performLayout method
Implementation
@override
void performLayout(Constraints constraints) {
final boxConstraints = constraints.asBoxConstraints;
final childConstraints = boxConstraints.loosen();
final dimensions = _layoutChildren(boxConstraints, childConstraints);
final int finalWidth =
dimensions.width > 0 ? dimensions.width : boxConstraints.minWidth;
final int finalHeight =
dimensions.height > 0 ? dimensions.height : boxConstraints.minHeight;
size = boxConstraints.constrain(Size(finalWidth, finalHeight));
_positionChildren(finalWidth, finalHeight);
}