Real World Generics In Swift

Short overview and examples of practical usage of generics in Swift.

Transcript

1. REAL WORLD GENERICS IN SWIFT @VADYMMARKOV
2. ABOUT ME let me = Developer( name: "Vadym Markov", specialisation: "iOS", company: "Hyper Interaktiv AS", interests: [ "Technology", "Open Source", "Music" ] )
3. GENERICS, HUH?
4. DEFINITION ▸ Generic programming is a way to write functions and data types without being specific about the types they use
 ▸ Generic code enables you to write flexible, reusable functions and types that can work with any type, subject to requirements that you define
5. GENERICS AND SWIFT ▸ Generics are one of the most powerful features of Swift ▸ Much of the Swift standard library is built with generic code ▸ You’ve already been using generics, even if you didn’t realise it
6. SWIFT STANDARD LIBRARY // Array var array = [String]() // Dictionary var dictionary = [String: String]() // Enum var string1 = Optional<String>.Some("Swift") var string2 = Optional<String>.None == nil
7. THE PROBLEM struct IntQueue { private var items = [Int]() mutating func enqueue(item: Int) { items.append(item) } mutating func dequeue() -> Int? { return items.removeFirst() } func peek() -> Int? { return items.first } func isEmpty() -> Bool { return items.isEmpty } }
8. SOLUTION? struct AnyQueue { private var items = [Any]() mutating func enqueue(item: Any) { items.append(item) } mutating func dequeue() -> Any? { return items.removeFirst() } func peek() -> Any? { return items.first } func isEmpty() -> Bool { return items.isEmpty } }
9. SO NOW WE ARE ABLE TO PUSH STRINGS TO THE STACK, RIGHT? YES BUT… ▸ We are losing type safety ▸ We also need to do a lot of casting
10. SOLUTION! struct Queue<T> { private var items = [T]() mutating func enqueue(item: T) { items.append(item) } mutating func dequeue() -> T? { return items.removeFirst() } func peek() -> T? { return items.first } func isEmpty() -> Bool { return items.isEmpty } }
11. SOLUTION! var stringQueue = Queue<String>() stringQueue.enqueue("String") stringQueue.dequeue() // "String" var intQueue = Queue<Int>() intQueue.enqueue(1) intQueue.dequeue() // 1
12. WELCOME TO REALITY
13. GENERIC TYPES class DataSource<Model, Cell: UITableViewCell> : NSObject, UITableViewDataSource, UITableViewDelegate { let cellIdentifier: String let configureCell: (Model, Cell) -> Void var cellHeight: CGFloat = 64 var items = [Model]() var action: (Model -> Void)? init(cellIdentifier: String, configureCell: (Model, Cell) -> Void) { self.cellIdentifier = cellIdentifier self.configureCell = configureCell } // ...
14. // MARK: - UITableViewDataSource func numberOfSectionsInTableView(tableView: UITableView) -> Int { return 1 } func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int { return items.count } func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell { let cell = tableView.dequeueReusableCellWithIdentifier( cellIdentifier, forIndexPath: indexPath) let item = items[indexPath.item] if let cell = cell as? Cell { configureCell(item, cell) } return cell }
15. // MARK: - UITableViewDelegate func tableView(tableView: UITableView, heightForRowAtIndexPath indexPath: NSIndexPath) -> CGFloat { return cellHeight } func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) { tableView.deselectRowAtIndexPath(indexPath, animated: true) guard let action = action else { return } let item = items[indexPath.row] action(item) }
16. class TeamController: UITableViewController { static let reusableIdentifier = "TeamCellIdentifier" var developers = [Developer]() lazy var dataSource: DataSource<Developer, TableViewCell> = { [unowned self] in let dataSource = DataSource( cellIdentifier: TeamController.reusableIdentifier, configureCell: self.configureCell) dataSource.action = self.selectCell return dataSource }() // ... func configureCell(developer: Developer, cell: TableViewCell) { cell.textLabel?.text = developer.name cell.imageView?.image = UIImage(named: "placeholder")) } func selectCell(developer: Developer) { UIApplication.sharedApplication().openURL(developer.githubURL) } }
17. protocol CacheAware { func object<T: Cachable>(key: String, completion: (object: T?) -> Void) } class DiskStorage: CacheAware { func object<T: Cachable>(key: String, completion: (object: T?) -> Void) { // ... } } // String must conform Cachable let storage = DiskStorage(name: "Unknown") storage.add("string", object: "My string”) storage.object("string") { (object: String?) in } GENERIC FUNCTIONS
18. struct ViewModel: Mappable { // ... var meta = [String : AnyObject]() func meta<T>(key: String, _ defaultValue: T) -> T { return meta[key] as? T ?? defaultValue } } let modelFoo = ViewModel(title: "foo", meta: ["id" : 1]) modelFoo.meta("id", 0) // 1 modelFoo.meta("foo", "bar") // bar GENERIC FUNCTIONS
19. WHAT ABOUT PROTOCOLS? ▸Protocols in Swift cannot be defined generically using type parameters.
 ▸Instead, protocols can define what are known as associated types.
20. ASSOCIATED TYPES protocol Cachable { typealias CacheType static func decode(data: NSData) -> CacheType? func encode() -> NSData? } extension String: Cachable { typealias CacheType = String static func decode(data: NSData) -> CacheType? { // ... } func encode() -> NSData? { // ... } }
21. ASSOCIATED TYPES let list: [Cachable] = [] ▸ Cachable represents a set of types rather than a single type ▸ In an array of Cachable, you are not be able to say anything about the return type of the decode() method
22. HOW COULD IT LOOK LIKE? let list : [Cachable<String>] = [] let’s imagine Swift supporting generic parameterised protocols
23. MORE EXAMPLES: OPERATORS infix operator ?= { associativity right precedence 90 } public func ?=<T>(inout left: T, right: T?) { guard let value = right else { return } left = value } public func ?=<T>(inout left: T?, right: T?) { guard let value = right else { return } left = value } // ... func testIfLetAssignment() { let hyper = NSURL(string: "hyper.no")! let faultyURL = NSURL(string: "\\/http") var testURL: NSURL? testURL ?= hyper // "hyper.no" testURL ?= faultyURL // "hyper.no" }
24. MORE EXAMPLES: PROTOCOL EXTENSIONS protocol Queueable { func process() -> Bool } extension Array where Element : Queueable { mutating func processQueue(from: Int, to: Int, process: ((element: Element) -> Void)) { // ... } } var testQueue = [Object(), Object()] testQueue.processQueue()
25. MORE EXAMPLES: ENUMS enum Result<T, ErrorType> { case Success(T) case Failure(ErrorType) } struct UFO { enum Error: ErrorType { case NotFound } } func recognizeUFO(completion: Result<UFO, UFO.Error> -> Void) { var ufo: UFO? // Some async complex calculations if let result = ufo { completion(Result.Success(result)) } else { completion(Result.Failure(UFO.Error.NotFound)) } }
26. WHY GENERICS? ▸ Type safety ▸ Less code duplication ▸ No type casting hell ▸ Public API flexibility ▸ Abstraction is fun
27. RESOURCES ▸ https://developer.apple.com/swift/ ▸ https://github.com/hyperoslo/Orchestra ▸ https://github.com/hyperoslo/Cache ▸ https://github.com/hyperoslo/Spots ▸ https://github.com/hyperoslo/Sugar
28. QUESTIONS? HTTPS://GITHUB.COM/VADYMMARKOV @VADYMMARKOV