Swifty importing of C functions

Paul Hudson      @twostraws

Swift 3 introduced attributes for C functions that allow library authors to specify new and beautiful ways their code should be imported into Swift. For example, all those functions that start with "CGContext" now get mapped to properties methods on a CGContext object, which makes for much more idiomatic Swift. Yes, this means the hideous wart that is CGContextSetFillColorWithColor() has finally been excised.

To demonstrate this, here's an example in Swift 2.2:

let ctx = UIGraphicsGetCurrentContext()

let rectangle = CGRect(x: 0, y: 0, width: 512, height: 512)
CGContextSetFillColorWithColor(ctx, UIColor.redColor().CGColor)
CGContextSetStrokeColorWithColor(ctx, UIColor.blackColor().CGColor)
CGContextSetLineWidth(ctx, 10)
CGContextAddRect(ctx, rectangle)
CGContextDrawPath(ctx, .FillStroke)

UIGraphicsEndImageContext()

In Swift 3 the CGContext can be treated as an object that you can call methods on rather than repeating CGContext again and again. So, we can rewrite that code like this:

if let ctx = UIGraphicsGetCurrentContext() {
    let rectangle = CGRect(x: 0, y: 0, width: 512, height: 512)
    ctx.setFillColor(UIColor.red.cgColor)
    ctx.setStrokeColor(UIColor.black.cgColor)
    ctx.setLineWidth(10)
    ctx.addRect(rectangle)
    ctx.drawPath(using: .fillStroke)

    UIGraphicsEndImageContext()
}

Note: in both Swift 2.2 and Swift 3.0 UIGraphicsGetCurrentContext() returns an optional CGContext, but because Swift 3 uses method calls we need to safely unwrap before it's used.

This mapping of C functions exists elsewhere, for example you can now read the numberOfPages property of a CGPDFDocument, and CGAffineTransform has been souped up quite dramatically. Here are some examples showing old and new:

CGAffineTransformIdentity
CGAffineTransform.identity

CGAffineTransformMakeScale(2, 2)
CGAffineTransform(scaleX: 2, y: 2)

CGAffineTransformMakeTranslation(128, 128)
CGAffineTransform(translationX: 128, y: 128)

CGAffineTransformMakeRotation(CGFloat(M_PI))
CGAffineTransform(rotationAngle: CGFloat(M_PI))

Other changes in Swift 3.0…

• All function parameters have labels unless you request otherwise
• Omit needless words
• UpperCamelCase has been replaced with lowerCamelCase for enums and properties
• Verbs and nouns

Download all Swift 3.0 changes as a playground Link to Swift 3.0 changes

Browse changes in all Swift versions