Swift’s design promotes language features like generics and first-class protocols as key architectural components in application development. However, many of the logical patterns that arise, including ones imported from Objective-C, don’t work as we expect them to. In many cases, Swift’s type system resists certain straightforward patterns, such as constraining a property to both a class and a protocol. This talk will highlight several of these challenges, discuss the underlying cause, and evaluate workarounds.

1. Protocols and the
Promised Land
@MicheleTitolo

2. Swift is shiny! And new!

3. Swift is not Objective-C

4. Interop

5. as? as? as?

6. as? lies

7. 🚫
let vendor = ObjcObject()
let items = vendor.giveMeItems() as? [Fruit]

8. let vendor = ObjcObject()
let items = vendor.giveMeItems().map { $0 as Fruit }

9. @objc spreads like a virus

10. 🚫
@objc protocol SomeProtocol {
func someFunc()
}
class MyClass: SomeProtocol {}

11. @objc protocol SomeProtocol {
func someFunc()
}
@objc class MyClass: NSObject, SomeProtocol {}

12. Swift > Objc > Swift

13. Can’t #import “MyApp-Swift.h” in
header

14. Can’t conform, can’t subclass

15. Bridging

16. many parts of Swift don’t bridge

17. Does this really need to be used in
Objective-C?

18. framework headers can’t be in
bridging header

19. use @import

20. Partial bridging

21. -Swift.h will leave out funcs/vars that
don’t bridge

22. Again: does this really need to be
bridged?

23. Type System

24. properties can have 1 type

25. protocol Themeable {}
class ListViewController: UIViewController, Themeable {}
var themedViewController: // UIViewController, Themeable ????

26. two workarounds

27. public protocol ViewControllerProtocol {
var view: UIView! { get }
var storyboard: UIStoryboard? { get }
init(nibName nibNameOrNil: String?, bundle nibBundleOrNil:
NSBundle?)
func viewDidLoad()
// etc
}
protocol ThemeableViewController: Themeable, ViewControllerProtocol {}
var themedViewController: ThemeableViewController
Swift-y but hack-y

28. var themed: Themeable {
get {
return self.viewController as! Themeable
}
set(newThemeable) {
if let themedViewController = newThemeable as? UIViewController{
viewController = themedViewController
}
}
}
var viewController: UIViewController
init<T where T: Themeable, T: UIViewController>(viewController: T) {
self.viewController = viewController
}
Also swfit-y also hack-y

29. Can’t override type with
inheritance

30. 🚫Deep object hierarchies

31. Protocols

32. Composition instead
of inheritance

33. Protocols and inheritance
don’t mix

34. use POP to mock

35. Generics

36. Can’t reference a generic protocol

37. protocol Holder {
typealias Item
var items: [Item] { get }
}
class Basket<Thing>: Holder{
var items: [Thing] = []
func add(item: Thing) {
items.append(item)
}
}

38. 🚫
🚫
var someHolder: Holder
var someBasket: Basket

39. 🚫 var someHolder: Holder<Peach>
var someBasket: Basket<Peach>

40. Covariance only works with generic
classes

41. Covariance: If a Cat is an Animal
func feed(animal: Animal)
can receive a Cat

42. 🚫
🚫
typealias Object
typealias Object: String
typealias RoundObject: Object

43. generic classes can specify type
requirements

44. class Basket<Thing: Fruit> {
var items: [Thing] = []
func add(item: Thing) {
items.append(item)
}
}

45. 🚫
class Basket<Thing: Fruit> {
var items: [Thing] = []
func add(item: Thing) {
items.append(item)
}
}
class FruitBasket: Basket<Fruit> {}
class PeachBasket: FruitBasket<Peach> {}

46. 🚫
class Bag<Stuff> {
class Basket<Thing: Stuff> {
var items: [Thing] = []
func add(item: Thing) {
items.append(item)
}
}
}

47. Generic non-NSObjects have
issues

48. Explicitly call out class for now

49. Swift introduces
awesome new patterns

50. Thanks!
@MicheleTitolo

51. Photo Credits
• https://www.flickr.com/photos/83073191@N00/21709516808/
• https://www.flickr.com/photos/64181484@N04/21946459678/
• https://www.flickr.com/photos/66904032@N05/18905223864/
• https://www.flickr.com/photos/27454212@N00/22120629112/
• https://www.flickr.com/photos/8725928@N02/24439195435/
• https://www.flickr.com/photos/28480761@N04/24143685631/