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I was reading the textbook "iOS 14 Programming for Beginners" by Armad Sahar. There are codes below in a MapDataManager class which I was stuck for days.

import Foundation
import MapKit

class MapDataManager: DataManager {

fileprivate var items: [RestaurantItem] = []

var annotations: [RestaurantItem] {
    return items
}

func fetch(completion:(_ annotations:[RestaurantItem]) ->()){
    if items.count > 0 { items.removeAll() }
    for data in load(file: "MapLocations") {
        items.append(RestaurantItem(dict: data))
    }
    completion(items)
}

func currentRegion(latDelta: CLLocationDegrees, longDelta: CLLocationDegrees) -> MKCoordinateRegion {
    guard let item = items.first else {
            return MKCoordinateRegion()
        }
        let span = MKCoordinateSpan(latitudeDelta: latDelta, longitudeDelta: longDelta)
        return MKCoordinateRegion(center: item.coordinate, span: span)
    }

}

My question is on the fetch function. According to my understanding, the fetch function is accepting another function as variable, which is named as completion, and I am supposed to declare the "type" of the function after the : after completion, which should be ([RestaurantItem]) -> ([RestaurantItem]) to me, because we try to input a [RestaurantItem] array and just copy and output a new [RestaurantItem] array.

But now it seems that (_ annotations:[RestaurantItem]) -> () becomes the "type" of the completion function and the input variable type is (_ annotations:[RestaurantItem]), which does not look like a type to me at all. I thought a type could be Int, String, [RestaurantItem] etc., what exactly is type _ annotations:[RestaurantItem]???

And why is the returning parameter Void ()? Apparently the completion handler has the return keyword that returns an item of type [RestaurantItem]!!!

Please help... I am novice with Swift and I don't have friends around who could help me with that...

Bosco

Sulthan
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Bosco Tsin
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1 Answers1

1

The parameter label _ annotations: is syntactic sugar.

It's Swift 2 legacy and the compiler is only interested in the actual type which is

([RestaurantItem]) -> ()

a closure with a parameter [RestaurantItem] and without a return type.

To make it clearer this is the closure written as function

func completion(_ annotations: [RestaurantItem]) {
   // do something with the restaurant items
}
vadian
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