rulerView(_:willMove:toLocation:)
Informs the client that aRulerView will move aMarker, an NSRulerMarker already on the ruler view.
Declaration
func rulerView(_ ruler: NSRulerView, willMove marker: NSRulerMarker, toLocation 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.