CGContext AND CGContextRef the same in Swift? How does this work?

Question

I was porting some ObjectiveC custom UIView subclasses to Swift this morning. Wanting to make it more "object oriented", I was following the example of extending CGContext with methods. E.g.

extension CGContext {
    func fillColor(color:UIColor) {
        CGContextSetFillColorWithColor(self, color.CGColor)
    }
}

Because I was converting the objective C style messages (e.g. -(void) drawOutline: (CGContextRef) cr {...}) to Swift style ones without paying to much attention (e.g. func drawOutline(cr:CGContextRef) {...}), I didn't realize at first that I was passing CGContextRef arguments, but had extended CGContext (not Ref). But it worked!

What was weirder, was when I changed those CGContextRef's to just CGContext's. And it still worked. Not just compiled, but ran and drew correctly. For the fun of it, I changed to extension CGContextRef. And yet it still continues to work. It's as if Swift is smart enough to boil them down to the same thing. Is it? Or is there something more subtle/cool going on here?

Obijective-C and Swift. And you added the C tag just for fun, I guess. — too honest for this site, Sep 01 '15 at 20:58
Not for fun. Maybe out of misguided confusion. The parts of `ObjectiveC` referred to here are strictly `C` (remember ObjectiveC is a superset of C). `CGContext` is a strict C opaque structure. And `CGContextRef` is the typedef for a pointer to the same. I guess I did C, because it's more of a question about how Swift translates the C parts than the ObjectiveC. — Travis Griggs, Sep 01 '15 at 21:24

score 5 · Accepted Answer · answered Sep 02 '15 at 00:32

There's nothing particularly magical here. CGContextRef and CGContext are just different names for (almost) the same thing. Swift just blurs the typedef for you.

<CoreGraphics/CGContext.h>

typedef struct CGContext *CGContextRef;

See also the Using Swift with Cocoa and Objective-C guide section on Core Foundation:

When Swift imports Core Foundation types, the compiler remaps the names of these types. The compiler removes Ref from the end of each type name because all Swift classes are reference types, therefore the suffix is redundant.

They just chose not to take away the Ref form of the name, I guess for old timers who are used to typing it :D

CGContext AND CGContextRef the same in Swift? How does this work?

1 Answers1