setViewSize method
Implementation
void setViewSize(double width, double height) {
_viewWidth = width;
_viewHalfWidth = width / 2.0;
_viewHeight = height;
_viewHalfHeight = height / 2.0;
viewRect = Rect.fromLTRB(-_viewHalfWidth, -viewHalfHeight, _viewHalfWidth, viewHalfHeight);
}