Embrace Immutability

Keith describes an open source library for parsing JSON written at Lyft, how it enables you to use immutable model objects, and why that's a good thing.

Transcript

1. EMBRACE IMMUTABILITY
2. GITHUB.COM/LYFT/MAPPER
3. NOT A JSON PARSER
4. NO MODEL -> JSON
5. { "user": { "id": "123", "photoURL": "http://example.com/123.jpg", "phone": { "verified": false } } }
6. struct User: Mappable { var id: String? var photoURL: NSURL? var verified: Bool = true init() {} mutating func map(mapper: Mapper) { id « mapper["id"] photoURL « mapper["photoURL"] verified « mapper["phone.verified"] } }
7. protocol Mapper { func baseType<T>(field: T?) -> T? func baseTypeArray<T>(field: [T]?) -> [T]? // ... subscript(keyPath: String) -> Mapper { get } }
8. func « <T>(inout left: T?, mapper: Mapper) { left = mapper.baseType(left) } func « <T>(inout left: [T]?, mapper: Mapper) { left = mapper.baseTypeArray(left) }
9. class MapperFromJSON: Mapper { var JSON: NSDictionary var currentValue: AnyObject? // ... subscript(keyPath: String) -> Mapper { get { self.currentValue = self.JSON.valueForKeyPath(keyPath) return self } } }
10. func baseType<T>(field: T?) -> T? { let value = self.currentValue switch T.self { case is NSURL.Type where value is String: return NSURL(string: value as! String) as? T // ... default: return value as? T } }
11. case is CLLocationCoordinate2D.Type: if let castedValue = value as? [String: Double], let latitude = castedValue["lat"], let longitude = castedValue["lng"] { return CLLocationCoordinate2D(latitude: latitude, longitude: longitude) as? T } return nil
12. id = JSON["id"].string
13. struct User: Mappable { let id: String let photoURL: NSURL? let verified: Bool init(map: Mapper) throws { try id = map.from("id") photoURL = map.optionalFrom("photoURL") verified = map.optionalFrom("phone.verified") ?? false } }
14. func from<T>(field: String) throws -> T { if let value = self.JSONFromField(field) as? T { return value } throw MapperError() }
15. extension NSURL: Convertible { static func fromMap(value: AnyObject?) throws -> NSURL { if let string = value as? String, let URL = NSURL(string: string) { return URL } throw MapperError() } }
16. struct AppInfo { let hints: [HintID: Hint] init(map: Mapper) throws { try hints = map.from("hints", transformation: Transform.toDictionary { $0.id }) } }
17. func from<T>(field: String, transformation: AnyObject? throws -> T) rethrows -> T { return try transformation(self.JSONFromField(field)) }
18. EMBRACE IMMUTABILITY
19. DUMB MODELS struct User: Mappable { let id: String let photoURL: NSURL? init(map: Mapper) throws { try id = map.from("id") photoURL = map.optionalFrom("photoURL") } }
20. CODE SMELL - let pickup: Place? + var pickup: Place?
21. SEPARATE CREATION struct Ride: Mappable { var pickup: Place? }
22. RideManager Observer1 Observer2 Observer3 UIChanges Network Call Evil Function
23. RideManager.ride.pickup = Place(name: "Realm")
24. RideManager Observer1 Observer2 Observer3 UIChanges Network Call Evil Function
25. RideManager Observer1 Observer2 Observer3 UIChanges Network Call Evil Function
26. SEPARATE CREATION struct RideRequest { var pickup: Place? }
27. 1. How can I do this? 2. How can I do this easier? 3. How can I do this simpler? 4. How can I not do this?
28. GITHUB.COM/LYFT/MAPPER
29. QUESTIONS