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#250: Improvements to C APIs in Swift 3 🐀


We’ve covered a little about working with C APIs in Swift back in Bite #189. Swift 3 brings us a ton of new goodies and improvements around C APIs, let's dive right in and take a look.

Swift 3 improves upon a number of areas of C API usage in Swift, but the biggest one is importing functions as members. Essentially, taking free floating C functions, renaming them, and shoving them onto a type as functions.

One place where this behavior really shines is when using CoreGraphics to draw into a view.

Here's some basic drawing code in Swift 2:

guard let context: CGContext = UIGraphicsGetCurrentContext() else { return }

CGContextSetStrokeColorWithColor(context, UIColor.redColor().CGColor)
CGContextSetLineWidth(context, 2)
CGContextDrawPath(context, .Stroke)

In Swift 3, the code to do the same would be:

guard let context: CGContext = UIGraphicsGetCurrentContext() else { return }

context.strokeColor = UIColor.red().cgColor
context.lineWidth = 2
context.drawPath(mode: .Stroke)

The mechanisms powering this conversion under the hood are pretty neat. The full (now implemented) proposal can be found here, but here's a quick overview:

The first is a bit of automatic inference. Many APIs (like CoreGraphics and CoreFoundation) use consistent (albeit often verbose) naming schemes for their functions. The Swift compiler can now exploit this to (for example) detect functions returning a specific type and convert them into init functions in Swift.

So this C function would come in to Swift 2 like this:

func CGColorCreate(space: CGColorSpace?, _ components: UnsafePointer<CGFloat>) -> CGColor?

But in Swift 3 it's an init function:

extension CGColor {
  init?(space: CGColorSpace?, components: UnsafePointer<CGFloat>)

There's tons more of these automatic inferences for things like detecting getter/setter pairs of functions, converting Boolean functions into Bool properties, etc.

Finally, authors of C libraries can customize exactly how their functions are imported into Swift using the new swift_name macro.

Here's an example from the proposal that demonstrates how we can define which type our function gets imported on to.

struct Point3D rotatePoint3D(Point3D point, float radians)

The string parameter we pass to swift_name has a bunch of little extra syntax to import things as inits, regular functions, getters/setters, etc.

As always when talking about Swift improvements, it's a good idea to check out the full proposal for more info.