performPaint method

Implementation

@override
void performPaint(PaintingContext context, Offset offset) {
  Offset _accumulateOffsetFromDescendant(RenderObject descendant, RenderObject ancestor) {
    Offset sum = Offset.zero;
    RenderObject? cur = descendant;
    while (cur != null && cur != ancestor) {
      final Object? pd = (cur is RenderBox) ? (cur.parentData) : null;
      if (pd is ContainerBoxParentData) {
        final Offset o = (pd as ContainerBoxParentData).offset;
        sum += o;
      } else if (pd is RenderLayoutParentData) {
        sum += (pd as RenderLayoutParentData).offset;
      }
      cur = cur.parent as RenderObject?;
    }
    return sum;
  }

  for (int i = 0; i < paintingOrder.length; i++) {
    RenderBox child = paintingOrder[i];
    if (isPositionPlaceholder(child)) continue;

    final RenderLayoutParentData childParentData = child.parentData as RenderLayoutParentData;
    if (!child.hasSize) continue;

    bool restoreFlag = false;
    bool previous = false;
    final bool promoteHere = (renderStyle as CSSRenderStyle).isDocumentRootBox();
    if (promoteHere && child is RenderBoxModel) {
      final CSSRenderStyle rs = child.renderStyle;
      final int? zi = rs.zIndex;
      final bool isPositive = zi != null && zi > 0;
      // Suppress painting of descendant positive stacking contexts when painting
      // normal-flow/auto/zero buckets; those positives are promoted to be painted
      // at this ancestor level to resolve cross-parent ordering.
      if (!isPositive) {
        previous = rs.suppressPositiveStackingFromDescendants;
        rs.suppressPositiveStackingFromDescendants = true;
        // Invalidate cached painting order on this child so suppression takes effect.
        if (child is RenderLayoutBox) {
          child.markChildrenNeedsSort();
        } else if (child is RenderWidget) {
          child.markChildrenNeedsSort();
        }
        restoreFlag = true;
      }
    }

    // Compute correct paint offset even if the render box to paint is not a direct child
    // of this container (e.g., promoted positive stacking contexts).
    final bool direct = identical(child.parent, this);
    final Offset localOffset = direct ? childParentData.offset : _accumulateOffsetFromDescendant(child, this);
    context.paintChild(child, localOffset + offset);

    if (restoreFlag && child is RenderBoxModel) {
      (child.renderStyle as CSSRenderStyle).suppressPositiveStackingFromDescendants = previous;
      if (child is RenderLayoutBox) {
        child.markChildrenNeedsSort();
      } else if (child is RenderWidget) {
        child.markChildrenNeedsSort();
      }
    }
  }
}