rulerView(_:willAdd:atLocation:)
Informs the client that aRulerView will add the new NSRulerMarker, aMarker.
Declaration
func rulerView(_ ruler: NSRulerView, willAdd marker: NSRulerMarker, atLocation location: CGFloat) -> CGFloatDiscussion
location is the marker’s tentative new location, expressed in the client view’s coordinate system. The value returned by the client view is actually used; the client can simply return location unchanged or adjust it as needed. For example, it may snap the location to a grid. This message is sent repeatedly to the client as the user drags the marker.