Breaking Up with Big ViewModels — Without Breaking Your Architecture (droidcon Berlin 2025)

In the past, Android developers struggled with bloated Activities and Fragments. Then MVVM, MVI, and Jetpack Compose helped us push presentation logic into ViewModels, business logic into UseCases, and data handling into Repositories — making the UI layer thin and reactive.

But a new problem has quietly emerged: we still tie ViewModels to entire screens, exposing the whole screen state and handling all user intents. As screens grow more complex, ViewModels become longer, harder to test, and harder to maintain.

What if we had multiple ViewModels per screen, each tied to a smaller UI component? This leads to two important questions that haven't been answered yet:

- Where do you hold the common state that multiple UI components of a screen might need?
- How can ViewModels communicate with each other when one needs to update the UI state that another ViewModel observes?

Repositories aren’t the answer — they expose domain models and shouldn’t know anything about the UI. Other state holder patterns that have been proposed do not fully address this problem either — they typically manage global or domain state, and they aren't designed to coordinate shared, screen-specific UI state across multiple ViewModels. Clearly, there’s a missing piece in our architecture.

Introducing the Mediator Pattern — a pattern that allows you to break a large ViewModel into smaller ones by moving the screen-specific UI state into a Mediator, with each smaller ViewModel deriving and updating only the part it needs.

Join this session to see the pattern applied in a real-world Android app and learn how to:

- Break a large ViewModel into smaller, independently testable ViewModels, each tied to a smaller component of the screen.
- Enable the smaller, screen-scoped ViewModels to reactively communicate with each other through a shared Mediator.
- Use Hilt or Koin to scope the Mediator to the screen’s lifecycle, automatically disposing it when the user exits that screen.

Whether you’re working with Jetpack Compose or traditional Views, this talk will give you the missing piece your architecture needs — so you can grow your apps without growing complexity.

Transcript

1. Breaking Up with Big ViewModels Without Breaking Your Architecture Stelios Frantzeskakis • Perry Street Software droidcon Berlin 2025
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5. Hi, I’m Stelios Frantzeskakis Staff Engineer at Perry Street Software Publisher of SCRUFF & Jack’d, serving more than 40M members Working with Android since the release of Android Gingerbread Passionate about Architecture & Testing @SteliosFran
6. Good-old Activities
7. Fragments
8. Architecture
9. Modern app architecture View ViewModel UseCase Repository Remote Data Source Local Data Source
10. Complexity per layer View ViewModel UseCase Repository Remote Data Source Local Data Source
11. Complexity per layer View ViewModel UseCase Repository Remote Data Source Local Data Source
12. Complexity per layer View ViewModel UseCase Repository Remote Data Source Local Data Source
13. Complexity per layer View ViewModel UseCase Repository Remote Data Source Local Data Source
14. Complexity per layer View ViewModel UseCase Repository Remote Data Source Local Data Source
15. We tie ViewModels to a whole screen
16. ViewModels handle user input and expose UI state
17. UseCases alone are not enough
18. Break apart your ViewModel!
21. GridViewModel TopBarViewModel BottomBarViewModel
22. GridViewModel TopBarViewModel BottomBarViewModel ? ?
23. ViewModels do not communicate with each other
24. The Mediator pattern Gang of Four, Design Patterns: Elements of Reusable Object-Oriented Software “An object that encapsulates how a set of other objects interact with each other.”
25. Modern app architecture revisited View ViewModel UseCase Repository Remote Data Source Local Data Source Mediator
26. GridViewModel GetMediaUseCase Mediator TopBarViewModel BottomBarViewModel
27. class GridViewModel( private val getMediaUseCase: GetMediaUseCase, private val gridMediator: GridMediator, ) : ViewModel() { val state: StateFlow<GridState> = gridMediator.state init { viewModelScope.launch(Dispatchers.Main) { getMediaUseCase() .map { GridState(GridMediaUiModelMapper.fromMediaList(it)) }.collect { newState -> gridMediator.updateState(newState) } } } fun onMediaLongTap(mediaId: String) { gridMediator.toggleSelection(mediaId) } fun onMediaTap(mediaId: String) { // ... } }
28. class GridViewModel( private val getMediaUseCase: GetMediaUseCase, private val gridMediator: GridMediator, ) : ViewModel() { val state: StateFlow<GridState> = gridMediator.state init { viewModelScope.launch(Dispatchers.Main) { getMediaUseCase() .map { GridState(GridMediaUiModelMapper.fromMediaList(it)) }.collect { newState -> gridMediator.updateState(newState) } } } fun onMediaLongTap(mediaId: String) { gridMediator.toggleSelection(mediaId) } fun onMediaTap(mediaId: String) { // ... } }
29. class GridViewModel( private val getMediaUseCase: GetMediaUseCase, private val gridMediator: GridMediator, ) : ViewModel() { val state: StateFlow<GridState> = gridMediator.state init { viewModelScope.launch(Dispatchers.Main) { getMediaUseCase() .map { GridState(GridMediaUiModelMapper.fromMediaList(it)) }.collect { newState -> gridMediator.updateState(newState) } } } fun onMediaLongTap(mediaId: String) { gridMediator.toggleSelection(mediaId) } fun onMediaTap(mediaId: String) { // ... } }
30. class GridMediator { private val _state = MutableStateFlow(GridState(emptyList())) val state: StateFlow<GridState> = _state.asStateFlow() fun updateState(newState: GridState) { _state.value = newState } fun toggleSelection(mediaId: String) { val currentState = _state.value val updatedMedia = currentState.media.map { media -> if (media.id == mediaId) { media.copy(selected = !media.selected) } else { media } } _state.value = currentState.copy(media = updatedMedia) } }
31. class GridMediator { private val _state = MutableStateFlow(GridState(emptyList())) val state: StateFlow<GridState> = _state.asStateFlow() fun updateState(newState: GridState) { _state.value = newState } fun toggleSelection(mediaId: String) { val currentState = _state.value val updatedMedia = currentState.media.map { media -> if (media.id == mediaId) { media.copy(selected = !media.selected) } else { media } } _state.value = currentState.copy(media = updatedMedia) } }
32. class TopBarViewModel( gridMediator: GridMediator, ) : ViewModel() { val selectedCount: Flow<Int> = gridMediator.state.map { state -> state.media.count { it.selected } } fun onSearchTap() { // ... } fun onCancelTap() { // ... } }
33. class TopBarViewModel( gridMediator: GridMediator, ) : ViewModel() { val selectedCount: Flow<Int> = gridMediator.state.map { state -> state.media.count { it.selected } } fun onSearchTap() { // ... } fun onCancelTap() { // ... } }
34. class BottomBarViewModel( gridMediator: GridMediator, ) : ViewModel() { val isVisible: Flow<Boolean> = gridMediator.state.map { state -> state.media.any { it.selected } } fun onCreateTap() { // ... } fun onShareTap() { // ... } fun onDeleteTap() { // ... } }
35. class BottomBarViewModel( gridMediator: GridMediator, ) : ViewModel() { val isVisible: Flow<Boolean> = gridMediator.state.map { state -> state.media.any { it.selected } } fun onCreateTap() { // ... } fun onShareTap() { // ... } fun onDeleteTap() { // ... } }
36. @Composable fun GridScreen(scope: Scope) { val gridViewModel: GridViewModel = koinViewModel(scope = scope) val gridState by gridViewModel.state.collectAsState() val topBarViewModel: TopBarViewModel = koinViewModel(scope = scope) val selectedCount by topBarViewModel.selectedCount.collectAsState(initial = 0) val bottomBarViewModel: BottomBarViewModel = koinViewModel(scope = scope) val isBottomBarVisible by bottomBarViewModel.isVisible.collectAsState(initial = false) AppTheme { Scaffold( topBar = { TopBar( selectedCount = selectedCount, onSearchTap = topBarViewModel::onSearchTap, onCancelTap = topBarViewModel::onCancelTap, ) }, bottomBar = { BottomBar( isVisible = isBottomBarVisible, onCreateTap = bottomBarViewModel::onCreateTap, onDeleteTap = bottomBarViewModel::onDeleteTap, ) }, ) { paddingValues -> Grid( state = gridState, modifier = Modifier.padding(paddingValues), onLongTap = gridViewModel::onMediaLongTap, ) } } }
37. GridViewModel TopBarViewModel BottomBarViewModel
38. GridViewModel TopBarViewModel BottomBarViewModel
39. Why not Repositories?
40. Repositories hold domain state, not UI state
41. Repository state is persistent, not temporary
42. Repositories are app-scoped, not feature-scoped
43. Dependency Injection
44. @Scope(name = Routes.GRID) @Scoped class GridMediator { private val _state = MutableStateFlow(GridState(emptyList())) val state: StateFlow<GridState> = _state.asStateFlow() // ... }
45. @KoinViewModel class GridViewModel( private val getMediaUseCase: GetMediaUseCase, private val gridMediator: GridMediator, ) : ViewModel() { // ... }
46. @Composable fun GridScreen(scope: Scope) { val gridViewModel: GridViewModel = koinViewModel(scope = scope) val gridState by gridViewModel.state.collectAsState() val topBarViewModel: TopBarViewModel = koinViewModel(scope = scope) val selectedCount by topBarViewModel.selectedCount.collectAsState(initial = 0) val bottomBarViewModel: BottomBarViewModel = koinViewModel(scope = scope) val isBottomBarVisible by bottomBarViewModel.isVisible.collectAsState(initial = false) // ... }
47. @Composable fun AppNavHost() { val navController = rememberNavController() NavHost( navController = navController, startDestination = Routes.GRID, ) { composable(Routes.GRID) { KoinScopedComposable(scopeName = Routes.GRID) { scope -> GridScreen(scope = scope) } } } }
48. @Composable fun GridScreen(scope: Scope) { val gridViewModel: GridViewModel = hiltViewModel() val gridState by gridViewModel.state.collectAsState() val topBarViewModel: TopBarViewModel = hiltViewModel() val selectedCount by topBarViewModel.selectedCount.collectAsState(initial = 0) val bottomBarViewModel: BottomBarViewModel = hiltViewModel() val isBottomBarVisible by bottomBarViewModel.isVisible.collectAsState(initial = false) // ... }
49. @HiltViewModel class GridViewModel @Inject constructor( private val getMediaUseCase: GetMediaUseCase, private val scopeHolder: ScopeHolder, @GridScopeId private val scopeId: String ) : ViewModel() { private val gridMediator by lazy { scopeHolder.getOrCreateMediator(scopeId) { GridMediator() } } // ... }
50. Testing
52. Test the behavior, not the implementation
53. Test the ViewModel, not the Mediator Martin Fowler, Refactoring: Improving the Design of Existing Code “Refactoring: A change made to the internal structure of software, without changing its observable behavior.”
54. class GridViewModelTest : BaseBehaviorSpec() { init { Given("I open the grid screen") { val gridViewModel: GridViewModel = gridScope.get() Then("I see a list of media") { with(gridViewModel.state.value) { media.size shouldBe 2 media[0].id shouldBeEqual "id1" media[0].title shouldBeEqual "Beautiful Landscape" media[1].id shouldBeEqual "id2" media[1].title shouldBeEqual "City View" } } } } }
55. class GridViewModelTest : BaseBehaviorSpec() { init { Given("I open the grid screen") { val gridViewModel: GridViewModel = gridScope.get() Then("I see a list of media") { with(gridViewModel.state.value) { media.size shouldBe 2 media[0].id shouldBeEqual "id1" media[0].title shouldBeEqual "Beautiful Landscape" media[1].id shouldBeEqual "id2" media[1].title shouldBeEqual "City View" } } } } }
56. class GridViewModelTest : BaseBehaviorSpec() { init { Given("I open the grid screen") { val gridViewModel: GridViewModel = gridScope.get() Then("I see a list of media") { with(gridViewModel.state.value) { media.size shouldBe 2 media[0].id shouldBeEqual "id1" media[0].title shouldBeEqual "Beautiful Landscape" media[1].id shouldBeEqual "id2" media[1].title shouldBeEqual "City View" } } } } }
57. class TopBarViewModelTest : BaseBehaviorSpec() { init { Given("I open the grid screen") { val gridViewModel: GridViewModel = gridScope.get() val topBarViewModel: TopBarViewModel = gridScope.get() Then("The top bar does not show any selected items”) { topBarViewModel.selectedCount.first() shouldBe 0 } When("I long tap on a media item") { gridViewModel.onMediaLongTap("id1") Then("The top bar shows 1 selected item") { topBarViewModel.selectedCount.first() shouldBe 1 } } } } }
58. class TopBarViewModelTest : BaseBehaviorSpec() { init { Given("I open the grid screen") { val gridViewModel: GridViewModel = gridScope.get() val topBarViewModel: TopBarViewModel = gridScope.get() Then("The top bar does not show any selected items”) { topBarViewModel.selectedCount.first() shouldBe 0 } When("I long tap on a media item") { gridViewModel.onMediaLongTap("id1") Then("The top bar shows 1 selected item") { topBarViewModel.selectedCount.first() shouldBe 1 } } } } }
59. class TopBarViewModelTest : BaseBehaviorSpec() { init { Given("I open the grid screen") { val gridViewModel: GridViewModel = gridScope.get() val topBarViewModel: TopBarViewModel = gridScope.get() Then("The top bar does not show any selected items”) { topBarViewModel.selectedCount.first() shouldBe 0 } When("I long tap on a media item") { gridViewModel.onMediaLongTap("id1") Then("The top bar shows 1 selected item") { topBarViewModel.selectedCount.first() shouldBe 1 } } } } }
60. class TopBarViewModelTest : BaseBehaviorSpec() { init { Given("I open the grid screen") { val gridViewModel: GridViewModel = gridScope.get() val topBarViewModel: TopBarViewModel = gridScope.get() Then("The top bar does not show any selected items”) { topBarViewModel.selectedCount.first() shouldBe 0 } When("I long tap on a media item") { gridViewModel.onMediaLongTap("id1") Then("The top bar shows 1 selected item") { topBarViewModel.selectedCount.first() shouldBe 1 } } } } }
61. class TopBarViewModelTest : BaseBehaviorSpec() { init { Given("I open the grid screen") { val gridViewModel: GridViewModel = gridScope.get() val topBarViewModel: TopBarViewModel = gridScope.get() Then("The top bar does not show any selected items”) { topBarViewModel.selectedCount.first() shouldBe 0 } When("I long tap on a media item") { gridViewModel.onMediaLongTap("id1") Then("The top bar shows 1 selected item") { topBarViewModel.selectedCount.first() shouldBe 1 } } } } }
62. // ... And("I long tap on another media item") { gridViewModel.onMediaLongTap("id2") Then("The top bar shows 2 selected items") { topBarViewModel.selectedCount.first() shouldBe 2 } And("I long tap again on the same media items") { gridViewModel.onMediaLongTap("id1") gridViewModel.onMediaLongTap("id2") Then("The top bar does not show any selected items”) { topBarViewModel.selectedCount.first() shouldBe 0 } } }
63. // ... And("I long tap on another media item") { gridViewModel.onMediaLongTap("id2") Then("The top bar shows 2 selected items") { topBarViewModel.selectedCount.first() shouldBe 2 } And("I long tap again on the same media items") { gridViewModel.onMediaLongTap("id1") gridViewModel.onMediaLongTap("id2") Then("The top bar does not show any selected items”) { topBarViewModel.selectedCount.first() shouldBe 0 } } }
64. // ... And("I long tap on another media item") { gridViewModel.onMediaLongTap("id2") Then("The top bar shows 2 selected items") { topBarViewModel.selectedCount.first() shouldBe 2 } And("I long tap again on the same media items") { gridViewModel.onMediaLongTap("id1") gridViewModel.onMediaLongTap("id2") Then("The top bar does not show any selected items”) { topBarViewModel.selectedCount.first() shouldBe 0 } } }
65. // ... And("I long tap on another media item") { gridViewModel.onMediaLongTap("id2") Then("The top bar shows 2 selected items") { topBarViewModel.selectedCount.first() shouldBe 2 } And("I long tap again on the same media items") { gridViewModel.onMediaLongTap("id1") gridViewModel.onMediaLongTap("id2") Then("The top bar does not show any selected items”) { topBarViewModel.selectedCount.first() shouldBe 0 } } }
66. class BottomBarViewModelTest : BaseBehaviorSpec() { init { Given("I open the grid screen") { val gridViewModel: GridViewModel = gridScope.get() val bottomBarViewModel: BottomBarViewModel = gridScope.get() Then("The bottom bar is hidden") { bottomBarViewModel.isVisible.first() shouldBe false } } } }
67. class BottomBarViewModelTest : BaseBehaviorSpec() { init { Given("I open the grid screen") { val gridViewModel: GridViewModel = gridScope.get() val bottomBarViewModel: BottomBarViewModel = gridScope.get() Then("The bottom bar is hidden") { bottomBarViewModel.isVisible.first() shouldBe false } } } }
68. class BottomBarViewModelTest : BaseBehaviorSpec() { init { Given("I open the grid screen") { val gridViewModel: GridViewModel = gridScope.get() val bottomBarViewModel: BottomBarViewModel = gridScope.get() Then("The bottom bar is hidden") { bottomBarViewModel.isVisible.first() shouldBe false } } } }
69. // ... When("I long tap on a media item") { gridViewModel.onMediaLongTap("id1") Then("The bottom bar is visible") { bottomBarViewModel.isVisible.first() shouldBe true } And("I long tap again on the same media item") { gridViewModel.onMediaLongTap("id1") Then("The bottom bar is hidden") { bottomBarViewModel.isVisible.first() shouldBe false } } }
70. // ... When("I long tap on a media item") { gridViewModel.onMediaLongTap("id1") Then("The bottom bar is visible") { bottomBarViewModel.isVisible.first() shouldBe true } And("I long tap again on the same media item") { gridViewModel.onMediaLongTap("id1") Then("The bottom bar is hidden") { bottomBarViewModel.isVisible.first() shouldBe false } } }
71. // ... When("I long tap on a media item") { gridViewModel.onMediaLongTap("id1") Then("The bottom bar is visible") { bottomBarViewModel.isVisible.first() shouldBe true } And("I long tap again on the same media item") { gridViewModel.onMediaLongTap("id1") Then("The bottom bar is hidden") { bottomBarViewModel.isVisible.first() shouldBe false } } }
72. // ... When("I long tap on a media item") { gridViewModel.onMediaLongTap("id1") Then("The bottom bar is visible") { bottomBarViewModel.isVisible.first() shouldBe true } And("I long tap again on the same media item") { gridViewModel.onMediaLongTap("id1") Then("The bottom bar is hidden") { bottomBarViewModel.isVisible.first() shouldBe false } } }
73. Enforcing the Mediator pattern
74. Code reviews
75. Code reviews ❌
76. Lint rules!
78. Konsist & Harmonize: Write lint rules as unit tests
79. class ViewModelLintRules : BaseBehaviorSpec() { init { Given("A ViewModel") { val viewModels = // ... Then("It exposes only one state”) { viewModels.assertTrue { // ... } } Then("It has maximum 5 dependencies") { viewModels.assertTrue { // ... } } } } }
80. class MediatorLintRules : BaseBehaviorSpec() { init { Given("A mediator") { val mediators = // ... Then("It is injected only in ViewModels") { mediators.assertTrue { // ... } } } } }
81. Resources Slide deck • https://speakerdeck.com/steliosf/breaking-up-with-big-viewmodels GitHub • https://github.com/perrystreetsoftware/breaking-up-with-big-viewmodels Put Your Tests on a Diet talk • https://www.steliosf.com/talks Start Enforcing with Lint Rules talk • https://www.steliosf.com/talks Konsist • https://github.com/LemonAppDev/konsist Harmonize • https://github.com/perrystreetsoftware/Harmonize Perry Street Engineering • https://medium.com/perry-street-software-engineering @SteliosFran • https://www.linkedin.com/in/SteliosFran
82. No more big ViewModels!
83. Nine hundred lines in one single file Just scrolling through will take a while Break that ViewModel in smaller parts Trust the Mediator to hold their hearts