rrow is a v a lid tr a nsition
52. UniDirectional Pipe State Machines Lo a ding onLoad d a t a lo a ded Editing Error S a ving Success Error Retry Success E a ch a rrow is a v a lid tr a nsition Missing a rrow = impossible tr a nsition
53. UniDirectional Pipe State Machines VIEW INTENT MODEL VIEW (UI) (Event) (State) (UI) Use r ──────> Submit ──────> Saving ──────> Show taps button Intent State Spinne r
54. State Anti-Patterns
55. ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } } State Anti-Patterns
56. var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } } ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns Timeline
57. var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } } ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns // ✅ Runs Timeline
58. var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } } ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns // ✅ Runs // 💀 Triggers c a ncell a tion Timeline
59. var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } } ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns // ✅ Runs // 💀 Triggers c a ncell a tion // ❌ NEVER RUNS Timeline
60. var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } } ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns // ✅ Runs // 💀 Triggers c a ncell a tion // ⏸ Suspension point // ❌ NEVER RUNS Timeline
61. ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns Timeline var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } }
62. ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns Timeline shouldProcess = true —> L a nchedE ff ect st a rts var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } }
63. ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns Timeline shouldProcess = true —> L a nchedE ff ect st a rts var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } } doSomething() —> completes
64. ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns Timeline shouldProcess = true —> L a nchedE ff ect st a rts var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } } doSomething() —> completes shouldProcess = f a lse —> Key Ch a nged, c a ncell a tion scheduled
65. ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns Timeline shouldProcess = true —> L a nchedE ff ect st a rts var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } } doSomething() —> completes shouldProcess = f a lse —> Key Ch a nged, c a ncell a tion scheduled del a y(200) —> Coroutine suspends
66. ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns Timeline shouldProcess = true —> L a nchedE ff ect st a rts var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } } doSomething() —> completes shouldProcess = f a lse —> Key Ch a nged, c a ncell a tion scheduled del a y(200) —> Coroutine suspends Recomposition runs (~16ms)
67. ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns Timeline shouldProcess = true —> L a nchedE ff ect st a rts var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } } doSomething() —> completes shouldProcess = f a lse —> Key Ch a nged, c a ncell a tion scheduled del a y(200) —> Coroutine suspends Recomposition runs (~16ms) Key is ch a nged
68. ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns Timeline shouldProcess = true —> L a nchedE ff ect st a rts var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } } doSomething() —> completes shouldProcess = f a lse —> Key Ch a nged, c a ncell a tion scheduled del a y(200) —> Coroutine suspends Recomposition runs (~16ms) Key is ch a nged C a ncels old L a unchedE ff ect
69. ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns Timeline shouldProcess = true —> L a nchedE ff ect st a rts var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(shouldProcess) { if (shouldProcess) { doSomething() shouldProcess = false delay(200) doImportantWork() } } doSomething() —> completes shouldProcess = f a lse —> Key Ch a nged, c a ncell a tion scheduled del a y(200) —> Coroutine suspends Recomposition runs (~16ms) Key is ch a nged C a ncels old L a unchedE ff ect doImport a ntWork() —> never re a ched
70. ANTI-PATTERN #1: LaunchedE ff ect Self-Cancellation Trap State Anti-Patterns Fix var shouldProcess by remember { mutableStateOf(false) } LaunchedE ff ect(Unit) { // Key never changes snapshotFlow { shouldProcess } . fi lter { it } .collect { doSomething() shouldProcess = false // Safe now delay(200) doImportantWork() // ✅ Runs } } Key is st a ble. St a te ch a nge doesn't kill the coroutine.
71. Rule State Anti-Patterns LAUNCHEDEFFECT RULE If you ch a nge the key inside the e ff ect...
72. Rule State Anti-Patterns LAUNCHEDEFFECT RULE If you ch a nge the key inside the e ff ect... ... a nd you h a ve a ny suspension point a fter...
73. Rule State Anti-Patterns LAUNCHEDEFFECT RULE If you ch a nge the key inside the e ff ect... Key = lifecycle Ch a nge key = rest a rt lifecycle Rest a rt = c a ncel previous ... a nd you h a ve a ny suspension point a fter... ...your code a fter th a t point won't run.
74. Transitions Are Functions
75. Transitions Are Functions data class T r ansitionResult( val newState: Sc r eenState, val effects: List<Effect> = emptyList() ) fun ScreenState.onSubmit(): TransitionResult = when (this) { is Loading -> TransitionResult(this) is Editing -> TransitionResult( newState = Saving(form), e fi ) is Saving -> TransitionResult(this) is Success -> TransitionResult(this)
76. Transitions Are Functions data class T r ansitionResult( val newState: Sc r eenState, val effects: List<Effect> = emptyList() ) fun ScreenState.onSubmit(): TransitionResult = when (this) { is Loading -> TransitionResult(this) is Editing -> TransitionResult( newState = Saving(form), e ff ects = listOf(E ff ect.SavePro fi le(form)) ) is Saving -> TransitionResult(this) is Success -> TransitionResult(this) is Error -> TransitionResult( newState = Saving(form), e ff ects = listOf(E ff ect.SavePro fi le(form)) ) }
77. Transitions Are Functions r val newState: Sc r val effects: List<Effect> = emptyList() ) fun ScreenState.onSubmit(): TransitionResult = when (this) { is Loading -> TransitionResult(this) is Editing -> TransitionResult( newState = Saving(form), e ff ects = listOf(E ff ect.SavePro fi le(form)) ) is Saving -> TransitionResult(this) is Success -> TransitionResult(this) is Error -> TransitionResult( newState = Saving(form), e ff ects = listOf(E ff ect.SavePro fi le(form)) ) }
78. Transitions Are Functions r val newState: Sc r val effects: List<Effect> = emptyList() ) fun ScreenState.onSubmit(): TransitionResult = when (this) { is Loading -> TransitionResult(this) is Editing -> TransitionResult( newState = Saving(form), e ff ects = listOf(E ff ect.SavePro fi le(form)) ) is Saving -> TransitionResult(this) is Success -> TransitionResult(this) is Error -> TransitionResult( newState = Saving(form), e ff ects = listOf(E ff ect.SavePro fi le(form)) ) }
79. Transitions Are Functions r val newState: Sc r val effects: List<Effect> = emptyList() ) fun ScreenState.onSubmit(): TransitionResult = when (this) { is Loading -> TransitionResult(this) is Editing -> TransitionResult( newState = Saving(form), e ff ects = listOf(E ff ect.SavePro fi le(form)) ) is Saving -> TransitionResult(this) is Success -> TransitionResult(this) is Error -> TransitionResult( newState = Saving(form), e ff ects = listOf(E ff ect.SavePro fi le(form)) ) } Pure function. No API calls inside. No side e ff ects. Just data in, data out. Why Pure functions?
80. Why Pure functions? Effects as Outputs
81. Why Pure functions? PREDICTABLE S a me input → s a me output. Alw a ys. Effects as Outputs
82. Why Pure functions? PREDICTABLE S a me input → s a me output. Alw a ys. TESTABLE No mocks. No coroutines. Just a ssertions. Effects as Outputs
83. Why Pure functions? PREDICTABLE S a me input → s a me output. Alw a ys. TESTABLE No mocks. No coroutines. Just a ssertions. DEBUGGABLE Log every tr a nsition. Repl a y a ny bug. Effects as Outputs
84. Effects as Outputs
85. Effects as Outputs Side E ff ect a re outputs, not inline code // ❌ E ff ect inside transition fun onSubmit() { _state.value = Saving(form) repository.save(form) // Side e ff ect here navigator.goTo(Success) // Another one }
86. Effects as Outputs Side E ff ect a re outputs, not inline code // ❌ E ff ect inside transition fun onSubmit() { _state.value = Saving(form) repository.save(form) // Side e ff ect here navigator.goTo(Success) // Another one } // ✅ E ff ects as data fun ScreenState.onSubmit() = TransitionResult( newState = Saving(form), e ff ects = listOf( E ff ect.SavePro fi le(form), E ff ect.Navigate(Routes.Success) ) )
87. Effect Types
88. Effect Types Common E ff ects sealed inte r face Effect { data class ShowSnackba r ( val message: St r ing, val id: UUID = UUID. r andomUUID() ) : Effect data class Navigate(val r oute: St r ing) : Effect data class SaveToDatabase(val data: Fo r m) : Effect data class T r ackAnalytics(val event: St r ing) : Effect data object HapticFeedback : Effect } ViewModel Executes
89. ViewModel Executes Effect Types class P r of i leViewModel : ViewModel() { p r ivate val _state = MutableStateF l ow<Sc r eenState>(Loading) val state = _state.asStateF l ow() fun onSubmit() { val r esult = _state.value.onSubmit() _state.value = r esult.newState r esult.effects.fo r Each { execute(it) } } p r ivate fun execute(effect: Effect) = when (effect) { is Effect.SaveP r of i le - > viewModelScope.launch { r eposito r y.save(effect.fo r m) } is Effect.ShowSnackba r - > _snackba r .emit(effect) is Effect.Navigate - > _navigation.emit(effect. r oute) } }
90. ViewModel Executes Effect Types class P r of i leViewModel : ViewModel() { p r ivate val _state = MutableStateF l ow<Sc r eenState>(Loading) val state = _state.asStateF l ow() fun onSubmit() { val r esult = _state.value.onSubmit() _state.value = r esult.newState r esult.effects.fo r Each { execute(it) } } p r ivate fun execute(effect: Effect) = when (effect) { is Effect.SaveP r of i le - > viewModelScope.launch { r eposito r y.save(effect.fo r m) } is Effect.ShowSnackba r - > _snackba r .emit(effect) is Effect.Navigate - > _navigation.emit(effect. r oute) } }
91. ViewModel Executes Effect Types class P r of i leViewModel : ViewModel() { p r ivate val _state = MutableStateF l ow<Sc r eenState>(Loading) val state = _state.asStateF l ow() fun onSubmit() { val r esult = _state.value.onSubmit() _state.value = r esult.newState r esult.effects.fo r Each { execute(it) } } p r ivate fun execute(effect: Effect) = when (effect) { is Effect.SaveP r of i le - > viewModelScope.launch { r eposito r y.save(effect.fo r m) } is Effect.ShowSnackba r - > _snackba r .emit(effect) is Effect.Navigate - > _navigation.emit(effect. r oute) } }
92. Async State Machines
93. Async State Machines ASYNC = EXPLICIT STATES sealed inte r face Async<out T> { data object Idle : Async<Nothing> data object Loading : Async<Nothing> data class Success<T>(val data: T) : Async<T> data class E r r o r (val e r r o r : Th r owable) : Async<Nothing> } / / Usage: data class P r of i leSc r eenState( val p r of i le: Async<Use r P r of i le> = Async.Idle, val saveStatus: Async<Unit> = Async.Idle )
94. Async State Machines WHY ASYNC<T>?
95. Async State Machines WHY ASYNC<T>? C a ncell a tion Job c a ncelled → b a ck to Idle Not stuck in Loading forever
96. Async State Machines WHY ASYNC<T>? C a ncell a tion Job c a ncelled → b a ck to Idle Not stuck in Loading forever MULTIPLE REQUESTS E a ch oper a tion gets its own Async<T> No boolean fl ags colliding
97. Async State Machines WHY ASYNC<T>? C a ncell a tion Job c a ncelled → b a ck to Idle Not stuck in Loading forever MULTIPLE REQUESTS E a ch oper a tion gets its own Async<T> No boolean fl ags colliding RETRY Error holds throw a ble. User taps retry → Loading again
98. Testing
99. Testing TESTING: NO MOCKS NEEDED @Test fun `submit f r om Editing t r ansitions to Saving`() { val state = Sc r eenState.Editing(fo r m = testFo r m) val r esult = state.onSubmit() asse r tThat( r esult.newState).isEqualTo(Saving(testFo r m)) asse r tThat( r esult.effects).containsExactly( Effect.SaveP r of i le(testFo r m) ) }
100. Testing TESTING: NO MOCKS NEEDED @Test fun `submit while Saving is igno r ed`() { val state = Sc r eenState.Saving(fo r m = testFo r m) val r esult = state.onSubmit() asse r tThat( r esult.newState).isEqualTo(Saving(testFo r m)) asse r tThat( r esult.effects).isEmpty() }
101. Avoiding AntiPatterns
102. Avoiding AntiPatterns Sn a ckb a r Shows Twice // Bad: ❌ State-based one-time event val error by viewModel.error.collectAsState() LaunchedE ff ect(error) { error?.let { snackbarHostState.showSnackbar(it) // Rotation → resubscribes → shows again } }
103. Avoiding AntiPatterns Sn a ckb a r Shows Twice // Good: ✅ E ff ect with unique ID data class ShowSnackbar( val message: String, val id: UUID = UUID.randomUUID() ) LaunchedE ff ect(e ff ect?.id) { e ff ect?.let { snackbarHostState.showSnackbar(it.message) viewModel.consumeE ff ect() } }
104. Avoiding AntiPatterns Forgotten Remember / / Bad: ❌ New instance eve r y r ecomposition @Composable fun Counte r () { va r count = mutableStateOf(0) / / No r emembe r ! Button(onClick = { count.value + + }) { Text("Count: ${count.value}") / / Always 0 } }
105. Avoiding AntiPatterns / / Good:✅ Su r vives r ecomposition @Composable fun Counte r () { va r count by r emembe r { mutableStateOf(0) } Button(onClick = { count + + }) { Text("Count: $count") } } Forgotten Remember
106. Avoiding AntiPatterns B a ckw a rds Write = In f inite Loop / / Bad:❌ W r ite afte r r ead = inf i nite r ecomposition @Composable fun B r oken() { va r count by r emembe r { mutableStateOf(0) } Text("$count") / / Read count + + / / W r ite → t r igge r s r ecomposition → r epeat }
107. Avoiding AntiPatterns / / Good: ✅ W r ite only in callbacks @Composable fun Fixed() { va r count by r emembe r { mutableStateOf(0) } Text("$count") Button(onClick = { count + + }) { / / W r ite in event Text("Inc r ement") } } Forgotten Remember
108. Avoiding AntiPatterns derivedSt a teOf Misue / / Useless: ❌ Output changes as often as input - no benef i t va r f i r stName by r emembe r { mutableStateOf("") } va r lastName by r emembe r { mutableStateOf("") } val fullName by r emembe r { de r ivedStateOf { "$f i r stName $lastName" } / / Pointless }
109. Avoiding AntiPatterns derivedSt a teOf Misue / / ✅ Just compute it val fullName = "$f i r stName $lastName" Co r r ect use: / / ✅ Output changes LESS than input val listState = r emembe r LazyListState() val showButton by r emembe r { de r ivedStateOf { listState.f i r stVisibleItemIndex > 0 } } / / Sc r oll position changes eve r y pixel / / Boolean changes r a r ely
110. Avoiding AntiPatterns collectAsSt a te vs collectAsSt a teWithLifecycle Wasteful: / / ❌ Keeps collecting when app backg r ounded val state by viewModel.state.collectAsState() Bette r : / / ✅ Lifecycle - awa r e - pauses when STOPPED val state by viewModel.state.collectAsStateWithLifecycle() collectAsState: App backg r ounded → still collecting → batte r y d r ain collectAsStateWithLifecycle: App backg r ounded → paused → r esumes when visible
111. Avoiding AntiPatterns Unst a ble L a mbd a C a ptures / / Bad: ❌ New lambda eve r y r ecomposition @Composable fun Pa r ent() { va r text by r emembe r { mutableStateOf("") } Child(onClick = { doSomething() }) / / New instance each time TextField(value = text, onValueChange = { text = it }) } / / TextField updates → Pa r ent r ecomposes → new lambda → Child r ecomposes
112. Avoiding AntiPatterns Unst a ble L a mbd a C a ptures / / Good: ✅ Stable lambda @Composable fun Pa r ent() { va r text by r emembe r { mutableStateOf("") } val onClick = r emembe r { { doSomething() } } Child(onClick = onClick) / / Same instance TextField(value = text, onValueChange = { text = it }) }
113. Climax Four Rules Sep a r a te Model Emit Test
114. Climax Four Rules Sep a r a te Model Emit Test State lives where its lifecycle matches
115. Climax Four Rules Sep a r a te Model Emit Test Sealed inte rf aces, not boolean bags State lives where its lifecycle matches
116. Climax Four Rules Sep a r a te Model Emit Test Sealed inte rf aces, not boolean bags State lives where its lifecycle matches E ff ects as outputs, not inline code
117. Climax Four Rules Sep a r a te Model Emit Test Sealed inte rf aces, not boolean bags State lives where its lifecycle matches E ff ects as outputs, not inline code Pure functions, no mocks
119. Thats all folks! https:/ /linktr.ee/aldefy