Composing an API the *right* way (Droidcon Italy 2023)

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@zsmb13 Composing an API the way Márton Braun Developer Advocate, JetBrains

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API design

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API design Framework Library App

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API design Framework Library App

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Guidelines

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Guidelines

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Guidelines jb.gg/compose

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Guidelines jb.gg/compose

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Agenda › Naming › Parameters › Default values › Modifiers › Slots › State handling

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Naming

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Constants const val SOME_LOUD_CONSTANT = 999999

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Constants const val SOME_LOUD_CONSTANT = 999999

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Constants const val DefaultKeyName = "__defaultKey"

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Constants const val DefaultKeyName = "__defaultKey" val StructurallyEqual: ComparisonPolicy = StructurallyEqualsImpl()

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Constants const val DefaultKeyName = "__defaultKey" val StructurallyEqual: ComparisonPolicy = StructurallyEqualsImpl() object ReferenceEqual : ComparisonPolicy {} sealed class LoadResult { object Loading : LoadResult() class Done(val result: T) : LoadResult() class Error(val cause: Throwable) : LoadResult() }

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Functions

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Functions It depends.

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Functions Return a value Emit content

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Functions Return a value Emit content

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Functions Return a value Emit content

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Functions Return a value Emit content @Composable fun Column(...) @Composable fun LaunchedEffect(...) @Composable fun Button(...)

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Functions Return a value @Composable fun stringResource( @StringRes id: Int ): String @Composable fun rememberScrollState( initial: Int = 0 ): ScrollState Emit content @Composable fun Column(...) @Composable fun LaunchedEffect(...) @Composable fun Button(...)

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Functions @Composable fun ColorScheme(darkTheme: Boolean): ColorScheme

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Functions @Composable fun ColorScheme(darkTheme: Boolean): ColorScheme

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Functions @Composable fun ColorScheme(darkTheme: Boolean): ColorScheme @Composable fun colorScheme(darkTheme: Boolean): ColorScheme

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Functions @Composable fun ColorScheme(darkTheme: Boolean): ColorScheme @Composable fun colorScheme(darkTheme: Boolean): ColorScheme @Composable fun rememberCoroutineScope(): CoroutineScope @Composable fun CoroutineScope(): CoroutineScope

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Component /kəmˈpəʊnənt/ noun

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Component /kəmˈpəʊnənt/ noun a @Composable function that returns Unit and emits exactly one Compose UI tree node.

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Component /kəmˈpəʊnənt/ noun a @Composable function that returns Unit and emits exactly one Compose UI tree node.

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Components @Composable fun Header() { Column { Text("Kotlin programming language") Text("Concise. Cross-platform. Fun.") Row { Button { Text("Docs") } Button { Text("Blog") } } } }

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Components @Composable fun Header() { Column { Text("Kotlin programming language") Text("Concise. Cross-platform. Fun.") Row { Button { Text("Docs") } Button { Text("Blog") } } } }

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Components @Composable fun Header() { Column { Row { Button { Text("Docs") } Button { Text("Blog") } } } } Text("Kotlin programming language") Text("Concise. Cross-platform. Fun.")

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Text("Kotlin programming language") Text("Concise. Cross-platform. Fun.") @Composable fun Title() { } Components @Composable fun Header() { Column { Row { Button { Text("Docs") } Button { Text("Blog") } } } }

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Components Text("Kotlin programming language") Text("Concise. Cross-platform. Fun.") @Composable fun Title() { } @Composable fun Header() { Column { Title() Row { Button { Text("Docs") } Button { Text("Blog") } } } }

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Components Kotlin programming language Concise. Cross-platform. Fun. Text("Kotlin programming language") Text("Concise. Cross-platform. Fun.") @Composable fun Title() { }

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Components Kotlin programming language Concise. Cross-platform. Fun. Text("Kotlin programming language") Text("Concise. Cross-platform. Fun.") @Composable fun Title() { Column { } }

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Components Concise. Cross-platform. Fun. Kotlin programming language Text("Kotlin programming language") Text("Concise. Cross-platform. Fun.") @Composable fun Title() { Column { } }

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Component /kəmˈpəʊnənt/ noun a @Composable function that returns Unit and emits exactly one Compose UI tree node.

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Component /kəmˈpəʊnənt/ noun a @Composable function that returns Unit and emits exactly one Compose UI tree node.

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Components @Composable fun InputField : { // ... } val inputState = InputField Button(onClick = { inputState.value = "" }) { Text("Clear input") } UserInputState () ()

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Components @Composable fun InputField : { // ... } Button(onClick = { inputState.value = "" }) { Text("Clear input") } InputField UserInputState val inputState = () ()

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Components @Composable fun InputField : UserInputState { // ... } InputField () () Button(onClick = { inputState.value = "" }) { Text("Clear input") } val inputState =

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Components @Composable fun InputField inputState: { // ... } remember { UserInputState() } InputField inputState UserInputState val inputState = ( ) ( ) Button(onClick = { inputState.value = "" }) { Text("Clear input") }

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Components @Composable fun InputField(inputState: UserInputState) { // ... } val inputState = remember { UserInputState() } InputField(inputState) Button(onClick = { inputState.value = "" }) { Text("Clear input") }

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Components @Composable fun InputField(inputState: UserInputState) { // ... } val inputState = remember { UserInputState() } Button(onClick = { inputState.value = "" }) { Text("Clear input") } InputField(inputState)

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Components Data UI

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Parameters

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CompositionLocal jb.gg/composition-locals

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CompositionLocal jb.gg/composition-locals

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CompositionLocal jb.gg/composition-locals

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CompositionLocal jb.gg/composition-locals

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CompositionLocal Data Styles jb.gg/composition-locals

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> Parameters CompositionLocal explicit implicit

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Default arguments @Composable fun Text( text: String, modifier: Modifier = Modifier, color: Color = Color.Unspecified, lineHeight: TextUnit = TextUnit.Unspecified, overflow: TextOverflow = TextOverflow.Clip, softWrap: Boolean = true, maxLines: Int = Int.MAX_VALUE, onTextLayout: (TextLayoutResult) -> Unit = {}, style: TextStyle = LocalTextStyle.current, )

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@Composable fun Text( text: String, modifier: Modifier = Modifier, color: Color = Color.Unspecified, lineHeight: TextUnit = TextUnit.Unspecified, overflow: TextOverflow = TextOverflow.Clip, softWrap: Boolean = true, maxLines: Int = Int.MAX_VALUE, onTextLayout: (TextLayoutResult) -> Unit = {}, style: TextStyle = LocalTextStyle.current, Default arguments )

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Default arguments @Composable fun Button( onClick: () -> Unit, modifier: Modifier = Modifier, enabled: Boolean = true, shape: Shape = ButtonDefaults.shape, colors: ButtonColors = ButtonDefaults.buttonColors(), elevation: ButtonElevation? = ButtonDefaults.buttonElevation(), border: BorderStroke? = null, contentPadding: PaddingValues = ButtonDefaults.ContentPadding, content: @Composable RowScope.() -> Unit, )

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Default arguments object ButtonDefaults { val ContentPadding: PaddingValues val MinWidth: Dp val MinHeight: Dp val IconSize: Dp val IconSpacing: Dp val shape: Shape @Composable get @Composable fun buttonElevation(...): ButtonElevation = ButtonElevation(...) } @Composable fun buttonColors(...): ButtonColors = ButtonColors(...) Button( onClick = { viewModel.savePage() }, colors = customColors ?: ButtonDefaults.buttonColors(),

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Default arguments object ButtonDefaults { } @Composable fun buttonColors(...): ButtonColors = ButtonColors(...) Button( onClick = { viewModel.savePage() }, colors = customColors ?: ButtonDefaults.buttonColors(), ) { Text("Save") }

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Default arguments internal object ButtonDefaults { @Composable fun buttonColors(...): ButtonColors = ButtonColors(...) } Button( onClick = { viewModel.savePage() }, colors = customColors ?: ButtonDefaults.buttonColors(), ) { Text("Save") }

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Modifiers

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modifier: Modifier = Modifier, Modifiers @Composable fun PageTitle( text: String, )

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modifier: Modifier = Modifier, Modifiers PageTitle( text = "Home", modifier = Modifier .width(100.dp) .height(40.dp) .background(Color.LightGray) .padding(horizontal = 12.dp) ) @Composable fun PageTitle( text: String, )

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Modifiers @Composable fun PageTitle( text: String, )

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Modifiers @Composable fun PageTitle( text: String, ) Box( Modifier .padding(12.dp) .background(Color.Blue) ) { PageTitle("Home") }

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Modifiers @Composable fun PageTitle( text: String, ) Box( Modifier .padding(12.dp) .background(Color.Blue) ) { PageTitle("Home") } @Composable fun PageTitle( text: String, modifier: Modifier = Modifier, ) PageTitle( "Home", Modifier .padding(12.dp) .background(Color.Blue), )

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Modifiers › Accept a Modifier parameter

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Modifiers › Accept a Modifier parameter › Named modifier

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Modifiers › Accept a Modifier parameter › Named modifier › First optional parameter

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Modifiers › Accept a Modifier parameter › Named modifier › First optional parameter › Applied to the root node

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Modifiers › Accept a Modifier parameter › Named modifier › First optional parameter › Applied to the root node › Default value Modifier

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Modifiers › Accept a Modifier parameter › Named modifier › First optional parameter › Applied to the root node › Default value Modifier

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Modifiers › Accept a Modifier parameter › Named modifier › First optional parameter › Applied to the root node › Default value Modifier

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Modifiers › Accept a Modifier parameter › Named modifier › First optional parameter › Applied to the root node › Default value Modifier

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(modifier) (modifier) Modifiers @Composable fun Header( modifier: Modifier = Modifier, ) { Column Title() Row Button(onClick = { ... }) { Text("Docs") } Button(onClick = { ... }) { Text("Blog") } } } } { {

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Modifiers @Composable fun MenuItems( smallScreen: Boolean, modifier: Modifier = Modifier, ) { if (smallScreen) { Column(modifier) { Text(...) Text(...) } } else { Row(modifier) { Text(...) Text(...) } } }

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Modifiers › Accept a Modifier parameter › Named modifier › First optional parameter › Applied to the root node › Default value Modifier

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Modifiers modifier: Modifier = Modifier

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Modifiers modifier: Modifier = Modifier interface Modifier { infix fun then(other: Modifier): Modifier = CombinedModifier(this, other) companion object : Modifier { ... override infix fun then(other: Modifier): Modifier = other override fun toString() = "Modifier" } }

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Modifiers @Composable fun Topics( topic: List, modifier: Modifier = Modifier, ) { Row( modifier = modifier ) { // ... } } .padding(12.dp),

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Modifiers @Composable fun Topics( topic: List, modifier: Modifier = Modifier, ) { Row( modifier = Modifier .then(modifier) ) { // ... } } .padding(12.dp) ,

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jb.gg/compose-modifiers

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Slots

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Slots @Composable () -> Unit

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Slots @Composable fun Button( content: @Composable () -> Unit, )

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Slots @Composable fun Button( content: @Composable , ) Button { } } () -> Unit Row { ) ) Icon(Icons.Default.Build, "Build" Text("Build project" this: RowScope

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Slots @Composable fun Button( content: @Composable RowScope. , ) { content() } } Button { } this: RowScope () -> Unit Row { ) ) Icon(Icons.Default.Build, "Build" Text("Build project"

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Slots Button { , Modifier.weight(1f) , Modifier.weight(4f) } this: RowScope ) ) @Composable fun Button( content: @Composable RowScope. , ) { content() } } () -> Unit Row { Icon(Icons.Default.Build, "Build" Text("Build project"

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Slots @Composable fun Button( content: @Composable RowScope.() -> Unit, ) { Row { content() } } Button { , Modifier.weight(1f)) , Modifier.weight(4f)) } this: RowScope Icon(Icons.Default.Build, "Build" Text("Build project"

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Slot lifecycle @Composable fun PreferenceItem( checked: Boolean, content: @Composable () -> Unit, ) { if (checked) { Row { Text("Checked") content() } } else { Column { Text("Unchecked") content() } } }

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Slot lifecycle val movableContent = remember(content) { movableContentOf(content) } @Composable fun PreferenceItem( checked: Boolean, content: @Composable () -> Unit, ) { if (checked) { Row { Text("Checked") content() } } else { Column { Text("Unchecked") content() } } } checked = false checked = true checked = false PreferenceItem content

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Slot lifecycle @Composable fun PreferenceItem( checked: Boolean, content: @Composable () -> Unit, ) { if (checked) { Row { Text("Checked") movableContent() } } else { Column { Text("Unchecked") movableContent() } } } val movableContent = remember(content) { movableContentOf(content) }

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Slot lifecycle @Composable fun PreferenceItem( checked: Boolean, content: @Composable () -> Unit, ) { val movableContent = remember(content) { movableContentOf(content) } if (checked) { Row { Text("Checked") movableContent() } } else { Column { Text("Unchecked") movableContent() } } } jb.gg/movable-content-of

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Ordering parameters

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Ordering parameters › Required parameters › Optional parameters › First one: Modifier › Trailing content lambda

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Ordering parameters fun NiaTopicTag( modifier: Modifier = Modifier, followed: Boolean, onClick: () -> Unit, enabled: Boolean = true, text: @Composable () -> Unit, ) › Required parameters › Optional parameters › First one: Modifier › Trailing content lambda

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Ordering parameters › Required parameters › Optional parameters › First one: Modifier › Trailing content lambda fun NiaTopicTag( modifier: Modifier = Modifier, followed: Boolean, onClick: () -> Unit, enabled: Boolean = true, text: @Composable () -> Unit, )

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Ordering parameters NiaTopicTag(true, { toggleFollowed(id) }) { Text("Compose") } › Required parameters › Optional parameters › First one: Modifier › Trailing content lambda fun NiaTopicTag( modifier: Modifier = Modifier, followed: Boolean, onClick: () -> Unit, enabled: Boolean = true, text: @Composable () -> Unit, )

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Ordering parameters NiaTopicTag(true, { toggleFollowed(id) } Text("Compose") } › Required parameters › Optional parameters › First one: Modifier › Trailing content lambda fun NiaTopicTag( modifier: Modifier = Modifier, followed: Boolean, onClick: () -> Unit, enabled: Boolean = true, text: @Composable () -> Unit, ) ) {

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Ordering parameters NiaTopicTag(true, { toggleFollowed(id) } Text("Compose") } › Required parameters › Optional parameters › First one: Modifier › Trailing content lambda fun NiaTopicTag( followed: Boolean, onClick: () -> Unit, modifier: Modifier = Modifier, enabled: Boolean = true, text: @Composable () -> Unit, ) ) {

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Ordering parameters NiaTopicTag(followed = true, onClick = { toggleFollowed(id) } Text("Compose") } › Required parameters › Optional parameters › First one: Modifier › Trailing content lambda fun NiaTopicTag( followed: Boolean, onClick: () -> Unit, text: @Composable () -> Unit, ) modifier: Modifier = Modifier, enabled: Boolean = true, ) {

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Ordering parameters NiaTopicTag(true, Text("Compose") } › Required parameters › Optional parameters › First one: Modifier › Trailing content lambda fun NiaTopicTag( followed: Boolean, onClick: () -> Unit, text: @Composable () -> Unit, ) modifier: Modifier = Modifier, enabled: Boolean = true, ) { { toggleFollowed(id) }

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Ordering parameters NiaTopicTag(true, , Modifier.padding(8.dp) Text("Compose") } fun NiaTopicTag( followed: Boolean, onClick: () -> Unit, modifier: Modifier = Modifier, enabled: Boolean = true, text: @Composable () -> Unit, ) › Required parameters › Optional parameters › First one: Modifier › Trailing content lambda { toggleFollowed(id) } ) {

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Ordering parameters NiaTopicTag(true, { toggleFollowed(id) }, Modifier.padding(8.dp)) { Text("Compose") } fun NiaTopicTag( followed: Boolean, onClick: () -> Unit, modifier: Modifier = Modifier, enabled: Boolean = true, text: @Composable () -> Unit, ) › Required parameters › Optional parameters › First one: Modifier › Trailing content lambda

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Ordering parameters NiaTopicTag(true, { toggleFollowed(id) }, Modifier.padding(8.dp)) { Text("Compose") } fun NiaTopicTag( followed: Boolean, onClick: () -> Unit, modifier: Modifier = Modifier, enabled: Boolean = true, content: @Composable () -> Unit, ) › Required parameters › Optional parameters › First one: Modifier › Trailing content lambda

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State

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State @Composable fun Scroller( offset: MutableState< >, ) Float

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State @Composable fun Scroller( offset: MutableState, )

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@Composable fun Scroller( offset: MutableState< >, ) { offset.value = 42f } State Float

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@Composable fun Scroller( offset: , onOffsetChange: (Float) -> Unit, ) State Float

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@Composable fun Scroller( offset: State< >, onOffsetChange: (Float) -> Unit, ) State Float

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@Composable fun Scroller( offset: State, onOffsetChange: (Float) -> Unit, ) @Composable fun ContextMenu(offset: Float, onOffsetChange: (Float) -> Unit) { Row { MenuItems() Scroller(offset, onOffsetChange) } } State

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@Composable fun Scroller( offset: State, onOffsetChange: (Float) -> Unit, ) @Composable fun ContextMenu(offset: Float, onOffsetChange: (Float) -> Unit) { val offsetState = remember { mutableStateOf(offset) } Row { MenuItems() Scroller(offsetState, onOffsetChange) } } State

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@Composable fun Scroller( offset: Float, onOffsetChange: (Float) -> Unit, ) State

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@Composable fun Scroller( offset: () -> Float, onOffsetChange: (Float) -> Unit, ) State

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@Composable fun Scroller( offset: () -> Float, onOffsetChange: (Float) -> Unit, ) // Constant Scroller(offset = { 0f }, ...) // Plain value Scroller(offset = { offset }, ...) // State Scroller(offset = { offsetState.value }, ...) // Value from an object Scroller(offset = { someObject.offset }, ...) State

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State holders

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State holders @Composable fun VerticalScroller( , , onScrollPositionChange: (Int) -> Unit, onScrollRangeChange: (Int) -> Unit, ) scrollPosition: Int scrollRange: Int

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State holders @Composable fun VerticalScroller( ) @Stable interface VerticalScrollerState { var var } scrollPosition: Int scrollRange: Int

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State holders @Composable fun VerticalScroller( verticalScrollerState: VerticalScrollerState ) @Stable interface VerticalScrollerState { } scrollPosition: Int scrollRange: Int var var

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State holders @Composable fun VerticalScroller( verticalScrollerState: VerticalScrollerState ) @Stable interface VerticalScrollerState { } scrollPosition: Int scrollRange: Int var var

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State holders @Stable interface { } jb.gg/compose-stability @Composable fun VerticalScroller( verticalScrollerState: VerticalScrollerState ) VerticalScrollerState scrollPosition: Int scrollRange: Int var var

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State holders class Impl( scrollPosition: Int = 0, scrollRange: Int = 0, ) : VerticalScrollerState { override by mutableStateOf(scrollPosition) override by mutableStateOf(scrollRange) } @Composable fun VerticalScroller( verticalScrollerState: VerticalScrollerState ) VerticalScrollerState scrollPosition: Int scrollRange: Int var var

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State holders @Composable fun VerticalScroller( verticalScrollerState: VerticalScrollerState ) fun VerticalScrollerState(): VerticalScrollerState = VerticalScrollerStateImpl() private class Impl( scrollPosition: Int = 0, scrollRange: Int = 0, ) : VerticalScrollerState { override var scrollPosition: Int by mutableStateOf(scrollPosition) override var scrollRange: Int by mutableStateOf(scrollRange) } VerticalScrollerState

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State holders @Composable fun VerticalScroller( verticalScrollerState: VerticalScrollerState = remember { VerticalScrollerState() } ) fun VerticalScrollerState(): VerticalScrollerState = VerticalScrollerStateImpl() private class Impl( scrollPosition: Int = 0, scrollRange: Int = 0, ) : VerticalScrollerState { override var scrollPosition: Int by mutableStateOf(scrollPosition) override var scrollRange: Int by mutableStateOf(scrollRange) } VerticalScrollerState

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State holders @Composable fun VerticalScroller( verticalScrollerState: VerticalScrollerState = remember { VerticalScrollerState() } ) fun VerticalScrollerState(): VerticalScrollerState = VerticalScrollerStateImpl() @Stable interface VerticalScrollerState { var scrollPosition: Int var scrollRange: Int } private class Impl( scrollPosition: Int = 0, scrollRange: Int = 0, ) : VerticalScrollerState { override var scrollPosition: Int by mutableStateOf(scrollPosition) override var scrollRange: Int by mutableStateOf(scrollRange) } VerticalScrollerState

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State holders @Stable class VerticalScrollerState { var scrollPosition: Int by mutableStateOf(0) var scrollRange: Int by mutableStateOf(0) }

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State holders @Stable class VerticalScrollerState { var scrollPosition: Int by mutableStateOf(0) var scrollRange: Int by mutableStateOf(0) }

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Conclusion

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Framework Library App

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Compose Multiplatform jb.gg/compose Guidelines jb.gg/compose-api-guidelines jb.gg/compose-component-api- guidelines In order of appearance jb.gg/composition-locals jb.gg/compose-modifiers jb.gg/movable-content-of jb.gg/debugging-recomposition jb.gg/compose-stability Composing an API the way zsmb.co/talks Márton Braun @zsmb13@androiddev.social