Animate Linear Gradient (Brush) infinitely and reversely in Compose

Question

Initial State looks like this (animatedOffset = 0f)

at 1f I want to get the reversed gradient:

Current code:

val transition = rememberInfiniteTransition(label = "NavIconGradientAnim")

val animatedOffset by transition.animateFloat(
    initialValue = 0f, targetValue = 1f,
    label = "NavIconGradientAnimOffset",
    animationSpec = infiniteRepeatable(
        animation = tween(1500, easing = LinearEasing),
        repeatMode = RepeatMode.Reverse
    )
)
Image(
    painterResource(id = item.icon),
    contentDescription = null,
    modifier = Modifier.drawWithCache {
        onDrawWithContent {
            with(drawContext.canvas.nativeCanvas) {
                val angle = 45f
                val endX = drawContext.size.width
                val endY = (endX * kotlin.math.tan(Math.toRadians(angle.toDouble()))).toFloat()

                val checkPoint = saveLayer(null, null)
                drawContent()
                val gradient = Brush.linearGradient(
                    colors = listOf(Color.Blue, Color.Yellow),
                    start = Offset(animatedOffset * endX, animatedOffset * endY),
                    end = Offset(endX, endX)
                )
                drawRect(
                    brush = gradient,
                    blendMode = BlendMode.SrcIn
                )
                restoreToCount(checkPoint)
            }
        }
    }
)

I tried to set

end = Offset(endX - animatedOffset * endX, endY - animatedOffset * endY),

but it doesn't look good (GIF preview):

it changes color abruptly

Answer 1

There are several ways to do it and you don't have to calculate tangent.

One way of doing it is

    val limit = 1.5f

    val transition = rememberInfiniteTransition(label = "shimmer")
    val progressAnimated by transition.animateFloat(
        initialValue = -limit,
        targetValue = limit,
        animationSpec = infiniteRepeatable(
            animation = tween(1500, easing = LinearEasing),
            repeatMode = RepeatMode.Reverse
        ), label = "shimmer"
    )

And set brush with

val width = size.width val height = size.height

val offset = width * progress
val gradientWidth = width

val brush = Brush.linearGradient(
    colors = gradientColors,
    start = Offset(offset, 0f),
    end = Offset(offset + gradientWidth, height)
)

Result

Demo

@Preview
@Composable
private fun SingleGradientTest() {

    Column(
        Modifier.padding(20.dp)
    ) {
        val painterStar = painterResource(id = R.drawable.star_foreground)

        val limit = 1.5f
        var progress by remember {
            mutableStateOf(-limit)
        }

        val transition = rememberInfiniteTransition(label = "shimmer")
        val progressAnimated by transition.animateFloat(
            initialValue = -limit,
            targetValue = limit,
            animationSpec = infiniteRepeatable(
                animation = tween(1500, easing = LinearEasing),
                repeatMode = RepeatMode.Reverse
            ), label = "shimmer"
        )

        Box(
            modifier = Modifier
                .fillMaxWidth()
                .aspectRatio(1f)
                .drawWithCache {
                    val width = size.width
                    val height = size.height

                    val offset = width * progress
                    val gradientWidth = width

                    val brush = Brush.linearGradient(
                        colors = gradientColors,
                        start = Offset(offset, 0f),
                        end = Offset(offset + gradientWidth, height)
                    )

                    onDrawBehind {
                        drawRect(
                            brush = brush,
                            blendMode = BlendMode.SrcIn
                        )
                    }
                }
        )

        Box(
            modifier = Modifier
                .size(100.dp)
                .graphicsLayer {
                    compositingStrategy = CompositingStrategy.Offscreen
                }
                .drawWithCache {
                    val width = size.width
                    val height = size.height

                    val offset = width * progress
                    val gradientWidth = width


                    val brush = Brush.linearGradient(
                        colors = gradientColors,
                        start = Offset(offset, 0f),
                        end = Offset(offset + gradientWidth, height)

                    )

                    onDrawBehind {
                        // Destination
                        with(painterStar) {
                            draw(
                                size = Size(width, width)
                            )
                        }

                        // Source
                        drawRect(
                            brush = brush,
                            blendMode = BlendMode.SrcIn
                        )
                    }
                }
        )

        Text("Progress: $progress")
        Slider(
            value = progress,
            onValueChange = { progress = it },
            valueRange = -limit..limit
        )

        Text(text = "Animated progress: $progressAnimated")
        Box(
            modifier = Modifier
                .size(100.dp)
                .graphicsLayer {
                    compositingStrategy = CompositingStrategy.Offscreen
                }
                .drawWithCache {
                    val width = size.width
                    val height = size.height

                    val offset = width * progressAnimated
                    val gradientWidth = width

                    val brush = Brush.linearGradient(
                        colors = gradientColors,
                        start = Offset(offset, 0f),
                        end = Offset(offset + gradientWidth, height)

                    )

                    onDrawBehind {
                        // Destination
                        with(painterStar) {
                            draw(
                                size = Size(width, width)
                            )
                        }

                        // Source
                        drawRect(
                            brush = brush,
                            blendMode = BlendMode.SrcIn
                        )
                    }
                }
        )
    }
}