Value-based animations

Animate a single value with animate*AsState

The animate*AsState functions are the simplest animation APIs in Compose for animating a single value. You only provide the target value (or end value), and the API starts animation from the current value to the specified value.

Below is an example of animating alpha using this API. By simply wrapping the target value in animateFloatAsState, the alpha value is now an animation value between the provided values (1f or 0.5f in this case).

var enabled by remember { mutableStateOf(true) }

val alpha: Float by animateFloatAsState(if (enabled) 1f else 0.5f, label = "alpha")
Box(
    Modifier
        .fillMaxSize()
        .graphicsLayer(alpha = alpha)
        .background(Color.Red)
)
AnimationSnippets.kt

Note that you don't need to create an instance of any animation class, or handle interruption. Under the hood, an animation object (namely, an Animatable instance) will be created and remembered at the call site, with the first target value as its initial value. From there on, any time you supply this composable a different target value, an animation is automatically started towards that value. If there's already an animation in flight, the animation starts from its current value (and velocity) and animates toward the target value. During the animation, this composable gets recomposed and returns an updated animation value every frame.

Out of the box, Compose provides animate*AsState functions for Float, Color, Dp, Size, Offset, Rect, Int, IntOffset, and IntSize. You can easily add support for other data types by providing a TwoWayConverter to animateValueAsState that takes a generic type.

You can customize the animation specifications by providing an AnimationSpec. See AnimationSpec for more information.

Animate multiple properties simultaneously with a transition

Transition manages one or more animations as its children and runs them simultaneously between multiple states.

The states can be of any data type. In many cases, you can use a custom enum type to ensure type safety, as in this example:

enum class BoxState {
    Collapsed,
    Expanded
}
AnimationSnippets.kt

updateTransition creates and remembers an instance of Transition and updates its state.

var currentState by remember { mutableStateOf(BoxState.Collapsed) }
val transition = updateTransition(currentState, label = "box state")
AnimationSnippets.kt

You can then use one of animate* extension functions to define a child animation in this transition. Specify the target values for each of the states. These animate* functions return an animation value that is updated every frame during the animation when the transition state is updated with updateTransition.

val rect by transition.animateRect(label = "rectangle") { state ->
    when (state) {
        BoxState.Collapsed -> Rect(0f, 0f, 100f, 100f)
        BoxState.Expanded -> Rect(100f, 100f, 300f, 300f)
    }
}
val borderWidth by transition.animateDp(label = "border width") { state ->
    when (state) {
        BoxState.Collapsed -> 1.dp
        BoxState.Expanded -> 0.dp
    }
}
AnimationSnippets.kt

Optionally, you can pass a transitionSpec parameter to specify a different AnimationSpec for each of the combinations of transition state changes. See AnimationSpec for more information.

val color by transition.animateColor(
    transitionSpec = {
        when {
            BoxState.Expanded isTransitioningTo BoxState.Collapsed ->
                spring(stiffness = 50f)

            else ->
                tween(durationMillis = 500)
        }
    }, label = "color"
) { state ->
    when (state) {
        BoxState.Collapsed -> MaterialTheme.colorScheme.primary
        BoxState.Expanded -> MaterialTheme.colorScheme.background
    }
}
AnimationSnippets.kt

Once a transition has arrived at the target state, Transition.currentState will be the same as Transition.targetState. This can be used as a signal for whether the transition has finished.

We sometimes want to have an initial state different from the first target state. We can use updateTransition with MutableTransitionState to achieve this. For example, it allows us to start animation as soon as the code enters composition.

// Start in collapsed state and immediately animate to expanded
var currentState = remember { MutableTransitionState(BoxState.Collapsed) }
currentState.targetState = BoxState.Expanded
val transition = rememberTransition(currentState, label = "box state")
// ……
AnimationSnippets.kt

For a more complex transition involving multiple composable functions, you can use createChildTransition to create a child transition. This technique is useful for separating concerns among multiple subcomponents in a complex composable. The parent transition will be aware of all the animation values in the child transitions.

enum class DialerState { DialerMinimized, NumberPad }

@Composable
fun DialerButton(isVisibleTransition: Transition<Boolean>) {
    // `isVisibleTransition` spares the need for the content to know
    // about other DialerStates. Instead, the content can focus on
    // animating the state change between visible and not visible.
}

@Composable
fun NumberPad(isVisibleTransition: Transition<Boolean>) {
    // `isVisibleTransition` spares the need for the content to know
    // about other DialerStates. Instead, the content can focus on
    // animating the state change between visible and not visible.
}

@Composable
fun Dialer(dialerState: DialerState) {
    val transition = updateTransition(dialerState, label = "dialer state")
    Box {
        // Creates separate child transitions of Boolean type for NumberPad
        // and DialerButton for any content animation between visible and
        // not visible
        NumberPad(
            transition.createChildTransition {
                it == DialerState.NumberPad
            }
        )
        DialerButton(
            transition.createChildTransition {
                it == DialerState.DialerMinimized
            }
        )
    }
}
AnimationSnippets.kt

Use transition with AnimatedVisibility and AnimatedContent

AnimatedVisibility and AnimatedContent are available as extension functions of Transition. The targetState for Transition.AnimatedVisibility and Transition.AnimatedContent is derived from the Transition, and triggering enter/exit transitions as needed when the Transition's targetState has changed. These extension functions allow all the enter/exit/sizeTransform animations that would otherwise be internal to AnimatedVisibility/AnimatedContent to be hoisted into the Transition. With these extension functions, AnimatedVisibility/AnimatedContent's state change can be observed from outside. Instead of a boolean visible parameter, this version of AnimatedVisibility takes a lambda that converts the parent transition's target state into a boolean.

See AnimatedVisibility and AnimatedContent for the details.

var selected by remember { mutableStateOf(false) }
// Animates changes when `selected` is changed.
val transition = updateTransition(selected, label = "selected state")
val borderColor by transition.animateColor(label = "border color") { isSelected ->
    if (isSelected) Color.Magenta else Color.White
}
val elevation by transition.animateDp(label = "elevation") { isSelected ->
    if (isSelected) 10.dp else 2.dp
}
Surface(
    onClick = { selected = !selected },
    shape = RoundedCornerShape(8.dp),
    border = BorderStroke(2.dp, borderColor),
    shadowElevation = elevation
) {
    Column(
        modifier = Modifier
            .fillMaxWidth()
            .padding(16.dp)
    ) {
        Text(text = "Hello, world!")
        // AnimatedVisibility as a part of the transition.
        transition.AnimatedVisibility(
            visible = { targetSelected -> targetSelected },
            enter = expandVertically(),
            exit = shrinkVertically()
        ) {
            Text(text = "It is fine today.")
        }
        // AnimatedContent as a part of the transition.
        transition.AnimatedContent { targetState ->
            if (targetState) {
                Text(text = "Selected")
            } else {
                Icon(imageVector = Icons.Default.Phone, contentDescription = "Phone")
            }
        }
    }
}
AnimationSnippets.kt

Encapsulate a transition and make it reusable

For simple use cases, defining transition animations in the same composable as your UI is a perfectly valid option. When you are working on a complex component with a number of animated values, however, you might want to separate the animation implementation from the composable UI.

You can do so by creating a class that holds all the animation values and an ‘update’ function that returns an instance of that class. The transition implementation can be extracted into the new separate function. This pattern is useful when there is a need to centralize the animation logic, or make complex animations reusable.

enum class BoxState { Collapsed, Expanded }

@Composable
fun AnimatingBox(boxState: BoxState) {
    val transitionData = updateTransitionData(boxState)
    // UI tree
    Box(
        modifier = Modifier
            .background(transitionData.color)
            .size(transitionData.size)
    )
}

// Holds the animation values.
private class TransitionData(
    color: State<Color>,
    size: State<Dp>
) {
    val color by color
    val size by size
}

// Create a Transition and return its animation values.
@Composable
private fun updateTransitionData(boxState: BoxState): TransitionData {
    val transition = updateTransition(boxState, label = "box state")
    val color = transition.animateColor(label = "color") { state ->
        when (state) {
            BoxState.Collapsed -> Color.Gray
            BoxState.Expanded -> Color.Red
        }
    }
    val size = transition.animateDp(label = "size") { state ->
        when (state) {
            BoxState.Collapsed -> 64.dp
            BoxState.Expanded -> 128.dp
        }
    }
    return remember(transition) { TransitionData(color, size) }
}
AnimationSnippets.kt

Create an infinitely repeating animation with rememberInfiniteTransition

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. You can create an instance of InfiniteTransition with rememberInfiniteTransition. Child animations can be added with animateColor, animatedFloat, or animatedValue. You also need to specify an infiniteRepeatable to specify the animation specifications.

val infiniteTransition = rememberInfiniteTransition(label = "infinite")
val color by infiniteTransition.animateColor(
    initialValue = Color.Red,
    targetValue = Color.Green,
    animationSpec = infiniteRepeatable(
        animation = tween(1000, easing = LinearEasing),
        repeatMode = RepeatMode.Reverse
    ),
    label = "color"
)

Box(
    Modifier
        .fillMaxSize()
        .background(color)
)
AnimationSnippets.kt

Low-level animation APIs

All the high-level animation APIs mentioned in the previous section are built on top of the foundation of the low-level animation APIs.

The animate*AsState functions are the simplest APIs, that render an instant value change as an animation value. It is backed by Animatable, which is a coroutine-based API for animating a single value. updateTransition creates a transition object that can manage multiple animating values and run them based on a state change. rememberInfiniteTransition is similar, but it creates an infinite transition that can manage multiple animations that keep on running indefinitely. All of these APIs are composables except for Animatable, which means these animations can be created outside of composition.

All of these APIs are based on the more fundamental Animation API. Though most apps will not interact directly with Animation, some of the customization capabilities for Animation are available through higher-level APIs. See Customize animations for more information on AnimationVector and AnimationSpec.

Animatable: Coroutine-based single value animation

Animatable is a value holder that can animate the value as it is changed via animateTo. This is the API backing up the implementation of animate*AsState. It ensures consistent continuation and mutual exclusiveness, meaning that the value change is always continuous and any ongoing animation will be canceled.

Many features of Animatable, including animateTo, are provided as suspend functions. This means that they need to be wrapped in an appropriate coroutine scope. For example, you can use the LaunchedEffect composable to create a scope just for the duration of the specified key value.

// Start out gray and animate to green/red based on `ok`
val color = remember { Animatable(Color.Gray) }
LaunchedEffect(ok) {
    color.animateTo(if (ok) Color.Green else Color.Red)
}
Box(
    Modifier
        .fillMaxSize()
        .background(color.value)
)
AnimationSnippets.kt

In the example above, we create and remember an instance of Animatable with the initial value of Color.Gray. Depending on the value of the boolean flag ok, the color animates to either Color.Green or Color.Red. Any subsequent change to the boolean value starts animation to the other color. If there's an ongoing animation when the value is changed, the animation is canceled, and the new animation starts from the current snapshot value with the current velocity.

This is the animation implementation that backs up the animate*AsState API mentioned in the previous section. Compared to animate*AsState, using Animatable directly gives us finer-grained control on several respects. First, Animatable can have an initial value different from its first target value. For example, the code example above shows a gray box at first, which immediately starts animating to either green or red. Second, Animatable provides more operations on the content value, namely snapTo and animateDecay. snapTo sets the current value to the target value immediately. This is useful when the animation itself is not the only source of truth and has to be synced with other states, such as touch events. animateDecay starts an animation that slows down from the given velocity. This is useful for implementing fling behavior. See Gesture and animation for more information.

Out of the box, Animatable supports Float and Color, but any data type can be used by providing a TwoWayConverter. See AnimationVector for more information.

You can customize the animation specifications by providing an AnimationSpec. See AnimationSpec for more information.

Animation: Manually controlled animation

Animation is the lowest-level Animation API available. Many of the animations we've seen so far build ontop of Animation. There are two Animation subtypes: TargetBasedAnimation and DecayAnimation.

Animation should only be used to manually control the time of the animation. Animation is stateless, and it does not have any concept of lifecycle. It serves as an animation calculation engine that the higher-level APIs use.

TargetBasedAnimation

Other APIs cover most use cases, but using TargetBasedAnimation directly allows you to control the animation play time yourself. In the example below, the play time of the TargetAnimation is manually controlled based on the frame time provided by withFrameNanos.

val anim = remember {
    TargetBasedAnimation(
        animationSpec = tween(200),
        typeConverter = Float.VectorConverter,
        initialValue = 200f,
        targetValue = 1000f
    )
}
var playTime by remember { mutableLongStateOf(0L) }

LaunchedEffect(anim) {
    val startTime = withFrameNanos { it }

    do {
        playTime = withFrameNanos { it } - startTime
        val animationValue = anim.getValueFromNanos(playTime)
    } while (someCustomCondition())
}
AnimationSnippets.kt

DecayAnimation

Unlike TargetBasedAnimation, DecayAnimation does not require a targetValue to be provided. Instead, it calculates its targetValue based on the starting conditions, set by initialVelocity and initialValue and the supplied DecayAnimationSpec.

Decay animations are often used after a fling gesture to slow elements down to a stop. The animation velocity starts at the value set by initialVelocityVector and slows down over time.