needsDisplay changes from true to false in Swift appkit

It's expected that the value will change back to false eventually:

https://developer.apple.com/documentation/appkit/nsview/1483360-needsdisplay/

Are you saying that setting it to true is being ignored (doesn't cause the view to update)? When (and on what thread) are you setting it to true? What custom drawing machinery are you using on your NSView subclass?

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.