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Snowfall Modifier
Author: Sebastiano Poggi
A Modifier that adds the effect of snowflakes falling on the composable that it's applied to.
| internal fun Modifier.snowfall() = composed { | |
| var snowflakesState by remember { | |
| mutableStateOf(SnowflakesState(-1, IntSize(0, 0))) | |
| } | |
| LaunchedEffect(Unit) { | |
| while (isActive) { | |
| withFrameNanos { newTick -> | |
| val elapsedMillis = | |
| (newTick - snowflakesState.tickNanos).nanoseconds.inWholeMilliseconds | |
| val wasFirstRun = snowflakesState.tickNanos < 0 | |
| snowflakesState.tickNanos = newTick | |
| if (wasFirstRun) return@withFrameNanos | |
| for (snowflake in snowflakesState.snowflakes) { | |
| snowflake.update(elapsedMillis) | |
| } | |
| } | |
| } | |
| } | |
| onSizeChanged { newSize -> snowflakesState = snowflakesState.resize(newSize) } | |
| .clipToBounds() | |
| .drawWithContent { | |
| drawContent() | |
| snowflakesState.draw(drawContext.canvas) | |
| } | |
| } | |
| fun ClosedRange<Float>.random() = | |
| ThreadLocalRandom.current().nextFloat() * (endInclusive - start) + start | |
| fun Float.random() = | |
| ThreadLocalRandom.current().nextFloat() * this | |
| fun Int.random() = | |
| ThreadLocalRandom.current().nextInt(this) | |
| fun IntSize.randomPosition() = | |
| Offset(width.random().toFloat(), height.random().toFloat()) | |
| private const val snowflakeDensity = 0.1 | |
| private val incrementRange = 0.4f..0.8f | |
| private val sizeRange = 5.0f..12.0f | |
| private const val angleSeed = 25.0f | |
| private val angleSeedRange = -angleSeed..angleSeed | |
| private const val angleRange = 0.1f | |
| private const val angleDivisor = 10000.0f | |
| internal data class SnowflakesState( | |
| var tickNanos: Long, | |
| val snowflakes: List<Snowflake>, | |
| ) { | |
| constructor(tick: Long, canvasSize: IntSize) : this(tick, createSnowflakes(canvasSize)) | |
| fun draw(canvas: Canvas) { | |
| snowflakes.forEach { it.draw(canvas) } | |
| } | |
| fun resize(newSize: IntSize) = copy(snowflakes = createSnowflakes(newSize)) | |
| companion object { | |
| private fun createSnowflakes(canvasSize: IntSize): List<Snowflake> { | |
| val canvasArea = canvasSize.width * canvasSize.height | |
| val normalizedDensity = snowflakeDensity.coerceIn(0.0..1.0) / 500.0 | |
| val snowflakesCount = (canvasArea * normalizedDensity).roundToInt() | |
| return List(snowflakesCount) { | |
| Snowflake( | |
| incrementFactor = incrementRange.random(), | |
| size = sizeRange.random(), | |
| canvasSize = canvasSize, | |
| position = canvasSize.randomPosition(), | |
| angle = angleSeed.random() / angleSeed * angleRange + (PI / 2.0) - (angleRange / 2.0) | |
| ) | |
| } | |
| } | |
| } | |
| } | |
| private val snowflakePaint = Paint().apply { | |
| isAntiAlias = true | |
| color = Color.White | |
| style = PaintingStyle.Fill | |
| } | |
| internal class Snowflake( | |
| private val incrementFactor: Float, | |
| private val size: Float, | |
| private val canvasSize: IntSize, | |
| position: Offset, | |
| angle: Double | |
| ) { | |
| private var position by mutableStateOf(position) | |
| private var angle by mutableStateOf(angle) | |
| fun update(elapsedMillis: Long) { | |
| val increment = incrementFactor * (elapsedMillis / baseFrameDurationMillis) * baseSpeedPxAt60Fps | |
| val xDelta = (increment * cos(angle)).toFloat() | |
| val yDelta = (increment * sin(angle)).toFloat() | |
| position = Offset(position.x + xDelta, position.y + yDelta) | |
| angle += angleSeedRange.random() / angleDivisor | |
| if (position.y > canvasSize.height + size) { | |
| position = Offset(position.x, -size) | |
| } | |
| } | |
| fun draw(canvas: Canvas) { | |
| canvas.drawCircle(position, size, snowflakePaint) | |
| } | |
| } |
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