Typumwandlung: is, as?, as! und as

Was ist Typ-Casting?

class Animal { var name: String = "Animal" }
class Dog: Animal { func bark() { print("Woof") } }
let a: Animal = Dog()
print(a.name)

Der Operator is

let a: Animal = Dog()
if a is Dog {
  print("It's a dog")
}

Downcasting mit as?

let a: Animal = Dog()
if let dog = a as? Dog {
  dog.bark()
} else {
  print("Not a dog")
}

Downcasting mit as!

let a: Animal = Dog()
let dog = a as! Dog
dog.bark()

Upcasting mit as

let dog = Dog()
let animal = dog as Animal
print(animal.name)

Typ-Casting in Collections

let items: [Any] = [1, "hello", Dog()]
for item in items {
  if let d = item as? Dog { d.bark() }
  if let s = item as? String { print(s) }
}

Typ-Casting mit AnyObject

let objs: [AnyObject] = [Dog(), Dog()]
for obj in objs {
  if let d = obj as? Dog { d.bark() }
}

Protokolltypen casten

protocol Runnable { func run() }
class Cat: Animal, Runnable { func run() { print("Cat runs") } }
let r: Runnable = Cat()
if let cat = r as? Cat { print(cat.name) }

Pattern Matching mit is in switch

let value: Any = 42
switch value {
case is Int: print("Integer")
case is String: print("String")
default: print("Other")
}

as? in switch-Patterns

let value: Any = "Swift"
switch value {
case let s as String: print("String: \(s)")
case let i as Int: print("Int: \(i)")
default: break
}

Best Practices für sicheres Casting

func describe(_ obj: AnyObject) {
  if let dog = obj as? Dog { dog.bark(); return }
  print("Unknown type")
}

Kurze Überprüfung

Zusammenfassung der Lektion