Question

Hi i am making a sample app in which i wanto create a square for which i used the following code

- (void)viewDidLoad {
    [super viewDidLoad];
    [self drawRect:CGRectMake(0, 0, 300, 200)];
    [[self view] setNeedsDisplay];
}
- (void) drawRect:(CGRect)rect
{
    NSLog(@"drawRect");

    CGFloat centerx = rect.size.width/2;
    CGFloat centery = rect.size.height/2;
    CGFloat half = 100/2;
    CGRect theRect = CGRectMake(-half, -half, 100, 100);

    // Grab the drawing context
    CGContextRef context = UIGraphicsGetCurrentContext();

    // like Processing pushMatrix
    CGContextSaveGState(context);
    CGContextTranslateCTM(context, centerx, centery);

    // Uncomment to see the rotated square
    //CGContextRotateCTM(context, rotation);

    // Set red stroke
    CGContextSetRGBStrokeColor(context, 1.0, 0.0, 0.0, 1.0);

    {
        CGContextSetRGBFillColor(context, 0.0, 1.0, 0.0, 1.0);
    }
    // Draw a rect with a red stroke
    CGContextFillRect(context, theRect);
    CGContextStrokeRect(context, theRect);

    // like Processing popMatrix
    CGContextRestoreGState(context);
    [[self view] setNeedsDisplay];

}

But nothing is drawn on screen , dont know wheres the issue is .When i debug it the CGContextRef context was always 0x0 , i dont know why its 0x0 always am i missing something in my code.

Answer 1

It looks like you’re trying to draw in a subclass of UIViewController. You need to subclass UIView to override the drawRect: method, which is then called automatically with a valid graphics context in place. You almost never call this method yourself.