Composing an API with Kotlin (Kotlin Budapest Meetup 2022 October)

Kotlin offers many powerful language features to API authors. We’ll explore how Jetpack Compose’s syntax builds on these features, and how its APIs were shaped by Kotlin idioms and conventions. These ideas and best practices give you a deeper understanding of the language, and they are valuable for any Kotlin developer designing APIs – which is actually every Kotlin developer!

More details and resources: https://zsmb.co/talks/composing-an-api-with-kotlin/

Transcript

1. Márton Braun @zsmb13 Developer Relations Engineer Google Composing an API with Kotlin
2. Jetpack Compose is Android’s modern toolkit for building native UI. d.android.com/compose
3. What is Jetpack Compose? @Composable fun Greeting(name: String) { Text(text = "Hello $name!") }
4. What is Jetpack Compose? @Composable fun Greeting(name: String) { Text(text = "Hello $name!") } Hello Android!
5. What is Jetpack Compose? @Composable fun Greeting(name: String) { Text(text = "Hello $name!") } Hello Compose!
6. class MainActivity : ComponentActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContent { DevDayDemoTheme { Surface( modifier = Modifier.fillMaxSize(), color = MaterialTheme.colors.background, ) { Greeting(name = "Android") } } } } } What is Jetpack Compose?
7. class MainActivity : ComponentActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContent { DevDayDemoTheme { Surface( modifier = Modifier.fillMaxSize(), color = MaterialTheme.colors.background, ) { Greeting(name = "Android") } } } } } What is Jetpack Compose?
8. What is Jetpack Compose?
9. • What is Jetpack Compose? Declarative
10. • • What is Jetpack Compose? Declarative Built with Kotlin
11. • • • What is Jetpack Compose? Declarative Built with Kotlin Open source
12. // ... Extension functions class MainActivity : ComponentActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContent { } } } DevDayDemoTheme { Surface( modifier = Modifier.fillMaxSize(), color = MaterialTheme.colors.background, ) { Greeting(name = "Android") } }
13. DevDayDemoTheme { Surface( modifier = Modifier.fillMaxSize(), color = MaterialTheme.colors.background, ) { Greeting(name = "Android") } } class MainActivity : ComponentActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContent { } } } public fun ComponentActivity.setContent( parent: CompositionContext? = null, content: @Composable () -> Unit ) Extension functions // ...
14. public fun CoroutineScope.launch( context: CoroutineContext = EmptyCoroutineContext, start: CoroutineStart = CoroutineStart.DEFAULT, block: suspend CoroutineScope.() -> Unit ) public fun ComponentActivity.setContent( parent: CompositionContext? = null, content: @Composable () -> Unit ) Extension functions
15. Box( modifier = Modifier .size(216.dp) .background( brush = Brush.radialGradient( colors = listOf(Color.Black, Color.White) ) ) .padding(bottom = 12.dp) ) Extension functions
16. Box( modifier = Modifier .size(216.dp) .background( brush = Brush.radialGradient( colors = listOf(Color.Black, Color.White) ) ) .padding(bottom = 12.dp) ) Extension functions @Composable fun Greeting( name: String, modifier: Modifier = ... )
17. Box( modifier = .size(216.dp) .background( brush = Brush.radialGradient( colors = listOf(Color.Black, Color.White) ) ) .padding(bottom = 12.dp) ) Modifier. Extension functions
18. Modifier. Extension functions
19. @Stable interface Modifier { infix fun then(other: Modifier): Modifier = if (other === Modifier) this else CombinedModifier(this, other) companion object : Modifier { override infix fun then(other: Modifier): Modifier = other } } Extension functions
20. @Stable interface Modifier { infix fun then(other: Modifier): Modifier = if (other === Modifier) this else CombinedModifier(this, other) companion object : Modifier { override infix fun then(other: Modifier): Modifier = other } } Extension functions
21. @Stable interface Modifier { infix fun then(other: Modifier): Modifier = if (other === Modifier) this else CombinedModifier(this, other) companion object : Modifier { override infix fun then(other: Modifier): Modifier = other } } Modifier. Extension functions
22. @Stable interface Modifier { infix fun then(other: Modifier): Modifier = if (other === Modifier) this else CombinedModifier(this, other) companion object : Modifier { override infix fun then(other: Modifier): Modifier = other } } Extension functions @Composable fun Greeting( name: String, modifier: Modifier = Modifier )
23. Modifier. Extension functions
24. package androidx.compose.foundation fun Modifier.background(color: Color, shape: Shape = RectangleShape) Extension functions
25. package androidx.compose.foundation fun Modifier.background(color: Color, shape: Shape = RectangleShape) package androidx.compose.foundation.gestures fun Modifier.scrollable(state: ScrollableState, orientation: Orientation) fun Modifier.draggable(state: DraggableState, orientation: Orientation) Extension functions
26. package androidx.compose.foundation fun Modifier.background(color: Color, shape: Shape = RectangleShape) package androidx.compose.foundation.gestures fun Modifier.scrollable(state: ScrollableState, orientation: Orientation) fun Modifier.draggable(state: DraggableState, orientation: Orientation) package androidx.compose.foundation.layout fun Modifier.padding(all: Dp) fun Modifier.size(width: Dp, height: Dp) Extension functions
27. package androidx.compose.foundation fun Modifier.background(color: Color, shape: Shape = RectangleShape) package androidx.compose.foundation.gestures fun Modifier.scrollable(state: ScrollableState, orientation: Orientation) fun Modifier.draggable(state: DraggableState, orientation: Orientation) package androidx.compose.foundation.layout fun Modifier.padding(all: Dp) fun Modifier.size(width: Dp, height: Dp) package androidx.compose.ui.draw fun Modifier.clip(shape: Shape) fun Modifier.rotate(degrees: Float) fun Modifier.scale(scaleX: Float, scaleY: Float) Extension functions
28. package androidx.compose.foundation fun Modifier.background(color: Color, shape: Shape = RectangleShape) package androidx.compose.foundation.gestures fun Modifier.scrollable(state: ScrollableState, orientation: Orientation) fun Modifier.draggable(state: DraggableState, orientation: Orientation) package androidx.compose.foundation.layout fun Modifier.padding(all: Dp) fun Modifier.size(width: Dp, height: Dp) package androidx.compose.ui.draw fun fun Modifier.rotate(degrees: Float) fun Modifier.scale(scaleX: Float, scaleY: Float) Extension functions Modifier.clip(shape: Shape)
29. Naming conventions Modifier.clip(shape: Shape)
30. Modifier.clip(CircleShape) Naming conventions
31. Modifier.clip(CircleShape) val CircleShape = RoundedCornerShape(50) Naming conventions
32. Modifier.clip(CircleShape) val circleShape = RoundedCornerShape(50) Naming conventions
33. Modifier.clip(CircleShape) val CIRCLE_SHAPE = RoundedCornerShape(50) Naming conventions
34. Modifier.clip(CircleShape) val CircleShape = RoundedCornerShape(50) Naming conventions
35. Modifier.clip(CircleShape) val CircleShape = RoundedCornerShape(50) class RoundedCornerShape( topStart: CornerSize, topEnd: CornerSize, bottomEnd: CornerSize, bottomStart: CornerSize ) Naming conventions
36. Modifier.clip(CircleShape) val CircleShape = RoundedCornerShape(50) fun RoundedCornerShape(percent: Int) = ... Naming conventions
37. Modifier.clip(CircleShape) val CircleShape = RoundedCornerShape(50) fun RoundedCornerShape(percent: Int) = ... Naming conventions
38. Modifier.clip(CircleShape) val CircleShape = RoundedCornerShape(50) fun RoundedCornerShape(percent: Int) = ... fun RoundedCornerShape(size: Float) = ... fun RoundedCornerShape(size: Dp) = ... Naming conventions
39. fun RoundedCornerShape( topStart: Dp = 0.dp, topEnd: Dp = 0.dp, bottomEnd: Dp = 0.dp, bottomStart: Dp = 0.dp ) = RoundedCornerShape( topStart = CornerSize(topStart), topEnd = CornerSize(topEnd), bottomEnd = CornerSize(bottomEnd), bottomStart = CornerSize(bottomStart) ) Naming conventions
40. fun RoundedCornerShape( topStart: Dp = 0.dp, topEnd: Dp = 0.dp, bottomEnd: Dp = 0.dp, bottomStart: Dp = 0.dp ) = RoundedCornerShape( topStart = CornerSize(topStart), topEnd = CornerSize(topEnd), bottomEnd = CornerSize(bottomEnd), bottomStart = CornerSize(bottomStart) ) Hello world Naming conventions
41. fun RoundedCornerShape( topStart: Dp = 0.dp, topEnd: Dp = 0.dp, bottomEnd: Dp = 0.dp, bottomStart: Dp = 0.dp ) = RoundedCornerShape( topStart = CornerSize(topStart), topEnd = CornerSize(topEnd), bottomEnd = CornerSize(bottomEnd), bottomStart = CornerSize(bottomStart) ) RoundedCornerShape(topStart = 16.dp, topEnd = 16.dp, bottomEnd = 16.dp) Hello world Naming conventions
42. fun RoundedCornerShape( topStart: Dp = 0.dp, topEnd: Dp = 0.dp, bottomEnd: Dp = 0.dp, bottomStart: Dp = 0.dp ) = RoundedCornerShape( topStart = CornerSize(topStart), topEnd = CornerSize(topEnd), bottomEnd = CornerSize(bottomEnd), bottomStart = CornerSize(bottomStart) ) RoundedCornerShape(topStart = 16.dp, topEnd = 16.dp, bottomEnd = 16.dp) Hello world RoundedCornerShape(16.dp).copy(bottomStart = CornerSize(0.dp)) Naming conventions
43. Column(modifier = modifier.fillMaxSize()) { JetNewsLogo(Modifier .padding(16.dp) .align(Alignment.CenterHorizontally) ) Divider() DrawerButton( icon = Icons.Filled.Home, label = stringResource(id = R.string.home_title), isSelected = currentRoute == JetnewsDestinations.HOME_ROUTE, action = { navigateToHome(); closeDrawer() } ) DrawerButton( icon = Icons.Filled.ListAlt, label = stringResource(id = R.string.interests_title), isSelected = currentRoute == JetnewsDestinations.INTERESTS_ROUTE, action = { navigateToInterests(); closeDrawer() } ) } Naming conventions
44. Column(modifier = modifier.fillMaxSize()) { JetNewsLogo(Modifier .padding(16.dp) .align(Alignment.CenterHorizontally) ) Divider() DrawerButton( icon = Icons.Filled.Home, label = stringResource(id = R.string.home_title), isSelected = currentRoute == JetnewsDestinations.HOME_ROUTE, action = { navigateToHome(); closeDrawer() } ) DrawerButton( icon = Icons.Filled.ListAlt, label = stringResource(id = R.string.interests_title), isSelected = currentRoute == JetnewsDestinations.INTERESTS_ROUTE, action = { navigateToInterests(); closeDrawer() } ) } Naming conventions
45. Scopes Column(modifier = modifier.fillMaxSize()) { JetNewsLogo(Modifier .padding(16.dp) .align(Alignment.CenterHorizontally) ) Divider() DrawerButton( icon = Icons.Filled.Home, label = stringResource(id = R.string.home_title), isSelected = currentRoute == JetnewsDestinations.HOME_ROUTE, action = { navigateToHome(); closeDrawer() } ) DrawerButton( icon = Icons.Filled.ListAlt, label = stringResource(id = R.string.interests_title), isSelected = currentRoute == JetnewsDestinations.INTERESTS_ROUTE, action = { navigateToInterests(); closeDrawer() } ) }
46. Column(modifier = modifier.fillMaxSize()) { JetNewsLogo(Modifier .padding(16.dp) .align(Alignment.CenterHorizontally) ) Divider() DrawerButton( icon = Icons.Filled.Home, label = stringResource(id = R.string.home_title), isSelected = currentRoute == JetnewsDestinations.HOME_ROUTE, action = { navigateToHome(); closeDrawer() } ) DrawerButton( icon = Icons.Filled.ListAlt, label = stringResource(id = R.string.interests_title), isSelected = currentRoute == JetnewsDestinations.INTERESTS_ROUTE, action = { navigateToInterests(); closeDrawer() } ) } this: ColumnScope Scopes
47. @LayoutScopeMarker @Immutable interface ColumnScope { @Stable fun Modifier.align(alignment: Alignment.Horizontal): Modifier } Scopes
48. @LayoutScopeMarker @Immutable interface ColumnScope { @Stable fun Modifier.align(alignment: Alignment.Horizontal): Modifier } @DslMarker annotation class LayoutScopeMarker Scopes
49. Column(modifier = modifier.fillMaxSize()) { JetNewsLogo( Modifier .padding(16.dp) .align(Alignment.CenterHorizontally) ) } Scopes this: ColumnScope
50. Column(modifier = modifier.fillMaxSize()) { Button(onClick = { /* ... */ }) { JetNewsLogo( Modifier .padding(16.dp) .align(Alignment.CenterHorizontally) ) } } Scopes this: ColumnScope
51. val sizeInPx = 16.dp.toPx() Scopes
52. Scopes val sizeInPx = 16.dp.toPx()
53. val sizeInPx = with(LocalDensity.current) { } this: Density Scopes 16.dp.toPx()
54. val sizeInPx = with(LocalDensity.current) { 16.dp.toPx() } this: Density @Immutable @Stable val density: Float @Stable Scopes interface Density { } fun Dp.toPx(): Float = value * density object CustomGridCells : GridCells { override fun Density.calculateCrossAxisCellSizes( availableSize: Int,
55. object CustomGridCells : GridCells { override fun Density.calculateCrossAxisCellSizes( availableSize: Int, spacing: Int ): List<Int> { val sideCellSize = 60.dp.roundToPx() return listOf( sideCellSize, availableSize - 2 * sideCellSize, sideCellSize, ) } } Scopes fun Dp.toPx(): Float = value * density interface Density { }
56. Not a real Compose API context(Density) object CustomGridCells : GridCells { context(Density) override fun calculateCrossAxisCellSizes( availableSize: Int, spacing: Int ): List<Int> { val sideCellSize = 60.dp.roundToPx() return listOf( sideCellSize, availableSize - 2 * sideCellSize, sideCellSize, ) } } Scopes – with context receivers? fun Dp.toPx(): Float = value * density
57. Inline classes
58. Inline classes @JvmInline value class Dp(val value: Float) { inline operator fun plus(other: Dp) inline operator fun minus(other: Dp) inline operator fun div(other: Float): Dp inline operator fun div(other: Int): Dp inline operator fun div(other: Dp): Float } inline val Int.dp: Dp get() = Dp(value = this.toFloat()) inline operator fun Int.times(other: Dp) = Dp(this * other.value)
59. val availableWidth: Dp = 200.dp val itemWidth: Dp = 30.dp val itemCount: Int = (availableWidth / itemWidth).toInt() val remaining: Dp = (availableWidth - itemCount * itemWidth) val separatorWidth: Dp = remaining / (itemCount - 1) Inline classes
60. float availableWidth = Dp.constructor-impl((float) 200); float itemWidth = Dp.constructor-impl((float) 30); int itemCount = (int)(availableWidth / itemWidth); float other$iv = Dp.constructor-impl((float) itemCount * itemWidth); float remaining = Dp.constructor-impl(availableWidth - other$iv); float separatorWidth = Dp.constructor-impl(remaining / (float) (itemCount - 1)); val availableWidth: Dp = 200.dp val itemWidth: Dp = 30.dp val itemCount: Int = (availableWidth / itemWidth).toInt() val remaining: Dp = (availableWidth - itemCount * itemWidth) val separatorWidth: Dp = remaining / (itemCount - 1) Inline classes
61. float availableWidth = Dp.constructor-impl((float) 200); float itemWidth = Dp.constructor-impl((float) 30); int itemCount = (int)(availableWidth / itemWidth); float other$iv = Dp.constructor-impl((float) itemCount * itemWidth); float remaining = Dp.constructor-impl(availableWidth - other$iv); float separatorWidth = Dp.constructor-impl(remaining / (float) (itemCount - 1)); val availableWidth: Dp = 200.dp val itemWidth: Dp = 30.dp val itemCount: Int = (availableWidth / itemWidth).toInt() val remaining: Dp = (availableWidth - itemCount * itemWidth) val separatorWidth: Dp = remaining / (itemCount - 1) Inline classes
62. float availableWidth = Dp.constructor-impl((float) 200); float itemWidth = Dp.constructor-impl((float) 30); int itemCount = (int)(availableWidth / itemWidth); float other$iv = Dp.constructor-impl((float) itemCount * itemWidth); float remaining = Dp.constructor-impl(availableWidth - other$iv); float separatorWidth = Dp.constructor-impl(remaining / (float) (itemCount - 1)); public final class Dp { private final float value; public static float constructor_impl(float value) { return value; } } Inline classes
63. Coroutines
64. Coroutines chris.banes.dev/suspending-views/
65. val listState = rememberLazyListState() val coroutineScope = rememberCoroutineScope() Box { LazyColumn(state = listState) { // ... } AnimatedVisibility(/* ... */) { ScrollToTopButton( onClick = { coroutineScope.launch { listState.animateScrollToItem(index = 0) } } ) } } Coroutines
66. val listState = rememberLazyListState() val coroutineScope = rememberCoroutineScope() Box { LazyColumn(state = listState) { // ... } AnimatedVisibility(/* ... */) { ScrollToTopButton( onClick = { coroutineScope.launch { listState.animateScrollToItem(index = 0) } } ) } } Coroutines
67. val listState = rememberLazyListState() val coroutineScope = rememberCoroutineScope() Box { LazyColumn(state = listState) { // ... } AnimatedVisibility(/* ... */) { } } Coroutines ScrollToTopButton( onClick = { coroutineScope.launch { listState.animateScrollToItem(index = 0) } } )
68. val listState = rememberLazyListState() val coroutineScope = rememberCoroutineScope() Box { LazyColumn(state = listState) { // ... } AnimatedVisibility(/* ... */) { } } Coroutines ScrollToTopButton( onClick = { coroutineScope.launch { listState.animateScrollToItem(index = 0) } } )
69. ScrollToTopButton( onClick = { coroutineScope.launch { listState.animateScrollToItem(index = 0) } } ) Coroutines
70. ScrollToTopButton( onClick = { coroutineScope.launch { listState.animateScrollToItem(index = 0) snackbarHostState.showSnackbar( message = "Scrolled to the top!", ) } } ) Coroutines
71. ScrollToTopButton( onClick = { coroutineScope.launch { listState.animateScrollToItem(index = 0) snackbarHostState.showSnackbar( message = "Scrolled to the top!", actionLabel = "Revert", ) } } ) Coroutines
72. ScrollToTopButton( onClick = { coroutineScope.launch { listState.animateScrollToItem(index = 0) val result = snackbarHostState.showSnackbar( message = "Scrolled to the top!", actionLabel = "Revert", ) if (result == SnackbarResult.ActionPerformed) { } } } ) Coroutines
73. ScrollToTopButton( onClick = { coroutineScope.launch { listState.animateScrollToItem(index = 0) val result = snackbarHostState.showSnackbar( message = "Scrolled to the top!", actionLabel = "Revert", ) if (result == SnackbarResult.ActionPerformed) { } } } ) Coroutines val firstItemIndex = listState.firstVisibleItemIndex val firstItemOffset = listState.firstVisibleItemScrollOffset
74. ScrollToTopButton( onClick = { coroutineScope.launch { listState.animateScrollToItem(index = 0) val result = snackbarHostState.showSnackbar( message = "Scrolled to the top!", actionLabel = "Revert", ) if (result == SnackbarResult.ActionPerformed) { listState.animateScrollToItem(firstItemIndex, firstItemOffset) } } } ) Coroutines val firstItemIndex = listState.firstVisibleItemIndex val firstItemOffset = listState.firstVisibleItemScrollOffset
75. Coroutines
76. Resources • Compose API guidelines • Kotlin for Jetpack Compose • Understanding Compose (ADS '19) • Extension oriented design • Suspending over Views by Chris Banes NEW! goo.gle/compose-api-guidelines goo.gle/kotlin-for-compose goo.gle/understanding-compose goo.gle/extension-oriented-design goo.gle/suspending-over-views
77. Thank You! Márton Braun @zsmb13 Composing an API with Kotlin