CGContextRef not drawing - objective-c

Question

I'm trying to draw a circle for my Mac Application. The Code is:
- (void)mouseMoved:(NSEvent*)theEvent {
NSPoint thePoint = [[self.window contentView] convertPoint:[theEvent locationInWindow] fromView:nil];
NSLog(#"mouse moved: %f % %f",thePoint.x, thePoint.y);
CGRect circleRect = CGRectMake(thePoint.x, thePoint.y, 20, 20);
CGContextRef context = [[NSGraphicsContext currentContext] graphicsPort];
CGContextSetRGBFillColor(context, 0, 0, 255, 1.0);
CGContextSetRGBStrokeColor(context, 0, 0, 255, 0.5);
CGContextFillEllipseInRect(context, CGRectMake(circleRect.origin.x, circleRect.origin.y, 25, 25));
CGContextStrokeEllipseInRect(context, circleRect);
[self needsDisplay];
}
- (void)mouseMoved: is called perfectly and I can see the correct x and y coordinates in NSLog. But I do not get any circles... surprisingly: If I'm minimizing my application and reopening it (so it "updates" NSView) the circles are perfectly drawn!

Answer 1

mouseMoved is NOT right place to draw anything, unless you're drawing onto the offscreen buffer. If you are going to draw on screen, save thePoint and any other necessary data, call [self setNeedsDisplay:YES] and draw in the drawRect:(NSRect)rect method.
Also, I can't see a reason to use CGContextRef while there is much more "friendly" NSGraphicsContext. Although, it's matter of taste.
An example of the drawing code:
- (void)mouseMoved:(NSEvent*)theEvent {
// thePoint must be declared as the class member
thePoint = [[self.window contentView] convertPoint:[theEvent locationInWindow] fromView:nil];
[self setNeedsDisplay:YES];
}
- (void)drawRect:(NSRect)rect
{
NSRect ovalRect = NSMakeRect(thePoint.x - 100, thePoint.y - 100, 200, 200);
NSBezierPath* oval = [NSBezierPath bezierPathWithOvalInRect:ovalRect];
[[NSColor blueColor] set];
[oval fill];
[[NSColor redColor] set];
[oval stroke];
}