swift-mirror/stdlib/stdlib.docc/customizing-your-c-code-for-swift.md

Customizing Your C Code for Swift

Use the CF_SWIFT_NAME macro to group functions that have related behavior.

Overview

Because structures in C can't have methods, property accessors, or custom initializers, you often need to write such functionality using global functions. Structures in Swift can declare methods, property accessors, and initializers. You use the CF_SWIFT_NAME macro to group together related global functions into a single structure type that's imported into Swift.

Apply CF_SWIFT_NAME to Related Functions

The example below shows several functions that are all related to a Color type. The CF_SWIFT_NAME macro is applied to each function, giving each one a new name for Swift that's nested together under the Color type:

Color ColorCreateWithCMYK(float c, float m, float y, float k) CF_SWIFT_NAME(Color.init(c:m:y:k:));

float ColorGetHue(Color color) CF_SWIFT_NAME(getter:Color.hue(self:));
void ColorSetHue(Color color, float hue) CF_SWIFT_NAME(setter:Color.hue(self:newValue:));

Color ColorDarkenColor(Color color, float amount) CF_SWIFT_NAME(Color.darken(self:amount:));

extern const Color ColorBondiBlue CF_SWIFT_NAME(Color.bondiBlue);

Color ColorGetCalibrationColor(void) CF_SWIFT_NAME(getter:Color.calibration());
Color ColorSetCalibrationColor(Color color) CF_SWIFT_NAME(setter:Color.calibration(newValue:));

The argument you pass to the CF_SWIFT_NAME macro uses the same syntax as the #selector expression. You use self in a CF_SWIFT_NAME argument to refer to the instance that the method belongs to.

Import Related Functions into Swift

Here's how Swift imports the related functions above into a single type:

extension Color {
    init(c: Float, m: Float, y: Float, k: Float)

    var hue: Float { get set }

    func darken(amount: Float) -> Color

    static var bondiBlue: Color

    static var calibration: Color
}

Note: You can't reorder or change the number of arguments for type members imported using the CF_SWIFT_NAME macro.