AttributesPlayground.swift

import UIKit

public struct CollectionViewCellProvider {
    @available(iOS 13, *)
    public static func compositionalCell() {
        // compositional collection view cell
    }
    public static func collectionCell() -> UICollectionViewCell {
        // collection view cell
        return UICollectionViewCell()
    }
}

class ParentViewController: UIViewController {
    override func viewDidLoad() {
        presentViewController() // No Xcode complains!
    }

    @discardableResult
    func presentViewController() -> UIViewController {
        return UIViewController()
    }
}

@dynamicCallable
struct RandomProvider {
    func dynamicallyCall(withArguments args: [Int]) -> Double {
        let numberOfZeroes = Double(args[0])
        let maximum = pow(10, numberOfZeroes)
        return Double.random(in: 0...maximum)
    }

    func dynamicallyCall(withKeywordArguments args: KeyValuePairs<String, Int>) -> Double {
        let numberOfZeroes = Double(args.first?.value ?? 0)
        let maximum = pow(10, numberOfZeroes)
        return Double.random(in: 0...maximum)
    }
}

let random = RandomProvider()
print(random(1)) // 8.15946134247
print(random(a:1)) // 5.14551692644

@dynamicMemberLookup
struct User {
    subscript(dynamicMember member: String) -> String {
        let properties = ["firstname": "Bob", "lastname": "Dylan"]
        return properties[member, default: ""]
    }
}
let user = User()
print(user.firstname) // Bob
print(user.lastname) // Dylan

@main
struct Repeat: ParsableCommand {
    @Argument(help: "The phrase to repeat.")
    var phrase: String

    mutating func run() throws {
        for _ in 1...5 {
            print(phrase)
        }
    }
}

class Service {

    var closure: (() -> Void)?

    func storageExample(with completion: @escaping (() -> Void)) {
        closure = completion // error: assigning non-escaping parameter 'completion' to an @escaping closure
    }

    func asyncExample(with completion: @escaping  (() -> Void)) {
        DispatchQueue.global().async { // error: escaping closure captures non-escaping parameter 'completion'
            completion()
        }
    }

    func runAsyncTask(completion: @escaping (() -> Void)) {
        DispatchQueue.global().async {
            completion()
        }
    }

}

enum UILayoutConstraintAxis: Int {
  case UILayoutConstraintAxisHorizontal = 0
  case UILayoutConstraintAxisVertical = 1
  case UILayoutConstraintAxisZ = 2
}

let axis = UILayoutConstraintAxis.UILayoutConstraintAxisHorizontal

switch axis {
case .UILayoutConstraintAxisHorizontal:
    print("")
case .UILayoutConstraintAxisVertical:
    print("")
@unknown default:
    print("")
}

@propertyWrapper
struct WhitespaceTrimmable {
    private(set) var value: String = ""

    var wrappedValue: String {
        get { value }
        set { value = newValue.trimmingCharacters(in: .whitespacesAndNewlines) }
    }

    init(wrappedValue: String) {
        self.wrappedValue = wrappedValue
    }
}

struct Service {
    @WhitespaceTrimmable(wrappedValue: "Default title") var title: String

    func accessTitle() {
        print("title: \(_title)") //Default title
    }
}

var temp = Service()
print(temp.title)
temp.accessTitle()