Question

This is probably a simple thing. I’m trying to draw my first shapes in my ViewDidLoad. However my context is always null, I get “con = 0x0”. I’m not sure if I’m using this properly. I’ve pasted my code below.

- (void)viewDidLoad
{
    [super viewDidLoad];

    // obtain the current graphics context

    CGContextRef con = UIGraphicsGetCurrentContext();

    // draw a black (by default) vertical line, the shaft of the arrow

    CGContextMoveToPoint(con, 100, 100);
    CGContextAddLineToPoint(con, 100, 19);
    CGContextSetLineWidth(con, 20);
    CGContextStrokePath(con);

    // draw a red triangle, the point of the arrow

    CGContextSetFillColorWithColor(con, [[UIColor redColor] CGColor]);
    CGContextMoveToPoint(con, 80, 25);
    CGContextAddLineToPoint(con, 100, 0);
    CGContextAddLineToPoint(con, 120, 25);
    CGContextFillPath(con);

    // snip a triangle out of the shaft by drawing in Clear blend mode

    CGContextMoveToPoint(con, 90, 101);
    CGContextAddLineToPoint(con, 100, 90);
    CGContextAddLineToPoint(con, 110, 101);
    CGContextSetBlendMode(con, kCGBlendModeClear);
    CGContextFillPath(con);

}

And when debugging, the value of “con” is:

con = (CGContext)0x0

Answer 1

The problem with your code is that you are trying to draw when you are not supposed to.

You can either move your drawing code to the drawRect: method in your view, because the operating system prepares a context into which you can draw before calling drawRect: or you can create your own context and draw into it, export it as an image and display it using an UIImageView.