computeTargetIndex static method

int computeTargetIndex({ required double currentRelativeX, required double velocityX, required double itemWidth, required int itemCount, double velocityThreshold = 0.5, double projectionTime = 0.3, })

Computes target index based on drag position and velocity.

Implementation

static int computeTargetIndex({
  required double currentRelativeX,
  required double velocityX,
  required double itemWidth,
  required int itemCount,
  double velocityThreshold = 0.5,
  double projectionTime = 0.3,
}) {
  if (currentRelativeX < 0) return 0;
  if (currentRelativeX > 1) return itemCount - 1;

  if (velocityX.abs() > velocityThreshold) {
    final projectedX =
        (currentRelativeX + velocityX * projectionTime).clamp(0.0, 1.0);
    var targetIndex =
        (projectedX / itemWidth).round().clamp(0, itemCount - 1);

    final currentIndex =
        (currentRelativeX / itemWidth).round().clamp(0, itemCount - 1);

    if (velocityX > velocityThreshold &&
        targetIndex <= currentIndex &&
        currentIndex < itemCount - 1) {
      targetIndex = currentIndex + 1;
    } else if (velocityX < -velocityThreshold &&
        targetIndex >= currentIndex &&
        currentIndex > 0) {
      targetIndex = currentIndex - 1;
    }

    return targetIndex;
  }

  return (currentRelativeX / itemWidth).round().clamp(0, itemCount - 1);
}