layout method
Implementation
void layout(LayoutConstraints constraints, {bool parentUsesSize = false}) {
_isRelayoutBoundary =
!parentUsesSize ||
sizedByParent ||
constraints.isTight ||
parent == null;
if (!_needsLayout && constraints == _constraints) {
return;
}
_constraints = constraints;
if (sizedByParent) {
performResize();
}
performLayout();
_needsLayout = false;
}