VisibilityTrackerNode.kt

private class VisibilityTrackerNode(
    var thresholdPercentage: Float,
    var onVisibilityChanged: (VisibilityInfo) -> Unit,
) : Modifier.Node(), GlobalPositionAwareModifierNode {
    private var previousVisibilityPercentage: Float? = null

    // Minimum visibility difference that triggers an update
    private val minimumVisibilityDelta = 0.01f

    override fun onGloballyPositioned(coordinates: LayoutCoordinates) {
        // Get information about this composable's position
        val boundsInWindow = coordinates.boundsInWindow()

        // Get information about the parent window
        val parentBounds = coordinates.parentLayoutCoordinates?.boundsInWindow()

        if (parentBounds == null) {
            // if parent bounds are null, it means that we are not added to the hierarchy, so we are not visible
            previousVisibilityPercentage = 0f
            return
        }

        // Calculate visible width
        val visibleLeft = max(boundsInWindow.left, parentBounds.left)
        val visibleRight = min(boundsInWindow.right, parentBounds.right)
        val visibleWidth = max(0f, visibleRight - visibleLeft)

        // Calculate visible height
        val visibleTop = max(boundsInWindow.top, parentBounds.top)
        val visibleBottom = min(boundsInWindow.bottom, parentBounds.bottom)
        val visibleHeight = max(0f, visibleBottom - visibleTop)

        // Calculate visible area and total area
        val visibleArea = visibleWidth * visibleHeight
        val totalArea = coordinates.size.width * coordinates.size.height

        // Calculate visibility percentage based on area
        val visibilityPercentage = (visibleArea / totalArea).coerceIn(0f, 1f)

        // Call visibility changed callback only if visibility changed significantly
        val visibilityDifference =
            previousVisibilityPercentage?.let { previous ->
                abs(visibilityPercentage - previous)
            } ?: Float.MAX_VALUE
        // Update only if changed enough
        if (visibilityDifference >= minimumVisibilityDelta) {
            onVisibilityChanged(
                VisibilityInfo(
                    isVisible = visibilityPercentage > 0f,
                    visiblePercentage = visibilityPercentage,
                    bounds = boundsInWindow,
                    isAboveThreshold = visibilityPercentage >= thresholdPercentage,
                ),
            )
            previousVisibilityPercentage = visibilityPercentage
        }
    }
}