Drawing circles in iOS touches moved

Question

My code was working when I was using UIViews and changing their background color. Now I try it with ellipses and they don't show up? Here is the code:

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
        overlayView = [[[UIView alloc] initWithFrame:self.view.bounds] autorelease];
        overlayView.backgroundColor = [UIColor colorWithWhite:255.0f alpha:1.0f];
    [self.view addSubview:overlayView];
    reda = 0;
    bluea = 0;
    greena = 255;
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    touchPoint = [touch locationInView:self.view];
    previousPoint = touchPoint;

}

- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    touchPoint = [touch locationInView:self.view];
    if (abs((previousPoint.x-touchPoint.x)) > 20) {
        UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Warning!" message:@"Too Fast!" delegate:self cancelButtonTitle:@"Okay." otherButtonTitles:nil];
        [alertView show];
        [alertView release];
    }
    CGContextRef contextRef = UIGraphicsGetCurrentContext();
    CGContextSetRGBFillColor(contextRef, reda/255.f, greena/255.f, bluea/255.f, 1.0f);
    CGContextFillEllipseInRect(contextRef, CGRectMake(touchPoint.x, touchPoint.y, 20, 20));
    [overlayView addSubview:contextRef];

    if ((greena != 0) && (bluea == 0)) {
        greena += -5;
        reda += +5;
    }
    if ((reda != 0) && (greena == 0)) {
        bluea += +5;
        reda += -5;
    }
    if ((bluea != 0) && (reda == 0)) {
        bluea += -5;
        greena += +5;
    }
}

Answer 1

You are calling UIGraphicsGetCurrentContext in touchesMoved:withEvent:. The system doesn't create a graphics context for you in touchesMoved:withEvent:. The system only creates a graphics context for you in -[UIView drawRect:].

You need to read about the view drawing cycle and the runtime interaction model for views in Apple's documentation.

You can either move your drawing code to drawRect:, or you can create your own graphics context (e.g. using UIGraphicsBeginImageContextWithOptions or CGBitmapContextCreate), draw to it, and then create an image from the context and assign the image to view.layer.contents.