Legado Compose Migration Project Patterns

Existing Shape

App module: app.
Main source root: app/src/main/java/io/legado/app.
UI package: ui/..., usually grouped by feature.
Data package: data/dao, data/entities, data/repository.
Domain package: domain/gateway, domain/model, domain/repository, domain/usecase.
DI: di/appModule.kt, using Koin singleOf, viewModelOf, and parameterized viewModel { ... }.
Main navigation: ui/main/MainActivity.kt, using Navigation3 NavKey, rememberNavBackStack, entryProvider, and NavDisplay, plus legacy route extras such as EXTRA_START_ROUTE.
Base Compose host: base/BaseComposeActivity.kt, which wraps Content() in AppTheme, configures system bars, locale/font, background image, and LiveBus recreation events.
Compose dependencies are already enabled in app/build.gradle.kts; do not add new UI libraries unless the target screen truly requires one.

Files to Inspect Before Migrating

Inspect these as local examples, not as APIs to copy blindly:

ui/book/info/BookInfoActivity.kt
ui/book/info/BookInfoContract.kt
ui/book/info/BookInfoViewModel.kt
ui/book/info/BookInfoScreen.kt
ui/book/search/SearchContract.kt
ui/book/search/SearchViewModel.kt
ui/book/search/SearchScreen.kt
ui/main/MainActivity.kt
ui/main/MainScreen.kt
ui/widget/components/AppScaffold.kt
ui/widget/components/list/ListScaffold.kt
ui/theme/LegadoTheme.kt
ui/theme/AppTheme.kt

Recommended Feature Layout

For a migrated feature, prefer colocated files:

ui/<area>/<feature>/
  FeatureActivity.kt       # only for legacy Intent compatibility or full-screen migration host
  FeatureContract.kt
  FeatureViewModel.kt
  FeatureScreen.kt
  FeatureSheets.kt        # only if sheets are substantial
  FeatureDialogs.kt       # only if dialogs are substantial

Use the smallest set of files. Do not split files just to match this shape.

MVI and UDF Rules

Use this shape for behavior-heavy screens:

data class FeatureUiState(
    val isLoading: Boolean = false,
    val items: List<Item> = emptyList(),
    val dialog: FeatureDialog? = null,
    val sheet: FeatureSheet = FeatureSheet.None,
)

sealed interface FeatureIntent {
    data object BackPressed : FeatureIntent
    data object Refresh : FeatureIntent
    data class ItemClick(val id: Long) : FeatureIntent
}

sealed interface FeatureEffect {
    data object Finish : FeatureEffect
    data class OpenDetail(val id: Long) : FeatureEffect
}

ViewModel rules:

Keep _uiState = MutableStateFlow(FeatureUiState()) private and expose uiState = _uiState.asStateFlow().
Keep _effects = MutableSharedFlow<FeatureEffect>(extraBufferCapacity = 8) private and expose effects = _effects.asSharedFlow().
Prefer fun onIntent(intent: FeatureIntent) as the UI entry point.
Treat FeatureIntent as an MVI/user-action type. When Android Intent launch/extras are also involved, name variables and explanations clearly enough that the two concepts are not confused.
Use _uiState.update { it.copy(...) } for state changes.
Emit one-shot work through effects, not booleans in state.
Keep cached current entities private in the ViewModel when existing project behavior needs mutation or incremental sync, but publish immutable render state.
Use existing BaseViewModel.execute { ... }.onSuccess { ... }.onError { ... } when the surrounding ViewModel already uses that pattern.

Activity rules:

Prefer MainActivity as the owner of Compose-first navigation. Add a Navigation3 route/key and entryProvider entry there for new destinations.
Extend BaseComposeActivity for migrated Activity screens only when a standalone Activity remains necessary.
Keep a legacy Activity for a migrated screen only when unreworked View code still calls it through Intent; in that case, make the Activity a thin compatibility host that translates extras/results and delegates to Compose state/effects.
Keep Activity Result launchers, file openers, clipboard, permission APIs, Android DialogFragments, and framework navigation in the Activity.
In Content(), collect state with collectAsStateWithLifecycle().value.
Collect effects in LaunchedEffect(Unit) { viewModel.effects.collectLatest { ... } }.
Pass only state and callbacks into the screen.

Navigation rules:

For new Compose-first destinations, add the route to MainActivity instead of adding a new Activity.
Follow MainActivity's current route style: serializable NavKey routes for in-process navigation and launcher Intent helper methods only when external or legacy callers need a stable entry point.
Keep legacy extra names stable when replacing an old Activity entry point.
When a compatibility Activity is retained, let it translate old Intent inputs/results; do not make it the source of truth for navigation or business behavior.

Screen rules:

Make FeatureScreen(state, onIntent, ...) stateless for business state.
Use remember and rememberSaveable only for local UI affordances such as menu expansion, scroll state, transient animation state, and text field drafts when committing through an intent.
Use BackHandler to route back actions through FeatureIntent.BackPressed when ViewModel decisions matter.
Keep expensive derived values in remember(key) or ViewModel state when they depend on repository data.
Prefer LazyColumn, LazyRow, stable keys, and existing fast-scroller/list components for adapter migrations.

Clean Architecture Boundaries

This project is mid-migration, so use Clean Architecture pragmatically. For newly created screens, default to standard modern Android architecture and avoid mixed legacy patterns unless they are needed to interoperate with View screens that have not been rewritten.

For new screens, keep UI, presentation, domain, and data boundaries explicit: Compose renders state, ViewModel owns state and intent handling, usecases hold reusable business actions, repositories mediate data access.
For new screens, do not place business rules in Activity/Fragment or composables.
For new screens, do not read DAOs directly from UI/presentation unless the project already has no reasonable repository/usecase boundary and adding one would be disproportionate.
For migrations, use existing DAOs/repositories directly if the old ViewModel already does and the migration is UI-only.
Prefer existing domain/usecase classes for reusable business actions such as cache, delete, group update, startup maintenance, reading progress, and WebDAV flows.
Add a usecase when a new screen has meaningful business rules, when logic would otherwise be duplicated across features, or when a UI migration needs to extract meaningful business rules from an Activity/adapter.
Keep UI-only formatting and Compose layout decisions out of domain.
Keep entity-to-display mapping near the UI or ViewModel unless it is reused business language.

Compose Components and Theme

Prefer project components before raw Material widgets:

Scaffold/top bars: AppScaffold, GlassTopAppBar..., DynamicTopAppBar, TopBarActionButton, TopBarNavigationButton.
Lists: ListScaffold, TopFloatingStickyItem, VerticalFastScroller, SelectionBottomBar.
Dialog/sheet: AppAlertDialog, AppModalBottomSheet, OptionSheet, text list input components.
Settings: ClickableSettingItem, SwitchSettingItem, SliderSettingItem, ListSettingItem, SettingCard.
Buttons/chips: AppIconButton, SmallIconButton, SmallTextButton, ToggleChip, AlertButton.
Covers/images: CoilBookCover, BookshelfCover, buildCoverImageRequest; use Coil for Compose image loading.
Theme: LegadoTheme.colorScheme, LegadoTheme.typography, ThemeResolver, ProvideThemeOverride where existing feature behavior needs theme override.

Avoid introducing a second visual system. If a raw Material 3 component is used, style it from LegadoTheme.

Migration Checklist

Before editing:

Identify old XML layouts, ViewBinding fields, adapters, decorations, menu XML, dialogs, and result contracts.
Capture behavior: empty/loading/error states, refresh, search, sort, selection, long click, swipe, menu actions, back behavior, result codes, events, and persisted preferences.
Decide whether the task is UI-only or needs domain extraction.
For new screens, explicitly decide the presentation/domain/data boundary before coding; do not default to legacy mixed architecture.

During editing:

Create/update Contract, then ViewModel, then Screen, then MainActivity route or compatibility host.
For new Compose destinations, wire navigation through MainActivity unless a legacy Intent caller must remain supported.
When retaining an Activity for compatibility, keep it small: parse legacy extras, collect effects, bridge result codes, and avoid placing feature business logic there.
Reuse existing string resources and add new resources for new user-facing text.
Register the ViewModel in di/appModule.kt.
Remove only resources made obsolete by this migration.
Keep existing public intent extra names and result codes unless explicitly changing an API.

Verification:

Run .\gradlew.bat :app:compileAppDebugKotlin for Kotlin-only changes.
Run .\gradlew.bat :app:assembleAppDebug when resources, manifests, XML deletion, or generated bindings are affected.
If a screen uses Room queries or migration-sensitive repositories, run the relevant existing Android/unit tests if practical.
For manual verification, open the migrated screen and compare core flows against the old behavior list.

Common Pitfalls

Do not put navigation or Activity launchers inside composables.
Do not store one-time navigation as nullable fields in UiState unless the existing local pattern already does; prefer SharedFlow effects.
Do not keep adapter selection state split between old adapter and new Compose state.
Do not delete XML/menu resources until all references and generated binding imports are gone.
Do not add speculative domain layers for a single migrated screen.
Do not use mixed architecture for new screens unless compatibility with unreworked View UI requires it.
Do not make a migrated screen's retained Activity the primary navigation model when MainActivity can own the Compose destination.
Do not bypass BaseComposeActivity unless the screen must remain inside a Fragment or legacy host.
Do not replace existing event bus or service interactions as part of a UI migration unless required for correctness.

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