Objective-C - How to draw perpendicular lines at center - objective-c

Question

This seems like a dumb question or maybe I'm just tired but I'm not getting the results I want. I'm not sure I'm getting any results since I can't see my lines. Can someone please tell me what I am doing wrong?
- (void)drawCenterPlus
{
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetLineWidth(context, 20);
[[UIColor redColor] setStroke];
CGPoint hStart;
CGPoint hEnd;
CGPoint vStart;
CGPoint vEnd;
hStart.x = self.center.x - 20.0;
hStart.y = self.center.y;
hEnd.x = self.center.x + 20.0;
hEnd.y = self.center.y;
vStart.x = self.center.x;
vStart.y = self.center.y - 20.0;
vEnd.x = self.center.x;
vEnd.y = self.center.y + 20.0;
//line 1
CGContextBeginPath(context);
CGContextMoveToPoint(context, hStart.x, hStart.y);
CGContextAddLineToPoint(context, hEnd.x, hEnd.y);
CGContextStrokePath(context);
//line 2
CGContextBeginPath(context);
CGContextMoveToPoint(context, vStart.x, vStart.y);
CGContextAddLineToPoint(context, vEnd.x, vEnd.y);
CGContextStrokePath(context);
}
Thank you in advance.
------------------------------------------------------- Additional Info ------------------------------------------
Actually this is called from viewDidLoad. I am just trying to draw a "+" in the center of the view. I've made the view a different color, so that the "+" can be seen. The "+" can be drawn in black, I just want to be able to see it. Yes, I could put a text "+" on the screen but ultimately I need to draw it inside of a rectangle.
I'm going to try again with a blank view to see if the drawing is hiding behind a subview. We'll see. Thank you again for your help.
------------------------------------------------------- Latest Method -----------------------------------------------
OK. Just a new project with one view that only has (hopefully) the red "+" in the middle of the screen. But nothing displays. What am I doing wrong?
#implementation TESTViewController
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetLineWidth(context, 20);
[[UIColor redColor] setStroke];
CGPoint hStart;
CGPoint hEnd;
CGPoint vStart;
CGPoint vEnd;
hStart.x = self.view.center.x - 20.0;
hStart.y = self.view.center.y;
hEnd.x = self.view.center.x + 20.0;
hEnd.y = self.view.center.y;
vStart.x = self.view.center.x;
vStart.y = self.view.center.y - 20.0;
vEnd.x = self.view.center.x;
vEnd.y = self.view.center.y + 20.0;
//line 1
CGContextBeginPath(context);
CGContextMoveToPoint(context, hStart.x, hStart.y);
CGContextAddLineToPoint(context, hEnd.x, hEnd.y);
CGContextStrokePath(context);
//line 2
CGContextBeginPath(context);
CGContextMoveToPoint(context, vStart.x, vStart.y);
CGContextAddLineToPoint(context, vEnd.x, vEnd.y);
CGContextStrokePath(context);
}

Answer 1

Check the value of context - I'm guessing it may be nil.
If you put this code inside a UIView subclass's drawRect method it should work as you will have a valid context.
Alternatively, you could create your own UIImage context, draw the shape in that and add the image as a subview. E.g.
- (void)drawCenterPlus {
UIGraphicsBeginImageContext(self.view.frame.size);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetLineWidth(context, 20);
[[UIColor redColor] setStroke];
CGPoint hStart;
CGPoint hEnd;
CGPoint vStart;
CGPoint vEnd;
hStart.x = self.view.center.x - 20.0;
hStart.y = self.view.center.y;
hEnd.x = self.view.center.x + 20.0;
hEnd.y = self.view.center.y;
vStart.x = self.view.center.x;
vStart.y = self.view.center.y - 20.0;
vEnd.x = self.view.center.x;
vEnd.y = self.view.center.y + 20.0;
//line 1
CGContextBeginPath(context);
CGContextMoveToPoint(context, hStart.x, hStart.y);
CGContextAddLineToPoint(context, hEnd.x, hEnd.y);
CGContextStrokePath(context);
//line 2
CGContextBeginPath(context);
CGContextMoveToPoint(context, vStart.x, vStart.y);
CGContextAddLineToPoint(context, vEnd.x, vEnd.y);
CGContextStrokePath(context);
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
UIImageView *imageView = [[UIImageView alloc] initWithImage:image];
[self.view addSubview:imageView];
}

Answer 2

CGContextRef context = UIGraphicsGetCurrentContext(); will be nil at this point... you basically want to add this code to your drawing code, unless you want to add static views or images to represent the cross.
The current Context wont be your view object unless it is has focus, which happens automatically by the time the drawRect: method is called.
you can alternately draw to an image and add it to an image view, you can do that in did load, and the drawing will happen automatically from that point on.