performLayout method

@override Size performLayout(BoxConstraints constraints)

Hook for subclasses to perform layout within the given constraints.

Implementation

@override
Size performLayout(BoxConstraints constraints) {
  final column = widget as Column;
  final width = constraints.maxWidth == BoxConstraints.infinity
      ? 0
      : constraints.maxWidth;
  final height = constraints.maxHeight == BoxConstraints.infinity
      ? 0
      : constraints.maxHeight;
  final area = Rect(0, 0, width, height);

  // Map directly over childElements to retrieve their widgets and calculate
  // constraints, avoiding index desyncs or out-of-bounds errors that could occur
  // if we relied on zip-indexing between column.children and childElements.
  final columnConstraints = childElements
      .map(
        (el) => getConstraint(
          el.widget,
          LayoutDirection.vertical,
          crossSize: width,
          element: el,
        ),
      )
      .toList();
  final rects = splitRect(
    area,
    columnConstraints,
    LayoutDirection.vertical,
    mainAxisAlignment: column.mainAxisAlignment,
  );

  var totalHeight = 0;

  for (var i = 0; i < childElements.length; i++) {
    final childEl = childElements[i];
    final childArea = rects[i];
    final minW = column.crossAxisAlignment == CrossAxisAlignment.stretch
        ? width
        : 0;
    final childSize = childEl.layout(
      BoxConstraints(
        minWidth: minW,
        maxWidth: width,
        minHeight: childArea.height,
        maxHeight: childArea.height,
      ),
    );
    childEl.relativeOffset = Offset(childArea.x, childArea.y);
    totalHeight += childSize.height;
  }
  return Size(width, totalHeight);
}