Customize animations

Many of the Animation APIs commonly accept parameters for customizing their behavior.

Customize animations with the AnimationSpec parameter

Most animation APIs allow developers to customize animation specifications by an optional AnimationSpec parameter.

val alpha: Float by animateFloatAsState(
    targetValue = if (enabled) 1f else 0.5f,
    // Configure the animation duration and easing.
    animationSpec = tween(durationMillis = 300, easing = FastOutSlowInEasing)
)
AnimationSnippets.kt

There are different kinds of AnimationSpec for creating different types of animation.

Create physics-based animation with spring

spring creates a physics-based animation between start and end values. It takes 2 parameters: dampingRatio and stiffness.

dampingRatio defines how bouncy the spring should be. The default value is Spring.DampingRatioNoBouncy.

Figure 1. Setting different spring damping ratios.

stiffness defines how fast the spring should move toward the end value. The default value is Spring.StiffnessMedium.

Figure 2. Setting different spring stiffness

val value by animateFloatAsState(
    targetValue = 1f,
    animationSpec = spring(
        dampingRatio = Spring.DampingRatioHighBouncy,
        stiffness = Spring.StiffnessMedium
    )
)
AnimationSnippets.kt

spring can handle interruptions more smoothly than duration-based AnimationSpec types because it guarantees the continuity of velocity when target value changes amid animations. spring is used as the default AnimationSpec by many animation APIs, such as animate*AsState and updateTransition.

For example, if we apply a spring config to the following animation that is driven by user touch, when interrupting the animation as its progressing, you can see that using tween doesn't respond as smoothly as using spring.

Figure 3. Setting tween vs spring specs for animation, and interrupting it.

Animate between start and end values with easing curve with tween

tween animates between start and end values over the specified durationMillis using an easing curve. tween is short for the word between - as it goes between two values.

You can also specify delayMillis to postpone the start of the animation.

val value by animateFloatAsState(
    targetValue = 1f,
    animationSpec = tween(
        durationMillis = 300,
        delayMillis = 50,
        easing = LinearOutSlowInEasing
    )
)
AnimationSnippets.kt

See Easing for more information.

Animate to specific values at certain timings with keyframes

keyframes animates based on the snapshot values specified at different timestamps in the duration of the animation. At any given time, the animation value will be interpolated between two keyframe values. For each of these keyframes, Easing can be specified to determine the interpolation curve.

It is optional to specify the values at 0 ms and at the duration time. If you do not specify these values, they default to the start and end values of the animation, respectively.

val value by animateFloatAsState(
    targetValue = 1f,
    animationSpec = 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
    }
)
AnimationSnippets.kt

Repeat an animation with repeatable

repeatable runs a duration-based animation (such as tween or keyframes) repeatedly until it reaches the specified iteration count. You can pass the repeatMode parameter to specify whether the animation should repeat by starting from the beginning (RepeatMode.Restart) or from the end (RepeatMode.Reverse).

val value by animateFloatAsState(
    targetValue = 1f,
    animationSpec = repeatable(
        iterations = 3,
        animation = tween(durationMillis = 300),
        repeatMode = RepeatMode.Reverse
    )
)
AnimationSnippets.kt

Repeat an animation infinitely with infiniteRepeatable

infiniteRepeatable is like repeatable, but it repeats for an infinite amount of iterations.

val value by animateFloatAsState(
    targetValue = 1f,
    animationSpec = infiniteRepeatable(
        animation = tween(durationMillis = 300),
        repeatMode = RepeatMode.Reverse
    )
)
AnimationSnippets.kt

In tests using ComposeTestRule, animations using infiniteRepeatable are not run. The component will be rendered using the initial value of each animated value.

Immediately snap to end value with snap

snap is a special AnimationSpec that immediately switches the value to the end value. You can specify delayMillis in order to delay the start of the animation.

val value by animateFloatAsState(
    targetValue = 1f,
    animationSpec = snap(delayMillis = 50)
)
AnimationSnippets.kt

Set a custom easing function

Duration-based AnimationSpec operations (such as tween or keyframes) use Easing to adjust an animation's fraction. This allows the animating value to speed up and slow down, rather than moving at a constant rate. Fraction is a value between 0 (start) and 1.0 (end) indicating the current point in the animation.

Easing is in fact a function that takes a fraction value between 0 and 1.0 and returns a float. The returned value can be outside the boundary to represent overshoot or undershoot. A custom Easing can be created like the code below.

val CustomEasing = Easing { fraction -> fraction * fraction }

@Composable
fun EasingUsage() {
    val value by animateFloatAsState(
        targetValue = 1f,
        animationSpec = tween(
            durationMillis = 300,
            easing = CustomEasing
        )
    )
    // ……
}
AnimationSnippets.kt

Compose provides several built-in Easing functions that cover most use cases. See Speed - Material Design for more information about what Easing to use depending on your scenario.

  • FastOutSlowInEasing
  • LinearOutSlowInEasing
  • FastOutLinearEasing
  • LinearEasing
  • CubicBezierEasing
  • See more

Animate custom data types by converting to and from AnimationVector

Most Compose animation APIs support Float, Color, Dp, and other basic data types as animation values by default, but you sometimes need to animate other data types including your custom ones. During animation, any animating value is represented as an AnimationVector. The value is converted into an AnimationVector and vice versa by a corresponding TwoWayConverter so that the core animation system can handle them uniformly. For example, an Int is represented as an AnimationVector1D that holds a single float value. TwoWayConverter for Int looks like this:

val IntToVector: TwoWayConverter<Int, AnimationVector1D> =
    TwoWayConverter({ AnimationVector1D(it.toFloat()) }, { it.value.toInt() })
AnimationSnippets.kt

Color is essentially a set of 4 values, red, green, blue, and alpha, so Color is converted into an AnimationVector4D that holds 4 float values. In this manner, every data type used in animations is converted to either AnimationVector1D, AnimationVector2D, AnimationVector3D, or AnimationVector4D depending on its dimensionality. This allows different components of the object to be animated independently, each with their own velocity tracking. Built-in converters for basic data types can be accessed using converters such as Color.VectorConverter or Dp.VectorConverter.

When you want to add support for a new data type as an animating value, you can create your own TwoWayConverter and provide it to the API. For example, you can use animateValueAsState to animate your custom data type like this:

data class MySize(val width: Dp, val height: Dp)

@Composable
fun MyAnimation(targetSize: MySize) {
    val animSize: MySize by animateValueAsState(
        targetSize,
        TwoWayConverter(
            convertToVector = { size: MySize ->
                // Extract a float value from each of the `Dp` fields.
                AnimationVector2D(size.width.value, size.height.value)
            },
            convertFromVector = { vector: AnimationVector2D ->
                MySize(vector.v1.dp, vector.v2.dp)
            }
        )
    )
}
AnimationSnippets.kt

The following list includes some built-in VectorConverters:

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Test animations