Protocols and the Promised Land

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 causes, and evaluate workarounds.

Transcript

1. Protocols and the Promised Land @MicheleTitolo
2. TRUTHS
3. Types at compile time
4. Swift and Objc have different language features
5. Protocols are first-class citizens
6. Swift is still very new
8. Interop
9. as? as? as?
10. as? lies
11. as? doesn’t work when bridging from Objective-C to Swift protocol
12. let vendor = ObjcObject() let items = vendor.giveMeItems() as? [Fruit]
13. let vendor = ObjcObject() let items = vendor.giveMeItems().map { $0 as Fruit }
14. @objc spreads like a virus
15. @objc protocol SomeProtocol { func someFunc() } class MyClass: SomeProtocol {}
16. @objc protocol SomeProtocol { func someFunc() } @objc class MyClass: NSObject, SomeProtocol {}
17. remember: some things can’t be bridged
18. -Swift.h will leave out methods/vars that don’t bridge
19. xctest is buggy
20. rdar://24200114: Bi-directional interop between Objective-C and Swift does not work in XCTest targets
21. Things you expect to work, but don’t
22. Protocols
23. class Fruit {} class Peach: Fruit {} protocol FruitHolder { var fruits: [Fruit] { get } } class PeachBasket: FruitHolder { var fruits: [Peach] = [] }
24. @interface PeachBasket : NSObject <FruitHolder> @property(nonatomic, strong) NSArray<Peach*>* fruits; @end
25. Generics
26. generic types are stand-ins
27. protocol Holder { typealias Item var items: [Item] { get } } class PeachBasket: Holder { var items: [Peach] = [] }
28. generic protocols
29. protocol Holder { typealias Item var items: [Item] { get } } class Basket<Thing>: Holder{ var items: [Thing] = [] func add(item: Thing) { items.append(item) } } class PeachBasket: Basket<Peach> {}
30. referencing generics
31. var someHolder: Holder var someBasket: Basket
32. var someHolder: Holder<Peach> var someBasket: Basket<Peach>
33. Covariance only works with generic classes
34. Covariance: If a Cat is an Animal func feed(animal: Animal) can receive a Cat
35. typealias Object typealias Object: String typealias RoundObject: Object
36. class Basket<Thing: Fruit>: Holder{ var items: [Thing] = [] func add(item: Thing) { items.append(item) } }
37. class Basket<Thing: Fruit>: Holder{ var items: [Thing] = [] func add(item: Thing) { items.append(item) } } class FruitBasket: Basket<Fruit> {} class PeachBasket: FruitBasket<Peach> {}
38. class Bag<Stuff> { class Basket<Thing: Stuff>: Holder{ var items: [Thing] = [] func add(item: Thing) { items.append(item) } } }
39. New language, new patterns
40. properties can have 1 type
41. protocol Themeable {} class ListViewController: UIViewController, Themeable {} var themedViewController: // UIViewController, Themeable ????
42. Two solutions
43. 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
44. 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
45. Can’t override type in a subclass (even if it’s a subtype)
46. class FruitBasket: Basket<Fruit> { var ribbonColor: UIColor } class PeachBasket: FruitBasket { var ribbonColor: Pattern }
47. Deep object hierarchies
48. Generic non-NSObjects have issues
49. Questions? @MicheleTitolo
50. Photo Credits • https://www.flickr.com/photos/61048402@N08/9528157702/ • https://www.flickr.com/photos/90157732@N03/15717442046 • https://www.flickr.com/photos/58595941@N00/1400248430/ • https://www.flickr.com/photos/34432248@N06/4122124763 • https://www.flickr.com/photos/16754363@N04/2289064123/