decompose-component

You are helping write a Decompose component. Follow these patterns exactly.

Component Structure

Always use interface + DefaultXxxComponent. Never extend a library base class.

interface CounterComponent {
    val model: Value<Model>
    fun onIncrementClicked()

    data class Model(val count: Int = 0)
}

class DefaultCounterComponent(
    componentContext: ComponentContext,
    private val onFinished: () -> Unit,          // callbacks to parent go via constructor
) : CounterComponent, ComponentContext by componentContext {   // <-- delegation, not inheritance

    private val _model = MutableValue(CounterComponent.Model())
    override val model: Value<CounterComponent.Model> = _model

    override fun onIncrementClicked() {
        _model.update { it.copy(count = it.count + 1) }
    }
}

Rules:

• ComponentContext by componentContext — always delegate, never extend
• Constructor receives componentContext: ComponentContext as first parameter
• Parent callbacks (navigation, results) come via constructor lambdas
• Expose Value<T> (immutable), hold MutableValue<T> privately
• MutableValue.update { } for state mutations — call only on the main thread

Value vs Other State Holders

Value<T> is Decompose's multiplatform observable. Prefer it for cross-platform components.

// Good — works on all platforms, observable in Compose/SwiftUI/React
val state: Value<State> = _state

// Also acceptable if you're coroutines-only (Android/JVM):
val state: StateFlow<State>

Value is NOT a coroutine — no collect, use subscribe/subscribeAsState() in Compose.

Lifecycle

Components get lifecycle automatically. Subscribe only in init or lifecycle callbacks.

class DefaultSomeComponent(componentContext: ComponentContext) : ComponentContext by componentContext {
    init {
        lifecycle.doOnStart { /* start polling */ }
        lifecycle.doOnStop { /* stop polling */ }
        lifecycle.doOnDestroy { /* cleanup */ }
    }
}

Lifecycle states: INITIALIZED → CREATED → STARTED → RESUMED → STOPPED → DESTROYED

• Active component is RESUMED
• Back-stack components are CREATED (still alive, stopped)

State Preservation (survives process death + config changes)

@Serializable
private data class State(val query: String = "", val selectedId: Long? = null)

class DefaultSearchComponent(componentContext: ComponentContext) : ComponentContext by componentContext {
    private var state: State by saveable(serializer = State.serializer(), init = ::State)
}

Manual version:

private var state = stateKeeper.consume("STATE", State.serializer()) ?: State()
init {
    stateKeeper.register("STATE", State.serializer()) { state }
}

Rules: @Serializable required, keep state small (<500KB on Android), consume() only once.

Instance Retaining (survives config changes, like ViewModel)

class DefaultTimerComponent(componentContext: ComponentContext) : ComponentContext by componentContext {

    private val timer = retainedInstance { Timer() }

    private class Timer : InstanceKeeper.Instance {
        override fun onDestroy() {}
    }
}

Rules: NOT inner class, no Activity/Context/View references, implement onDestroy().

Combining state preservation + instance retaining

class DefaultComponent(componentContext: ComponentContext) : ComponentContext by componentContext {

    private val logic by saveable(serializer = Logic.State.serializer(), state = { it.state }) { savedState ->
        retainedInstance { Logic(savedState) }
    }

    private class Logic(savedState: Logic.State?) : InstanceKeeper.Instance {
        var state = savedState ?: Logic.State()
            private set
        @Serializable data class State(val items: List<String> = emptyList())
        override fun onDestroy() {}
    }
}

Back Button Handling

class DefaultEditorComponent(componentContext: ComponentContext) : ComponentContext by componentContext {

    private val backCallback = BackCallback(isEnabled = false) {
        showDiscardDialog()
    }

    init { backHandler.register(backCallback) }

    fun onFormChanged() { backCallback.isEnabled = true }
}

Priority: last-registered wins. Use priority = Int.MAX_VALUE to always intercept first.

Preview / Test Doubles

class PreviewCounterComponent : CounterComponent {
    override val model: Value<Model> = MutableValue(Model(count = 42))
    override fun onIncrementClicked() {}
}

internal val PreviewContext: ComponentContext = DefaultComponentContext(LifecycleRegistry())

Critical Warnings

• Root component must be created on the UI/Main thread, NEVER inside a @Composable function
• defaultComponentContext() must be called only once per Activity/Fragment lifetime
• retainedComponent() has the same restriction — call only once in onCreate
• Navigate only on Main thread — background navigation causes crashes
• MutableValue: subscribe and update only on the main thread — off-thread calls cause race conditions
• Don't make retained inner classes — captures outer component → memory leak

Quick Reference

Need	API
Component context delegation	class Foo(ctx: ComponentContext) : ComponentContext by ctx
Observable state	MutableValue<T> / Value<T>
Update state	_state.update { it.copy(...) }
Survive config change	retainedInstance { }
Survive process death	saveable(serializer = ...) or stateKeeper
Lifecycle events	lifecycle.doOnStart/Stop/Destroy { }
Custom back handling	backHandler.register(BackCallback { })
Auto back in navigation	handleBackButton = true in childStack()/childSlot()