Как сделать окно прозрачным?

@implementation MyCustomWindow

- (id)initWithContentRect:(NSRect)contentRect
                styleMask:(NSUInteger)windowStyle
                  backing:(NSBackingStoreType)bufferingType
                    defer:(BOOL)deferCreation
{
    self = [super
            initWithContentRect:contentRect
            styleMask:NSBorderlessWindowMask
            backing:bufferingType
            defer:deferCreation];
    if (self)
    {
        [self setOpaque:NO];
        [self setBackgroundColor:[NSColor clearColor]];
    }
    return self;
}

@end