The problem was solved by a posting on Stack Overflow. Basically, you were correct - the problem was that I was creating an additional instance of the GameViewController. In order to force graphRef to refer to the original instance, the suggestion was to re-bind it to the appropriate instance by placing this statement in the Renderer init: self.graphRef = graphRef Then the guard statement in GameViewController needed to include the change so that graphRef referred to self and the init in Renderer needed to be notified of the same thing. After these changes, I was able to call dodraw() using graphRef.dodraw() and everything worked well. Thanks for your help!

After prompting from others, I created a protocol and delegate and removed the graphRef instantiation of GameViewController so there should be only one instance. The protocol was: protocol link { dodraw() } I then created a delegate by putting the following code in the Renderer class: var delegate: link? I invoked the dodraw() from the draw in Renderer using the following statement: self.delegate?.dodraw() Now, there is presumably only one instance of GameViewController. The protocol/delegate worked in accessing dodraw() in GameViewController but I still got nil for graph. Things are still a mystery.

The storyboard identifies GameViewController as the controller for the single window that is created by the app. I have no idea how to access methods in this object.

I tried to make dodraw() public but it is still not recognized by a call from the Renderer class. I tried calling it using GameViewController.dodraw() but this doesn't work either. Sorry for being so obtuse, but I can't see how to do this.

For example, making dodraw() a public method might work, but I understand that then GameViewController would also need to be public as well as all its methods. I have found that using global variables makes things much easier but it is not in the spirit of object oriented programming.

Is there some way to access the method dodraw() from the Renderer class without creating another instance of GameViewController? I am fairly new to Swift and have been told that a delegate might serve but I have no idea how to do this. Thanks!

I am not quite sure how I could easily post a minimal project. I think it would be very time consuming to parse it down to its basic components. I could post a zip archive of the entire project - I am not interested in selling it and have written it for my own use (It is a physics demo which I originally wrote in Obj. C and OpenGL and used it in a lecture demo at my University about 16 years ago. Swift and Metal are much, much more difficult to use, in my opinion).

In addition to getting the "unexpectedly found nil" message, I also got in the output window "[Nib Loading] Failed to connect (controller) outlet from (Vectors.GameViewController) to (MTKView): missing setter or instance variable". This indicates that the linking between the classes didn't occur.

I borrowed the Metal part from a tutorial on the web - it drew a 3D box with lighting, etc. I simply replaced the vertices with the much more complicated structures I wanted to draw and included mouse interaction so one could rotate the objects. This all worked well. It turns out the the author included an instance of the GameViewController in the Renderer class: public var graphRef = GameViewController() My code at the beginning of the GameViewController class is: @IBOutlet weak var graph: GraphView! func dodraw(){ graph.needsDisplay = true } In the draw method from Renderer I invoke dodraw using: graphRef.dodraw() When I run this, I get a message: Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value. If I include a test for nil in dodraw(), I am back to where I was: graph is always nil. The outlet seemed OK - it was the only one connecting the GameViewClass to the GraphView rect and the circle was filled in. Any ideas?

Just a quick question: I can make a public method in the GameViewController, as follows: IBOutlet weak var graph:GraphView! public func viewData(){ graph.needsDisplay = true } where the outlet is connected to the GraphView rect in the StoryBoard. If I try to refer to this method from a Renderer class method, I get "cannot find viewData in scope". I am just using a line such as: viewData() in the Renderer method. Do you know why this happens? Do I need to call the method in some other way, since it is in a different class? Thanks!

Thanks, that is what I am looking into. Despite using Obj. C and Swift a fair amount, there is a lot I don't know about - for some reason, object oriented programming never made much sense to me. (As a retired physicist, I was brought up in Fortran, and APL, which I loved).

OK, the scope problem is back again. I need to get an outlet called "graph" in GameViewController to be recognized by a draw method in Renderer which is instantiated in GameViewController. It would be easier if I could put the outlet in the Renderer class, but Xcode won't allow this. Is there a way to force this?

I needed to use time machine to go back 2 days to get a version of the app which didn't complain about scope issues. So far as I know, this is identical to the version which did complain. Obviously, there is some subtle difference (maybe the GraphView in the StoryBoard was a little different?). I put both print statements at the beginning of the draw function. Here is what they give me: draw stared rect (0.0, 0.0, 700.0, 102.0) draw stared rect (0.0, 0.0, 700.0, 102.0) tag -1 By the way, I don't instantiate the GraphView class - should I do this? (I don't think that Hillegass' book on Cocoa does this either for his drawing example).

Thanks! I verified that draw is not called by putting in a breakpoint at some executable part of the draw code and it breaks once just after the views are loaded and never again. This gives it enough time to draw a few things - the cyan color, the line down the middle, etc. In order to get it to draw the graph, I need to resize the whole window (which is mainly an MTKView with the GraphView on the bottom over part of the MTKView). The graph then draws more or less correctly, although it will be distorted by the irregular movements of my hand as I drag the corner of the window. The Metal part of the display uses a GameViewController and a Renderer class which contains the Metal draw method. The latter is called at 60 fps and I put the needsDisplay in that method. It is linked to the GraphView draw by an outlet which appears in the GameViewController and connects to the GraphView in the StoryBoard. OK, now things are very strange. Formerly, putting the outlet in GameViewController and the needsDisplay in the draw method in the (Metal) Renderer class (which is instantiated by the GameViewController class) worked fine (i.e., no complier complaints and nothing in the output panel in Xcode). Now, it gives me an out of scope error for the needsDisplay. I don't understand that change in behavior. Anyway, when it did work (no scope issues), I was able to get the graph to display by putting needsDisplay in one of the mouse functions connected to the GameViewController (moving the mouse gave me beautiful graphs!) I tried to put the outlet in the Renderer class but the small circle never closed when I connected it to the GraphView in the StoryBoard. I also got messages in the output window that I needed an instance variable to link GameView to the MTKView view. I am totally confused by all of this. Any suggestions welcome.

Thanks very much for the response. After some experimentation, I discovered that needsDisplay quickly gets reset. My problem is that the draw method doesn't respond to needsDisplay. If I resize the view, it redraws while I am doing that, but I guess that is what draw is expected to do. I just want it to respond to needsDisplay. My drawing class looks like this: class GraphView: NSView { override func draw(_ dirtyRect:NSRect) { super.draw(dirtyRect) var width:Float = 0.0, height:Float let backgroundColor = NSColor.cyan var iGraph, iData, i1:Int var x,y:Int backgroundColor.set() NSBezierPath.fill(bounds) height=Float(dirtyRect.size.height) width=Float(dirtyRect.size.width) let path=NSBezierPath() NSColor.black.setStroke() path.lineWidth = 0.5 path.move(to:NSPoint(x:0, y:Int(height)/2)) path.line(to:NSPoint(x:Int(width), y:Int(height)/2)) path.stroke() current+=1 if current>200 { current = 0 } points[current]=jz i1=current x = 0 y = Int(0.25*height*(1.0-points[i1]/height_vector)) path.move(to:NSPoint(x:0, y:Int(y))) for i in 0..<200 { x = i*Int(width/200.0) if i1<=0 {i1+=200} y = Int(0.5*height - 0.45*height*points[i1]/height_vector) path.line(to:NSPoint(x:x,y:y)) i1 -= 1 } path.stroke() } } The needsDisplay is issued in another class (an animated MTKView) and needsDisplay=true is placed so that it is called at each iteration of the inner loop of the Metal view (60 fps). Everything compiles fine, but I can't get the above draw function to execute. This app has an interesting history. I first wrote in about 20 years ago on the Mac using Obj. C and OpenGL. About 5 years ago I ported it to iOS using the OpenGL ES, a much more difficult version of OpenGL. I have now ported it back to the Mac writing it in Swift and using Metal. The Metal code is a modification of code I found on the web and works very well, displaying some 3D vectors rotating around a magnetic field vector (to illustrate nuclear magnetic resonance). The iOS code was my template and doesn't seem to have this problem (though it is written in a UIViewController). I hope this sheds some light on my problem and sorry for the length of this post. I must confess that I never had a deep understanding of object oriented programming, even in the old days of OS X (I also developed on a NeXT). Much of it is trial and error with a little help from Hillegass' book on Cocoa programming.