Flowmvi

Contract and state design

State

• Make state immutable and comparable. Use data class / data object.
• Use sealed interfaces for LCE or multi-branch screens. Avoid placeholder fields.
• Use EmptyState if no state is needed.

Example (state family):

sealed interface CounterState : MVIState {
    data object Loading : CounterState
    data class Content(val count: Int) : CounterState
    data class Error(val cause: Exception) : CounterState
}

Intent and action

• MVIIntent: inputs/events. Use sealed interfaces for MVI style.
• MVIAction: one-off effects, never rely on them for critical logic.
• If no actions, use Nothing or the no-action store overload.

sealed interface CounterIntent : MVIIntent {
    data object Tap : CounterIntent
    data class Set(val value: Int) : CounterIntent
}

sealed interface CounterAction : MVIAction {
    data class ShowToast(val text: String) : CounterAction
}

Store creation patterns

Container pattern

Prefer a container per screen/feature and expose the store as a property.

class CounterContainer : Container<CounterState, CounterIntent, CounterAction> {
    override val store = store(initial = CounterState.Loading) {
        configure { debuggable = BuildFlags.debuggable; name = "CounterStore" }
        reduce { intent ->
            when (intent) {
                is CounterIntent.Set -> updateState<_, CounterState.Content> { copy(count = intent.value) }
                is CounterIntent.Tap -> action(CounterAction.ShowToast("tap"))
            }
        }
    }
}

Use lazyStore when construction is heavy or should be deferred.

MVVM+ Intents

Intents can be lambdas (Orbit MVI-style):

// contract
typealias CounterIntent = LambdaIntent<CounterState, CounterAction>

// sending
fun onTap() = store.intent { updateState<State.Content, _> { copy(count = count + 1) } }

When using LambdaIntent:

• Mandatory: Install reduceLambdas plugin to invoke intent blocks.
• Prefer ImmutableStore/ImmutableContainer at the edges to avoid leaking context.
• Plugins lose observability into intent contents, prefer regular intents unless codebase already uses lambdas.

State management (SST)

FlowMVI serializes state transactions by default.

Use:

• updateState { } for atomic updates.
• withState { } for safe reads.
• Typed overloads for state families: updateState<T, R> { } and withState<T, R> { } (note always two parameters, of which first is target state, and second is always underscore _). Also these need an import.
• updateStateImmediate only for performance-critical hot paths; it bypasses plugins and safety.

PipelineContext API

You get PipelineContext receiver inside most of the store DSLs and plugin callbacks. Key operations:

• updateState { }, withState { }, updateStateImmediate { }
• action(action) and intent(intent)
• config for store config and logging
• Flow.consume(context) for safe collection in store context

Exact signatures in references/api-signatures.md.

Store configuration

Configure inside store { configure { ... } }. Defaults are defined in StoreConfigurationBuilder.

Key flags:

• debuggable: enable validations + verbose logging.
• name: used for logging/debugging and store identity.
• parallelIntents: process intents concurrently (state safety becomes critical).
• coroutineContext: merged with start scope.
• actionShareBehavior: Distribute, Share, Restrict, Disabled.
• intentCapacity + onOverflow: buffer sizing and backpressure.
• stateStrategy: Atomic(reentrant = true/false) or Immediate.
• allowIdleSubscriptions, allowTransientSubscriptions.
• logger, verifyPlugins.

See exact defaults in references/api-signatures.md and use references/docs-index.md for the latest state docs.

Exact signatures: references/api-signatures.md.

Plugins (system)

Plugins are the core of FlowMVI. Order matters.

• Install plugins in the order you want them to intercept events.
• Plugins can consume, modify, or veto intents/actions/states.
• Use reduce early if you want later plugins to observe processed intents.

All plugin signatures are in references/plugin-signatures.md. All callback signatures and behavior are in references/plugin-callbacks.md.

Prebuilt plugin catalog

Open references/docs-index.md and references/plugin-signatures.md for the full list. Core plugins include:

• reduce, init, asyncInit, recover, whileSubscribed
• enableLogging
• cache / asyncCache
• manageJobs
• awaitSubscribers
• undoRedo
• disallowRestart
• timeTravel
• consumeIntents
• deinit, resetStateOnStop
• saved state plugins (savedstate module)
• remote debugger plugins (debugger modules)

Creating custom plugins

Use the plugin DSL to intercept store events. Use lazyPlugin when you need config at creation time.

val analytics = plugin<State, Intent, Action> {
    onIntent { intent -> log { "intent = $intent" }; intent }
}

Rules:

• Do not call updateState inside onState to avoid loops.
• Never throw from onUndeliveredIntent/onUndeliveredAction.

Use references/plugin-callbacks.md for full behavior rules.

Decorators (plugins for plugins)

Decorators wrap plugins and can short-circuit the entire chain.

• decorators run after plugins; they wrap all previously installed plugins.
• If you do not invoke child.onX(...) in a decorator, the chain stops there.
• Decorators return final values, not intermediate chain values.

Use decorators only when:

• You need to instrument, retry, debounce, or batch the whole chain.
• You want cross-cutting behavior that is hard to express as a plugin without chain control.

Child stores

Tie lifecycle of child stores to a parent store:

val child = store(ChildState.Loading) { /* ... */ }
val parent = store(ParentState.Loading) {
    this hasChild child
}

Delegation

Delegate state/actions from another store and project them:

val feedState by delegate(feedStore) { action -> /* handle */ }
whileSubscribed { feedState.collect { state -> /* render */ } }

Use DelegationMode.Immediate when you need always-hot projections. Default WhileSubscribed mode yields stale projections when no subscribers.

Prefer building tree-like store hierarchies using children and delegates for complex business logic.

Other

Use savedstate module for persistence, metrics module to set up metrics, and essenty for decompose integration.

More info in references/docs-index.md