Setting composite drawing mode causing black holes/patches on NSWindow - objective-c

Question

I wanted to draw a shape, over it I wanted to draw the smaller transparent shape.
-(void) drawRect:(NSRect)dirtyRect
{
//clear everything
{
[[NSColor whiteColor] set];
[[NSBezierPath bezierPathWithRect:dirtyRect] fill];
}
NSBezierPath *thePath = [NSBezierPath bezierPathWithOvalInRect: self.bounds];
[thePath setLineWidth: 30];
[[NSColor blueColor] set];
[thePath stroke];
[[NSGraphicsContext currentContext] setCompositingOperation: NSCompositeCopy];
[[NSColor clearColor] set];
[thePath setLineWidth: 4];
[thePath stroke];
}
I've also tried it with an NSImage forst and then on view, but still getting the same result.
-(void) drawRect:(NSRect)dirtyRect
{
//clear everything
{
[[NSColor whiteColor] set];
[[NSBezierPath bezierPathWithRect:dirtyRect] fill];
}
NSImage* image = [[NSImage alloc] initWithSize:self.frame.size];
[image lockFocus];
//clear everything
{
[[NSColor whiteColor] set];
[[NSBezierPath bezierPathWithRect:dirtyRect] fill];
}
NSBezierPath *thePath = [NSBezierPath bezierPathWithOvalInRect: self.bounds];
[thePath setLineWidth: 30];
[[NSColor blueColor] set];
[thePath stroke];
[[NSGraphicsContext currentContext] setCompositingOperation: NSCompositeCopy];
[[NSColor clearColor] set];
[thePath setLineWidth: 4];
[thePath stroke];
[image unlockFocus];
[image drawInRect:self.frame fromRect:NSZeroRect operation:NSCompositeCopy fraction:1.0];
}

Answer 1

The oval appears black because windows are opaque by default. You've successfully cut an oval track in your window, but because the window is opaque, without an industrial laser to cut the same oval in your monitor, your Mac has to show some color there instead. The color it shows is the color of clearColor: black.
The solution is to set the window's opaque to NO.
The default is YES, which is nice and efficient but (well, actually, because) it prevents other windows' contents from showing through. Setting it to NO will allow you to see what is behind the window through that thin oval track. (It'll work better if you fill the oval, giving yourself a larger window in the… er… window.)
(Why does it look like the track is filled with gray? That's the window's shadow you're seeing there. When you try it for real, you'll be able to see the other windows on your system through the track as you move the window around.)