layout method
Implementation
void layout(Constraints constraints, {bool parentUsesSize = true}) {
if (!_needsLayout && _constraints == constraints) return;
_constraints = constraints;
performLayout(constraints);
_needsLayout = false;
_needsPaint = true;
if (parentUsesSize && _parent != null && !_relayoutBoundary) {
_parent!._needsLayout = true;
_parent!._needsPaint = true;
}
}