describeOverflowSemantics method

Implementation

void describeOverflowSemantics(SemanticsConfiguration config) {
  final CSSOverflowType overflowX = renderStyle.effectiveOverflowX;
  final CSSOverflowType overflowY = renderStyle.effectiveOverflowY;

  final bool xScrollable =
      (overflowX == CSSOverflowType.scroll || overflowX == CSSOverflowType.auto) && scrollOffsetX != null;
  final bool yScrollable =
      (overflowY == CSSOverflowType.scroll || overflowY == CSSOverflowType.auto) && scrollOffsetY != null;

  if (!xScrollable && !yScrollable) {
    return;
  }

  // Mirror Flutter's _RenderScrollSemantics: scrolling regions are semantic
  // boundaries with explicit child nodes, and expose scroll metrics.
  config.isSemanticBoundary = true;
  config.explicitChildNodes = true;
  config.hasImplicitScrolling = true;

  final Size? viewport = _viewportSize;
  final Size? content = _scrollableSize;
  if (viewport == null || content == null) {
    return;
  }

  final double maxX = math.max(0.0, content.width - viewport.width);
  final double maxY = math.max(0.0, content.height - viewport.height);

  // Expose primary-axis scroll metrics (prefer vertical when available).
  if (yScrollable) {
    config.scrollExtentMin = 0.0;
    config.scrollExtentMax = maxY;
    config.scrollPosition = scrollTop.clamp(0.0, maxY).toDouble();
  } else if (xScrollable) {
    config.scrollExtentMin = 0.0;
    config.scrollExtentMax = maxX;
    config.scrollPosition = scrollLeft.clamp(0.0, maxX).toDouble();
  }

  // Best-effort child count: used by some platforms to provide "x of y".
  try {
    config.scrollChildCount = renderStyle.target.childNodes.length;
  } catch (_) {}

  final bool canScroll = (yScrollable && maxY > 0.0) || (xScrollable && maxX > 0.0);
  if (!canScroll) {
    return;
  }

  // Semantic scroll-to-offset for implicit scrolling.
  config.onScrollToOffset = (Offset targetOffset) {
    if (yScrollable && scrollOffsetY != null) {
      final double targetY = targetOffset.dy.clamp(0.0, maxY).toDouble();
      scrollOffsetY!.jumpTo(targetY);
    }
    if (xScrollable && scrollOffsetX != null) {
      final double targetX = targetOffset.dx.clamp(0.0, maxX).toDouble();
      scrollOffsetX!.jumpTo(targetX);
    }
  };

  // Semantic scroll actions (RenderSemanticsGestureHandler-like behavior).
  if (yScrollable && scrollOffsetY != null && maxY > 0.0 && hasSize) {
    config.onScrollUp = () {
      final double delta = size.height * -_kSemanticsScrollFactor;
      final double next = (scrollTop + delta).clamp(0.0, maxY).toDouble();
      scrollOffsetY!.jumpTo(next);
    };
    config.onScrollDown = () {
      final double delta = size.height * _kSemanticsScrollFactor;
      final double next = (scrollTop + delta).clamp(0.0, maxY).toDouble();
      scrollOffsetY!.jumpTo(next);
    };
  }

  if (xScrollable && scrollOffsetX != null && maxX > 0.0 && hasSize) {
    config.onScrollLeft = () {
      final double delta = size.width * -_kSemanticsScrollFactor;
      final double next = (scrollLeft + delta).clamp(0.0, maxX).toDouble();
      scrollOffsetX!.jumpTo(next);
    };
    config.onScrollRight = () {
      final double delta = size.width * _kSemanticsScrollFactor;
      final double next = (scrollLeft + delta).clamp(0.0, maxX).toDouble();
      scrollOffsetX!.jumpTo(next);
    };
  }
}