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Mohit Sarveiya Building State Flows with Compose @heyitsmohit

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Building State Flows with Compose ● How to setup and use Molecule

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Building State Flows with Compose ● How to setup and use Molecule ● Design Patterns

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Building State Flows with Compose ● How to setup and use Molecule ● Design Patterns ● Testing with Molecule

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Building State Flows with Compose ● How to setup and use Molecule ● Design Patterns ● Testing with Molecule ● Molecule Internals

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Building State Flows with Compose

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Building State Flows with Compose

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Compose vs Compose UI

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Compose ● General purpose tool for managing tree of nodes.

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Compose ● General purpose tool for managing tree of nodes.

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Compose ● General purpose tool for managing tree of nodes. Node can be anything

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Compose ● Compose Runtime ● Kotlin Complier Plugin ● State Snapshot System

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Compose UI ● UI Toolkit Views

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Molecule @Composable fun MoleculePresenter( events: Flow, ): Model

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Molecule @Composable fun MoleculePresenter( events: Flow, ): Model Compose Runtime

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Molecule @Composable fun MoleculePresenter( events: Flow, ): Model Compose Runtime State Flow 
 
 Flow

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val userFlow = db.userObservable() val balanceFlow = db.balanceObservable()

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@Composable fun ProfileScreen() { val user by userFlow.subscribeAsState(null) val balance by balanceFlow.subscribeAsState(0L) if (user == null) { Text("Loading…") } else { Text("${user.name} - $balance") } }

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@Composable fun ProfileScreen() { val user by userFlow.subscribeAsState(null) val balance by balanceFlow.subscribeAsState(0L) if (user == null) { Text("Loading…") } else { Text("${user.name} - $balance") } }

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@Composable fun ProfileScreen() { val user by userFlow.subscribeAsState(null) val balance by balanceFlow.subscribeAsState(0L) if (user == null) { Text("Loading…") } else { Text("${user.name} - $balance") } } Undesired coupling

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@Composable fun ProfileScreen() { val user by userFlow.subscribeAsState(null) val balance by balanceFlow.subscribeAsState(0L) if (user == null) { Text("Loading…") } else { Text("${user.name} - $balance") } } Not Reusable

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class ProfilePresenter(val db: Db) { fun transform(events: Flow) : Flow { } }

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class ProfilePresenter(val db: Db) { fun transform(events: Flow) : Flow { } } sealed interface ProfileModel { object Loading : ProfileModel data class Data( val name: String, val balance: Long, ): ProfileModel }

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class ProfilePresenter(val db: Db) { fun transform(events: Flow) : Flow { }

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class ProfilePresenter(val db: Db) { fun transform(events: Flow) : Flow { return combine( db.users().onStart { emit(null) }, db.balances().onStart { emit(0L) }, ) { user, balance -> } } }

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class ProfilePresenter(val db: Db) { fun transform(events: Flow) : Flow { return combine( db.users().onStart { emit(null) }, db.balances().onStart { emit(0L) }, ) { user, balance -> } } } Can get complex if we add more streams

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Molecule @Composable fun MoleculePresenter( ... ): Model Compose Runtime State Flow 
 
 Flow

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@Composable fun ProfilePresenter( userFlow: Flow, balanceFlow: Flow, ): ProfileModel { val user by userFlow.collectAsState(null) val balance by balanceFlow.collectAsState(0L) return if (user == null) { Loading } else { Data(user.name, balance) } }

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@Composable fun ProfilePresenter( userFlow: Flow, balanceFlow: Flow, ): ProfileModel { val user by userFlow.collectAsState(null) val balance by balanceFlow.collectAsState(0L) return if (user == null) { Loading } else { Data(user.name, balance) } }

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@Composable fun ProfilePresenter( userFlow: Flow, balanceFlow: Flow, ): ProfileModel { val user by userFlow.collectAsState(null) val balance by balanceFlow.collectAsState(0L) return if (user == null) { Loading } else { Data(user.name, balance) } }

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val scope = CoroutineScope(Main)

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val scope = CoroutineScope(Main) scope.launchMolecule( 
 clock = RecompositionClock.ContextClock 
 ) { ProfilePresenter(events, randomService) }

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val scope = CoroutineScope(Main) val models: StateFlow = scope.launchMolecule( 
 clock = RecompositionClock.ContextClock 
 ) { ProfilePresenter(events, randomService) }

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val scope = CoroutineScope(Main) val models: StateFlow = scope.launchMolecule( 
 clock = RecompositionClock.ContextClock 
 ) { ProfilePresenter(events, randomService) }

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val scope = CoroutineScope(Main) val models: StateFlow = scope.launchMolecule( 
 clock = RecompositionClock.ContextClock 
 ) { ProfilePresenter(events, randomService) }

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val scope = CoroutineScope(Main) 
 scope.launchMolecule( 
 clock = RecompositionClock.ContextClock, 
 emitter = { value -> } 
 ) { ProfilePresenter(events, randomService) }

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val scope = CoroutineScope(Main) 
 scope.launchMolecule( 
 clock = RecompositionClock.ContextClock, 
 emitter = { value -> } 
 ) { ProfilePresenter(events, randomService) } Will not respect back-pressure

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val models: Flow = moleculeFlow( 
 clock = RecompositionClock.ContextClock 
 ) { ProfilePresenter(events, randomService) }

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val scope = CoroutineScope(Main) 
 val models: StateFlow = scope.launchMolecule( 
 clock = RecompositionClock.ContextClock 
 ) { ProfilePresenter(events, randomService) } Frame Clock

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Composable Recomposition Enter the composition Leave the composition

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Composable Wait for next frame Enter the composition Leave the composition

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Composable Wait for next frame Enter the composition Leave the composition Monotonic Frame Clock

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Monotonic 
 Frame Clock Pull Based System Pull Signal

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Monotonic 
 Frame Clock Pull Based System Pull Signal Choreographer Performs Pull for UI

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val scope = CoroutineScope(Main) 
 
 fun CoroutineScope.launchMolecule( ... ) { with(this + clockContext) { 
 // Pass context element to Recomposer } }

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interface MonotonicFrameClock : CoroutineContext.Element { suspend fun withFrameNanos(onFrame): R override val key: CoroutineContext.Key <*> get() = Key companion object Key : CoroutineContext.Key }

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Recomposition Clocks ● Context clock ● Immediate

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Context Clock ● Use MonotonicFrameClock from calling Coroutine 
 
 Context ● Use Molecule with built in Android Frame Clock

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fun CoroutineScope.launchMolecule( ... ) { val clockContext = when (clock) { RecompositionClock.ContextClock -> EmptyCoroutineContext RecompositionClock.Immediate -> GatedFrameClock(this) } ... }

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fun CoroutineScope.launchMolecule( ... ) { val clockContext = when (clock) { RecompositionClock.ContextClock -> EmptyCoroutineContext RecompositionClock.Immediate -> GatedFrameClock(this) } ... }

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val dispatcher = TestCoroutineDispatcher() val clock = BroadcastFrameClock() 
 Frame clock provided by Compose

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val dispatcher = TestCoroutineDispatcher() val clock = BroadcastFrameClock() 
 val scope = CoroutineScope(dispatcher + clock)

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var value = 0 
 scope.launchMolecule(RecompositionClock.ContextClock, { value = it }) { var count by remember { mutableStateOf(0) } LaunchedEffect(Unit) { while (true) { delay(100) count ++ } } count }

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scope.launchMolecule( ... ) { var count by remember { mutableStateOf(0) } LaunchedEffect(Unit) { while (true) { delay(100) count ++ } } count }

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scope.launchMolecule( ... ) { var count by remember { mutableStateOf(0) } LaunchedEffect(Unit) { while (true) { delay(100) count ++ } } count }

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dispatcher.advanceTimeBy(100) assertEquals(1, value)

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scope.launchMolecule(RecompositionClock.ContextClock) { var count by remember { mutableStateOf(0) } LaunchedEffect(Unit) { while (true) { delay(100) count ++ } } count }

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scope.launchMolecule(RecompositionClock.ContextClock) { var count by remember { mutableStateOf(0) } LaunchedEffect(Unit) { while (true) { delay(100) count ++ } } count }

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assertEquals(0, value) Initial Composition

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assertEquals(0, value) 
 dispatcher.advanceTimeBy(100) 
 


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scope.launchMolecule(RecompositionClock.ContextClock) { var count by remember { mutableStateOf(0) } LaunchedEffect(Unit) { while (true) { delay(100) count ++ } } count }

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assertEquals(0, value) 
 dispatcher.advanceTimeBy(100) 
 
 assertEquals(1, value)

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val dispatcher = TestCoroutineDispatcher() val clock = BroadcastFrameClock() 
 val scope = CoroutineScope(dispatcher + clock)

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assertEquals(0, value) 
 dispatcher.advanceTimeBy(100) 
 
 clock.sendFrame(0) 
 
 assertEquals(1, value)

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Context Clock ● Use MonotonicFrameClock from calling Coroutine 
 
 Context ● Use Molecule with built in Android Frame Clock

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Recomposition Clocks ● Context clock ● Immediate

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Gated Frame Clock ● Request for a frame immediately succeeds ● Can be stopped

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Immediate class GatedFrameClock(scope: CoroutineScope) : MonotonicFrameClock { 
 val frameSends = Channel(CONFLATED) init { scope.launch { for (send in frameSends) sendFrame() } } 
 ... }

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Immediate class GatedFrameClock(scope: CoroutineScope) : MonotonicFrameClock { 
 val clock = BroadcastFrameClock { if (isRunning) frameSends.trySend(Unit).getOrThrow() } 
 ... }

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Recomposition Clocks ● Context clock ● Immediate

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val scope = CoroutineScope(Main) 
 val models: StateFlow = scope.launchMolecule( 
 clock = RecompositionClock.ContextClock 
 ) { ProfilePresenter(events, randomService) }

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Launching Molecules ● Create coroutine scope ● Specify frame clock ● Provide Composable function

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Design Patterns

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View

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View Presenter Events

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View Presenter Events Model

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View Presenter Events Model Remote Service Result

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interface MoleculePresenter { @Composable fun models(events: Flow): Model }

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class ProfilePresenter: MoleculePresenter { @Composable override fun models(events: Flow): ProfileModel { } }

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class ProfilePresenter(val repo: Repo): MoleculePresenter< ... > { @Composable override fun models(events: Flow): ProfileModel { } }

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@Composable fun models(events: Flow): ProfileModel { var data by remember { mutableStateOf(null) } LaunchedEffect(Unit) { data = repo.getProfileData() } }

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@Composable fun models(events: Flow): ProfileModel { 
 ... return if (data == null) { ProfileModel.Loading } else { ProfileModel.Success(data) } }

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val models = scope.launchMolecule( ... ) { presenter.models(events) } Events Flow

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sealed interface ProfileEvents { object Edit: ProfileEvents }

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Channel Event Presenter

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@Composable fun models(events: Flow): ProfileModel { LaunchedEffect(events) { events.collect { when (it) { Events.Edit -> { ... } } } } }

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@Composable fun models(events: Flow): ProfileModel { CollectEffect(events) { when (it) { Events.ClickedSubmit -> { ... } } } }

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View Presenter Events Model Remote Service Result

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Molecule Presenter ● Separation of concerns ● Composite presenters

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Component 1 Component 2 Component 3

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class ProfilePresenter: MoleculePresenter { @Composable override fun models(events: Flow): ProfileModel { } } Can get complex

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class Component1Presenter: MoleculePresenter { @Composable override fun models(events: Flow): Model { } }

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class ProfilePresenter( 
 val component1Presenter: Component1Presenter, 
 ...
 ): MoleculePresenter { @Composable override fun models(events: Flow): ProfileModel { } }

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class ProfilePresenter( ... ): MoleculePresenter { @Composable override fun models(events: Flow): ProfileModel { val model = component1Presenter.models(events) } }

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View Presenter Events Model Remote Service Result

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Testing

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val models: StateFlow = scope.launchMolecule( 
 clock = RecompositionClock.ContextClock 
 ) { ProfilePresenter(randomService) } Testing

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Testing ● Turbine ● Molecule Testing Dependency [Deprecated]

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Turbine ● Queue based testing ● Use to be an extension in SQL Delight

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No content

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Turbine interface ReceiveTurbine { fun awaitEvent(): Event fun awaitError(): Throwable suspend fun awaitItem(): T ... }

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Turbine flow { emit("one") emit("two") }.test { 
 assertEquals("one", awaitItem()) 
 assertEquals("two", awaitItem()) }

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Turbine flow { emit("one") emit("two") }.test { 
 assertEquals("one", awaitItem()) 
 assertEquals("two", awaitItem()) }

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Turbine suspend fun Flow.test( validate: suspend ReceiveTurbine.() -> Unit, ) { coroutineScope { collectTurbineIn(this).apply { } } }

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Turbine fun Flow.collectTurbineIn(scope: CoroutineScope): Turbine { 
 lateinit var channel: Channel val job = scope.launch(start = UNDISPATCHED) { channel = collectIntoChannel(this) } return ChannelTurbine(channel, job) }

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Turbine fun Flow.collectTurbineIn(scope: CoroutineScope): Turbine { 
 lateinit var channel: Channel val job = scope.launch(start = UNDISPATCHED) { channel = collectIntoChannel(this) } return ChannelTurbine(channel, job) }

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Turbine fun Flow.collectTurbineIn(scope: CoroutineScope): Turbine { 
 lateinit var channel: Channel val job = scope.launch(start = UNDISPATCHED) { channel = collectIntoChannel(this) } return ChannelTurbine(channel, job) }

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Turbine class ChannelTurbine( channel: Channel = Channel(UNLIMITED) ) : Turbine { suspend fun awaitItem(): T = channel.awaitItem() }

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Turbine val channel = Turbine() listOf(1, 2, 3).forEach { channel.add(it) } channel.skipItems(2) assertEquals(3, channel.awaitItem())

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val channel = Turbine() listOf(1, 2, 3).forEach { channel.add(it) } channel.skipItems(2) assertEquals(3, channel.awaitItem()) Turbine

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Turbine val channel = Turbine() listOf(1, 2, 3).forEach { channel.add(it) } channel.skipItems(2) assertEquals(3, channel.awaitItem())

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val models: StateFlow = scope.launchMolecule( 
 clock = RecompositionClock.ContextClock 
 ) { profilePresenter.models(events) } Testing

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Testing Cases ● Verify correct models are created ● Verify side effects from events

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Testing ● runBlocking ● Immediate Frame Clock ● Use Turbine

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Molecule Testing @Test fun `should get success model`() = runBlocking { launchMolecule(RecompositionClock.Immediate) { profilePresenter.models(events) }.test { } }

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Molecule Testing @Test fun `should get success model`() = runBlocking { launchMolecule(RecompositionClock.Immediate) { profilePresenter.models(events) }.test { assertEquals(ProfileModel.Loading, awaitItem()) 
 ... } }

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Molecule Testing @Test fun `should get success model`() = runBlocking { 
 val events = MutableSharedFlow() 
 launchMolecule(RecompositionClock.Immediate) { profilePresenter.models(events) }.test { } }

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Molecule Testing @Test fun `should get success model`() = runBlocking { launchMolecule(RecompositionClock.Immediate) { profilePresenter.models(events) }.test { 
 events.emit(EditProfile) } }

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Molecule Testing @Test fun `should get success model`() = runBlocking { launchMolecule(RecompositionClock.Immediate) { profilePresenter.models(events) }.test { 
 events.emit(EditProfile) 
 // Assertions } }

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Testing ● runBlocking ● Immediate Frame Clock ● Use Turbine

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Molecule Internals

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val scope = CoroutineScope(Main) 
 
 fun CoroutineScope.launchMolecule( ... ) { // Setup Recomposer }

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Internals ● Compose Compiler & Runtime ● How Molecule hooks into Compose Runtime

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Compose Components ● Composition ● Recomposer ● Applier

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Compose Internals class Foo { fun bar() { 
 ...
 } } Code Parsing Analysis 
 & 
 Resolve IR

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@Composable fun Greeting(name: String) { Text(name) } Compose Internals

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@Composable fun Greeting(name: String) { Text(name) } Creates Deferred Change Compose Internals

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@Composable fun Greeting(name: String, $composer: Composer <*> ) { Text(name) } Connects to Compose Runtime Compose Internals

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@Composable fun Greeting(name: String, $composer: Composer <*> ) { Text(name, $composer) } Calling context is passed along Compose Internals

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suspend fun getData(payload: Payload) { ... } Compose Internals

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suspend fun getData(payload: Payload, $cont: Continuation) { anotherMethod($cont) } Compose Internals

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@Composable fun Greeting(name: String) { Text(name) } Creates Deferred Change Composition

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@Composable inline fun Layout( ... ) { 
 ReusableComposeNode> ( update = { set(measurePolicy, ComposeUiNode.SetMeasurePolicy) set(density, ComposeUiNode.SetDensity) ... }, ) } Composition Teaching the runtime

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@Composable inline fun ResuableComposeNode( ... ) { 
 ... currentComposer.startReusableNode() 
 ...
 currentComposer.createNode(factory) } Composition Delegate to composer

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Composition @Composable inline fun remember( ... ) { 
 ... currentComposer.cache( ... ) 
 }

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@Composable inline fun ResuableComposeNode( ... ) { 
 ... currentComposer.startReusableNode() 
 ...
 currentComposer.createNode(factory) } Composition Changes List Change(…) Change(…) Change(…)

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Composition internal typealias Change = ( applier: Applier <*> , slots: SlotWriter, rememberManager: RememberManager ) -> Unit

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Composition Changes List Change(…) Change(…) Change(…) Slot table Applier

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Composition class UiApplier(…): AbstractApplier { 
 fun insertBottomUp( ... ) fun insertTopDown( ... ) fun move( ... ) fun onClear( ... ) fun onEndChanges( ... ) 
 fun remove( ... ) } Applier in Compose UI

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Entry Points for Client Libraries ● Create a recomposer ● Specify Applier ● Call setContent with composable function

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Recomposer ● Trigger recompositions ● Determine which thread to compose or recompose on

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val contextWithClock = currentThreadContext + 
 (pausableClock ?: EmptyCoroutineContext) 
 
 val recomposer = Recomposer(contextWithClock) 
 Compose UI Example

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fun launchMolecule(clock: RecompositionClock, body: @Composable () -> T) { with(this + clockContext) { val recomposer = Recomposer(coroutineContext) val composition = Composition(UnitApplier, recomposer) launch(start = UNDISPATCHED) { recomposer.runRecomposeAndApplyChanges() } } } Molecule Internals

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fun launchMolecule( ... ) { with(this + clockContext) { val recomposer = Recomposer(coroutineContext) val composition = Composition(UnitApplier, recomposer) launch(start = UNDISPATCHED) { recomposer.runRecomposeAndApplyChanges() } } } Molecule Internals

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Recomposer ● Creates coroutine Job ● Applies changes for recomposition using context ● Cancels composition or recompositions when shutdown

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fun launchMolecule( ... ) { with(this + clockContext) { val recomposer = Recomposer(coroutineContext) val composition = Composition(UnitApplier, recomposer) launch(start = UNDISPATCHED) { recomposer.runRecomposeAndApplyChanges() } } } Molecule Internals

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fun launchMolecule(@Composable body: () -> T) { with(this + clockContext) { 
 ... composition.setContent { body() } } } Molecule Internals

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fun launchMolecule(emitter: (value: T) -> Unit) { with(this + clockContext) { 
 ... composition.setContent { emitter(body()) } } } Molecule Internals

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Molecule Internals var flow: MutableStateFlow? = null launchMolecule( emitter = { value -> outputFlow.value = value }, ... )

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val models: StateFlow = scope.launchMolecule(…) { profilePresenter.models(events) } models.collect { ... }

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Building State Flows with Compose ● How to setup and use Molecule ● Design Patterns ● Testing with Molecule ● Molecule Internals

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Thank You! www.codingwithmohit.com @heyitsmohit