I’ve spent the last few weeks learning about compose and decided to write a post to share my notes. This post is not meant to teach you everything about Compose but rather it’ll be more like a roadmap that you can use to learn Compose or to see what you still don’t know about Compose. This article will be constantly updated as I learn new things. State hoisting State hoisting is a pattern of moving state up to make a component stateless. When applied to composables, this often means introducing two parameters to the composable. value: T – the current value to display onValueChange: (T) -> Unit – an event that requests the value to change, where T is the proposed new value CompositionLocal Tool for passing data down through the Composition implicitly. CompositionLocalProvider(LocalContentAlpha provides ContentAlpha.medium) {
  Text(...)
}
CompositionLocalProvider(LocalContentAlpha provides ContentAlpha.disabled) {
  Icon(...)
  Text(...)
} compositionLocalOf Changing the value provided during recomposition invalidates only the content that reads its current value. data class Elevations(val card: Dp = 0.dp, val default: Dp = 0.dp)
val LocalElevations = compositionLocalOf { Elevations() } // Create composition with default value

val elevations = Elevations(card = 4.dp, default = 2.dp) // Provide different value here
CompositionLocalProvider(LocalElevations provides elevations) { ... } staticCompositionLocalOf Reads of a are not tracked by Compose. Changing the value causes the entirety of the content lambda where the CompositionLocal is provided to be recomposed. staticCompositionLocalOf If the value provided to the CompositionLocal is highly unlikely to change or will never change, use to get performance benefits. staticCompositionLocalOf Keeping track of changes remember A value computed by remember will be stored in the composition tree, and only be recomputed if the keys to remember change. When adding memory to a composable, always ask yourself “will some caller reasonably want to control this?” If the answer is yes, make a parameter instead. If the answer is no, keep it as a local variable. Remember stores values in the Composition, and will forget them if the composable that called remember is removed. This means you shouldn’t rely upon remember to store important things inside of composables that add and remove children such as . LazyColumn rememberSaveable It behaves similarly to remember, but the stored value will survive the activity or process recreation using the saved instance state mechanism rememberUpdatedState In some situations you might want to capture a value in your effect that, if it changes, you do not want the effect to restart. Create a reference to this value which can be captured and updated. rememberLauncherForActivityResult val result = remember { mutableStateOf<Bitmap?>(null) }
val launcher = rememberLauncherForActivityResult(ActivityResultContracts.TakePicturePreview()) {
  result.value = it
}

Button(onClick = { launcher.launch() }) { ... }

result.value?.let { image ->
  Image(image.asImageBitmap(), null, modifier = Modifier.fillMaxWidth())
} rememberCoroutineScope In order to launch a coroutine outside of a composable, but scoped so that it will be automatically canceled once it leaves the composition is used for scoping jobs initiated by the composition. is for scoping jobs initiated by a user interaction. LaunchedEffect rememberCoroutineScope LaunchedEffect Call suspend functions safely from inside a composable. The coroutine will be cancelled if LaunchedEffect leaves the composition. If LaunchedEffect is recomposed with different keys, the existing coroutine will be cancelled and the new suspend function will be launched in a new coroutine. val currentOnTimeout by rememberUpdatedState(onTimeout)

LaunchedEffect(true) {
  delay(SplashWaitTimeMillis)
  currentOnTimeout()
} snapshotFlow Convert Compose State<T> objects into a Flow. DisposableEffect DisposableEffect is meant for side effects that need to be cleaned up after the keys change or the composable leaves the Composition. DisposableEffect(dispatcher) {
  dispatcher.addCallback(backCallback)
  onDispose {
    backCallback.remove()
  }
} derivedStateOf derivedStateOf is used when you want a Compose State that’s derived from another State. State Holder State holders always need to be in order to keep them in the Composition and not create a new one every time. It’s a good practice to create a method in the same file that does this to remove boilerplate and avoid any mistakes that might occur. remembered Layouts LazyColumn and LazyRow Don’t recycle their children like RecyclerView. It emits new Composables as you scroll through it and is still performant as emitting Composables is relatively cheap compared to instantiating Android Views. val listState = rememberLazyListState()

LazyColumn(
  contentPadding,
  verticalArrangement / horizontalArrangement = Arrangement.spacedBy
) {
  stickyHeader { Header()  } // Experimental
  items(5) { index -> Text(text = "Item: $index") } // key = ...
  item { Text(text = "Last item") }
}

LaunchedEffect(listState) {
  snapshotFlow { listState.firstVisibleItemIndex }
}

firstVisibleItemIndex
firstVisibleItemScrollOffset

scrollToItem()
animateScrollToItem() ConstraintLayout ConstraintLayout {
  val (button, text) = createRefs()

  Button(
    modifier = Modifier.constrainAs(button) {
      top.linkTo(parent.top, margin = 16.dp)
    }
  )

  Text("Text", Modifier.constrainAs(text) {
    top.linkTo(button.bottom, margin = 16.dp)
  })
}

ConstraintSet {
  val button = createRefFor("button")
  val text = createRefFor("text")

  constrain(button) { ... }
  constrain(text) { ... }
} Layout Instead of controlling how a single composable gets measured and laid out on the screen, you might have the same necessity for a group of composables. For that, you can use the Layout composable to manually control how to measure and position the layout’s children. Layout(
  modifier = modifier,
  content = content
) { measurables, constraints ->
  // Don't constrain child views further, measure them with given constraints
  val placeables = measurables.map { measurable -> measurable.measure(constraints) }

  // Set the size of the layout as big as it can
  layout(constraints.maxWidth, constraints.maxHeight) {
    var yPosition = 0

    placeables.forEach { placeable ->
      placeable.placeRelative(x = 0, y = yPosition)
      yPosition += placeable.height
    }
  }
} Intrinsic measurements Row(modifier = modifier.height(IntrinsicSize.Min)) Navigation val navController = rememberNavController()
val backstackEntry = navController.currentBackStackEntryAsState()

NavHost(navController = navController, startDestination = "profile") {
  composable(
    "profile/{userId}/?mode={mode}",
    arguments = listOf(
      navArgument("userId") { type = NavType.StringType },
      navArgument("mode") { defaultValue = "lite" },
    ),
    deepLinks = listOf(navDeepLink {
      uriPattern = "rally://$accountsName/{name}"
    })
  ) { 
    Profile(/*...*/) 
  }
  
  loginGraph()
}

fun NavGraphBuilder.loginGraph(navController: NavController) {
  navigation(startDestination = "username", route = "login") {
    composable("username") { ... }
  }
}

navController.navigate("friends") {
  popUpTo("home") { inclusive = true }
}

navController.navigate(screen.route) {
  // Pop up to the start destination of the graph to avoid building up a large stack 
  // of destinations on the back stack as users select items
  popUpTo(navController.graph.findStartDestination().id) {
    saveState = true
  }
  // Avoid multiple copies of the same destination when reselecting the same item
  launchSingleTop = true
  // Restore state when reselecting a previously selected item
  restoreState = true
} ViewModel @HiltViewModel
class ExampleViewModel @Inject constructor(
  ...
) : ViewModel() { /* ... */ }

@AndroidEntryPoint
class ExampleActivity : AppCompatActivity()

@Composable
fun ExampleScreen(
  exampleViewModel: ExampleViewModel = viewModel()
) { /* ... */ } Animation You can create an animation value by simply wrapping the changing value with the corresponding variant of composables. animate*AsState Float, Color, Dp, Size, Bounds, Offset, Rect, Int, IntOffset, and IntSize You can combine multiple transition objects with a + operator. MutableTransitionState(false).apply {
  targetState = true  // Start the animation immediately
  // isIdle, currentState
} AnimatedVisibility Content within (direct or indirect children) can use the animateEnterExit modifier to specify different animation behavior for each of them. AnimatedVisibility AnimatedContent defines how the size should animate between the initial and the target contents. SizeTransform animateContentSize Animates a size change Crossfade Animates between two layouts with a crossfade animation. Animatable Animatable is a value holder that can animate the value as it is changed via . animateTo sets the current value to the target value immediately. snapTo starts an animation that slows down from the given velocity. animateDecay val color = remember { Animatable(Color.Gray) }
LaunchedEffect(ok) { color.animateTo(if (ok) Color.Green else Color.Red) }InfiniteTransition InfiniteTransition Holds one or more child animations like Transition, but the animations start running as soon as they enter the composition and do not stop unless they are removed. Use the function. rememberInfiniteTransition Transition Transition manages one or more animations as its children and runs them simultaneously . between multiple states var currentState by remember { mutableStateOf(BoxState.Collapsed) }
val transition = updateTransition(currentState)

// Start in collapsed state and immediately animate to expanded
var currentState = remember { MutableTransitionState(BoxState.Collapsed) }
currentState.targetState = BoxState.Expanded
val transition = updateTransition(currentState)

transition.AnimatedVisibility
transition.AnimatedContent AnimationSpec spring(
  dampingRatio = Spring.DampingRatioHighBouncy,
  stiffness = Spring.StiffnessMedium
)

tween(
  durationMillis = 300,
  delayMillis = 50,
  easing = LinearOutSlowInEasing
)

// animates based on the snapshot values specified at different timestamps
keyframes {
  durationMillis = 375
  0.0f at 0 with LinearOutSlowInEasing // for 0-15 ms
  0.2f at 15 with FastOutLinearInEasing // for 15-75 ms
  0.4f at 75 // ms
  0.4f at 225 // ms
}

// runs a animation repeatedly until it reaches the specified iteration count
repeatable(
  iterations = 3,
  animation = tween(durationMillis = 300),
  repeatMode = RepeatMode.Reverse
)

// like repeatable, but it repeats for an infinite amount of iterations
infiniteRepeatable(
  animation = tween(durationMillis = 300),
  repeatMode = RepeatMode.Reverse
)

// immediately switches the value to the end value
snap(delayMillis = 50) Theming When defining colors, we name them “literally”, based on the color value, rather than “semantically” e.g. Red500 not primary. This enables us to define multiple themes e.g. another color might be considered primary in dark theme or on a differently styled screen. isSystemInDarkTheme() / MaterialTheme.colors.isLight color: Color = MaterialTheme.colors.surface, contentColor: Color = contentColorFor(color) When setting the color of any elements, prefer using a Surface to do this as it sets an appropriate content color value, be wary of direct Modifier.background calls which do not set an appropriate content color. CompositionLocal TextFieldDefaults textFieldColors outlinedTextFieldColors ProvideTextStyle So how do components set a theme typography style? Under the hood they use the composable (which itself uses a CompositionLocal) to set a “current” TextStyle. The Text composable defaults to querying this “current” style if you do not provide a concrete textStyle parameter. ProvideTextStyle Resources stringResource(R.string.congratulate, "New Year", 2021)
dimensionResource(R.dimen.padding_small)

painterResource(id = R.drawable.ic_logo)
animatedVectorResource(id = R.drawable.animated_vector)

Icon(Icons.Rounded.Menu) Fonts private val light = Font(R.font.raleway_light, FontWeight.W300)
private val regular = Font(R.font.raleway_regular, FontWeight.W400)
private val medium = Font(R.font.raleway_medium, FontWeight.W500)
private val semibold = Font(R.font.raleway_semibold, FontWeight.W600)

// Create a font family to use in TextStyles
private val craneFontFamily = FontFamily(light, regular, medium, semibold) buildAnnotatedString buildAnnotatedString {
  append("This is some unstyled text\\n")
  withStyle(SpanStyle(color = Color.Red)) {
    append("Red text\\n")
  }
  pushStringAnnotation(tag = "URL", annotation = "<https://developer.android.com>")
  withStyle(SpanStyle(fontSize = 24.sp)) {
    append("Large text")
  }
  pop()
} Generic Modifiers align
alignBy

// animateEnterExit modifier can be used for any direct or indirect children 
// of AnimatedVisibility to create a different enter/exit animation than 
// what's specified in AnimatedVisibility.
animateEnterExit(enter, exit)

border
background
clip
clipToBounds

drawBehind
drawWithCache
drawWithContent

// The draw layer can be invalidated separately from parents. 
// scaleX, scaleY, rotationXYZ, alpha, shadowElevation, shape, clip, shape
// Use with state values such as ScrollState or LazyListState
graphicsLayer

shadow
zIndex

onKeyEvent

// Creates a LayoutModifier that allows changing how the wrapped element is 
// measured and laid out.
layout

absoluteOffset
offset

fillMaxHeight, fillMaxWidth, fillMaxSize
// matches the size of the Box after all other children have been measured 
// to obtain the Box's size.
matchParentSize

heighIn(min, max), widthIn(min, max) Gesture Modifiers combinedClickable(onLongClick, onDoubleClick, onClick)

draggable(
  orientation = Orientation.Horizontal,
  state = rememberDraggableState { delta ->
    val newValue = offsetPosition.value + delta
    offsetPosition.value = newValue.coerceIn(minPx, maxPx)
  }
)

swipeable(
  state = swipeableState,
  anchors = anchors,
  thresholds = { _, _ -> FractionalThreshold(0.3f) },
  orientation = Orientation.Horizontal
)

pointerInput(Unit) {
  detectTapGestures(onPress, onDoubleTap, onLongPress, onTap)
  detectDragGestures { change, dragAmount ->
    change.consumeAllChanges()
    offsetX += dragAmount.x
    offsetY += dragAmount.y
  }
}

horizontalScroll, verticalScroll

// Detects the scroll gestures, but does not offset its contents.
// Has nested scroll built in
scrollable

nestedScroll(nestedScrollConnection, nestedScrollDispatcher) Miscelaneous LocalSoftwareKeyboardController val keyboardController = LocalSoftwareKeyboardController.current
// Calling this function is considered a side-effect and should not be called directly from recomposition
keyboardController.hide() LocalOnBackPressedDispatcherOwner and BackHandler var backHandlingEnabled by remember { mutableStateOf(true) }
var backPressedCount by remember { mutableStateOf(0) }
BackHandler(backHandlingEnabled) { backPressedCount++ }

val dispatcher = LocalOnBackPressedDispatcherOwner.current!!.onBackPressedDispatcher

Button(onClick = { dispatcher.onBackPressed() }) {
  Text("Press Back count $backPressedCount")
} LocalDensity val sizeInPx = with(LocalDensity.current) { 16.dp.toPx() }
val (minPx, maxPx) = with(LocalDensity.current) { min.toPx() to max.toPx() } FocusRequester val focusRequester = remember { FocusRequester() }
var color by remember { mutableStateOf(Black) }
Box(
  Modifier
    .clickable { focusRequester.requestFocus() }
    .border(2.dp, color)
    // The focusRequester should be added BEFORE the focusable.
    .focusRequester(focusRequester)
    // The onFocusChanged should be added BEFORE the focusable that is being observed.
    .onFocusChanged { color = if (it.isFocused) Green else Black }
    .focusable()
) Thank you for reading. If you have any suggestions feel free to contact me. Resources https://developer.android.com/jetpack/compose/architecture https://developer.android.com/jetpack/compose/resources https://developer.android.com/jetpack/compose/animation https://developer.android.com/jetpack/compose/gestures https://developer.android.com/jetpack/compose/navigation https://developer.android.com/jetpack/compose/modifiers-list https://developer.android.com/jetpack/compose/mental-model https://developer.android.com/jetpack/compose/compositionlocal https://developer.android.com/codelabs/jetpack-compose-basics https://developer.android.com/codelabs/jetpack-compose-layouts https://developer.android.com/codelabs/jetpack-compose-state https://developer.android.com/codelabs/jetpack-compose-theming https://developer.android.com/codelabs/jetpack-compose-animation https://developer.android.com/codelabs/jetpack-compose-navigation https://developer.android.com/codelabs/jetpack-compose-advanced-state-side-effects https://developer.android.com/reference/kotlin/androidx/compose/runtime/saveable/package-summary Also published . here

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