How do I draw a point inside of an instance of UIImageView?

Question

How do I use Core Graphics to draw inside of a instantiated UIImageView (No class files to override drawrect with -- just a reference to the instance)? I want to draw a simple point of fixed width (say, 10 pixels diameter) and of a certain color.

Answer 1

How do I use Core Graphics to … draw a simple point of fixed width (say, 10 pixels diameter) and of a certain color[?]

1. Set the fill color.
2. Move to the point.
3. Plot an arc, centered at the point, with the desired radius, giving a start angle of 0 and an end angle of 2 * M_PI.
4. Close the path.
5. Fill.

… draw inside of a instantiated UIImageView (No class files to override drawrect with -- just a reference to the instance)

I know that on the Mac, that's really not a good idea, as an NSView may redraw at any time if anything has set it as needing display (or told it directly to display), and would then clobber anything you'd drawn over it. No idea about the iPhone, but I do notice that UIView doesn't have methods like NSView's lockFocus and unlockFocus.

There's a more architectural reason not to do this: Drawing is the exclusive domain of views, and you're proposing to break that exclusivity and sprinkle some drawing code in (I'm assuming) a controller. Scattering code for a purpose in an object that doesn't have that purpose is a path to messy, unnavigable code.

Better to make a subview for each point and add those as subviews of the image view, or subclass UIImageView, give your instance of that subclass an array of the points to draw, and have the subclass's drawRect: call up to super before drawing its points.

The above solution, then, goes in drawRect:.

Answer 2

You can also just add a Core Animation layer to the view. Set the corner radius to half the size of the point you want (assuming a square) and it will render as a circle. Something like:

CALayer *layer = [CALayer layer];
[layer setBounds:CGRectMake(0.0f, 0.0f, 10.0f, 1.0f)];
[layer setCornerRadius:5.0f];
[layer setMasksToBounds:YES];
[layer setBackgroundColor:[[UIColor redColor] CGColor]];

// Center the layer in the view.
[layer setPosition:CGPointMake([view bounds].size.width/2, 
                            [view bounds].size.height/2)];

// Add the layer to your view
[[view layer] addSublayer:layer];

Not Core Graphics exactly, but it's easy to implement.

Best Regards,