detectTransformGestures

DetectTransformGestures

@Composable
fun DetectTransformGestures() {
    /**
     * Rotates the given offset around the origin by the given angle in degrees.
     *
     * A positive angle indicates a counterclockwise rotation around the right-handed 2D Cartesian
     * coordinate system.
     *
     * See: [Rotation matrix](https://en.wikipedia.org/wiki/Rotation_matrix)
     */
    fun Offset.rotateBy(angle: Float): Offset {
        val angleInRadians = angle * (PI / 180)
        val cos = cos(angleInRadians)
        val sin = sin(angleInRadians)
        return Offset((x * cos - y * sin).toFloat(), (x * sin + y * cos).toFloat())
    }
    var offset by remember { mutableStateOf(Offset.Zero) }
    var zoom by remember { mutableStateOf(1f) }
    var angle by remember { mutableStateOf(0f) }
    Box(
        Modifier.pointerInput(Unit) {
                detectTransformGestures(
                    onGesture = { centroid, pan, gestureZoom, gestureRotate ->
                        val oldScale = zoom
                        val newScale = zoom * gestureZoom
                        // For natural zooming and rotating, the centroid of the gesture should
                        // be the fixed point where zooming and rotating occurs.
                        // We compute where the centroid was (in the pre-transformed coordinate
                        // space), and then compute where it will be after this delta.
                        // We then compute what the new offset should be to keep the centroid
                        // visually stationary for rotating and zooming, and also apply the pan.
                        offset =
                            (offset + centroid / oldScale).rotateBy(gestureRotate) -
                                (centroid / newScale + pan / oldScale)
                        zoom = newScale
                        angle += gestureRotate
                    }
                )
            }
            .graphicsLayer {
                translationX = -offset.x * zoom
                translationY = -offset.y * zoom
                scaleX = zoom
                scaleY = zoom
                rotationZ = angle
                transformOrigin = TransformOrigin(0f, 0f)
            }
            .background(Color.Blue)
            .fillMaxSize()
    )
}