1. override
void performResize()

Updates the render objects size using only the constraints.

Do not call this function directly: call layout instead. This function is called by layout when there is actually work to be done by this render object during layout. The layout constraints provided by your parent are available via the constraints getter.

Subclasses that set sizedByParent to true should override this method to compute their size.

This function is called only if sizedByParent is true.

Source 

@override
void performResize() {
  size = constraints.constrain(const Size(_kMaxWidth, _kMaxHeight));
}