computeDistanceToActualBaseline()

computeDistanceToActualBaseline() returns the distance from the top of the render object to its text baseline, measured in the render object's own local coordinate space. It is called after layout by getDistanceToBaseline(), which handles caching and the onlyReal flag before delegating here.

Override this method — not getDistanceToBaseline() — in RenderBox subclasses. It is called once per layout cycle; the result is cached until the next layout.

Because layout has already run when this is called, child sizes and positions are known. Query children with child.getDistanceToBaseline() and add the child's vertical offset from its parent data to convert the result into the parent's coordinate space.

Default Implementation#

Returns null — no baseline. Correct for any render object that contains no text and should not participate in baseline alignment.

@override
double? computeDistanceToActualBaseline(TextBaseline baseline) {
  return null;
}

Leaf — Fixed Baseline#

Return a constant when the baseline is determined by a fixed font or design specification rather than live text layout.

@override
double? computeDistanceToActualBaseline(TextBaseline baseline) {
  return baseline == TextBaseline.alphabetic ? 14.0 : null;
}

Single Child — Passthrough#

Delegate to the child when the render object places the child at its own origin. The child's baseline is already expressed in the parent's coordinate space.

@override
double? computeDistanceToActualBaseline(TextBaseline baseline) {
  return child?.getDistanceToBaseline(baseline, onlyReal: true);
}

Single Child — With Offset#

When the child is positioned at a non-zero offset (due to padding or explicit placement computed during layout), add the child's vertical offset to its baseline distance.

@override
double? computeDistanceToActualBaseline(TextBaseline baseline) {
  if (child == null) return null;
  final BoxParentData childParentData = child!.parentData! as BoxParentData;
  final double? childBaseline =
      child!.getDistanceToBaseline(baseline, onlyReal: true);
  if (childBaseline == null) return null;
  return childBaseline + childParentData.offset.dy;
}

Multiple Children — First Baseline#

For vertically stacked children, report the first baseline found, adjusted by that child's vertical position within the parent.

@override
double? computeDistanceToActualBaseline(TextBaseline baseline) {
  RenderBox? child = firstChild;
  while (child != null) {
    final BoxParentData childParentData = child.parentData! as BoxParentData;
    final double? childBaseline =
        child.getDistanceToBaseline(baseline, onlyReal: true);
    if (childBaseline != null) {
      return childBaseline + childParentData.offset.dy;
    }
    child = childParentData.nextSibling;
  }
  return null;
}

Multiple Children — Baseline Alignment#

For a row whose children were aligned to a shared baseline during layout, each child's offset already encodes the alignment. Any child with a baseline gives the same answer — its baseline plus its vertical offset equals the shared baseline distance from the parent's top edge.

@override
double? computeDistanceToActualBaseline(TextBaseline baseline) {
  RenderBox? child = firstChild;
  while (child != null) {
    final BoxParentData childParentData = child.parentData! as BoxParentData;
    final double? childBaseline =
        child.getDistanceToBaseline(baseline, onlyReal: true);
    if (childBaseline != null) {
      return childBaseline + childParentData.offset.dy;
    }
    child = childParentData.nextSibling;
  }
  return null;
}