performLayout method

@override Size performLayout(BoxConstraints constraints)

Hook for subclasses to perform layout within the given constraints.

Implementation

@override
Size performLayout(BoxConstraints constraints) {
  final sb = widget as ScrollBar;
  final width = constraints.hasBoundedWidth
      ? constraints.maxWidth
      : (sb.direction == LayoutDirection.vertical ? 1 : 80);
  final height = constraints.hasBoundedHeight
      ? constraints.maxHeight
      : (sb.direction == LayoutDirection.vertical ? 80 : 1);

  trackHeight = sb.direction == LayoutDirection.vertical ? height : width;

  final total = sb._totalExtent;
  final view = sb._viewportExtent;
  if (total <= 0) {
    thumbHeight = 0;
    thumbPos = 0;
  } else {
    final double ratio = (view / total).clamp(0.0, 1.0);
    thumbHeight = (ratio * trackHeight).round().clamp(1, trackHeight);

    final int maxScrollOffset = total - view;
    thumbPos = 0;
    if (maxScrollOffset > 0) {
      final double scrollRatio = (sb._scrollOffset / maxScrollOffset).clamp(
        0.0,
        1.0,
      );
      thumbPos = (scrollRatio * (trackHeight - thumbHeight)).round().clamp(
        0,
        trackHeight - thumbHeight,
      );
    }
  }

  return constraints.constrain(Size(width, height));
}