Need explaining on the double label function declaration

Question

I was reading the apple doc when I fell upon this piece of syntax :

struct Point {
    var x = 0.0, y = 0.0
    mutating func moveBy(x deltaX: Double, y deltaY:          


        

Answer 1

Short answer: first argument label is for external caller, second one for local in-method use.

func moveBy(x deltaX: Double, y deltaY: Double) when calling looks following: moveBy(x: 1, y: 1), but inside the method deltaX and deltaY labels are used.

This naming style is not necessary, you can declare the method func moveBy(x: Double, y: Double) so x and y will be used inside the method.

To support legacy style, so from caller scope your method looks like moveBy(1, 1), you should place _ as first argument label: func moveBy(_ deltaX: Double, _ deltaY: Double). Such declarations are used in CocoaTouch to support legacy obj-c interface (ex. func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool).