performBoxLayout method

@override void performBoxLayout(BoxConstraints constraints)

Perform layout with typed box constraints

Subclasses override this instead of performLayout. The default implementation sizes to the maxima, treating an unbounded axis as the corresponding lower bound.

Implementation

@override
void performBoxLayout(BoxConstraints constraints) {
  if (_child != null) {
    // Combine parent constraints with additional constraints. `null`
    // maxima are unbounded, so picking the tighter of two means: the
    // non-null one wins when only one side specifies a limit.
    final combinedMinWidth = math.max(
      constraints.minWidth,
      _additionalConstraints.minWidth,
    );
    final combinedMaxWidth = _minNullable(
      constraints.maxWidth,
      _additionalConstraints.maxWidth,
    );
    final combinedMinHeight = math.max(
      constraints.minHeight,
      _additionalConstraints.minHeight,
    );
    final combinedMaxHeight = _minNullable(
      constraints.maxHeight,
      _additionalConstraints.maxHeight,
    );

    final childConstraints = BoxConstraints(
      minWidth: combinedMaxWidth == null
          ? combinedMinWidth
          : math.min(combinedMinWidth, combinedMaxWidth),
      maxWidth: combinedMaxWidth,
      minHeight: combinedMaxHeight == null
          ? combinedMinHeight
          : math.min(combinedMinHeight, combinedMaxHeight),
      maxHeight: combinedMaxHeight,
    );

    // Layout child with combined constraints
    _child!.layout(childConstraints);

    // Position child at origin
    positionChild(_child!, 0, 0);

    // Size ourselves to the child
    size = _child!.size;
  } else {
    // No child, size to minimum of additional constraints
    final constrainedSize = Size(
      _additionalConstraints.constrainWidth(0),
      _additionalConstraints.constrainHeight(0),
    );
    size = Size(
      constraints.constrainWidth(constrainedSize.width),
      constraints.constrainHeight(constrainedSize.height),
    );
  }
}