การแปลงชนิดแบบมีเงื่อนไขด้วย as?

การแคสต์แบบมีเงื่อนไข

class Animal {
    let name: String
    init(name: String) { self.name = name }
}
class Dog: Animal {
    func bark() -> String { return name + " says Woof" }
}
class Cat: Animal {
    func meow() -> String { return name + " says Meow" }
}
let a: Animal = Dog(name: "Rex")
let maybe = a as? Dog
print(maybe != nil)

ผลลัพธ์เป็นค่าทางเลือก

class Animal {
    let name: String
    init(name: String) { self.name = name }
}
class Dog: Animal {
    func bark() -> String { return name + " says Woof" }
}
class Cat: Animal {
    func meow() -> String { return name + " says Meow" }
}
let a: Animal = Dog(name: "Rex")
let maybe: Dog? = a as? Dog
print(type(of: maybe))

การแกะค่าด้วย if let

class Animal {
    let name: String
    init(name: String) { self.name = name }
}
class Dog: Animal {
    func bark() -> String { return name + " says Woof" }
}
class Cat: Animal {
    func meow() -> String { return name + " says Meow" }
}
let a: Animal = Dog(name: "Rex")
if let dog = a as? Dog {
    print(dog.bark())
} else {
    print("Not a dog")
}

การแคสต์ที่ล้มเหลวคืนค่า nil

class Animal {
    let name: String
    init(name: String) { self.name = name }
}
class Dog: Animal {
    func bark() -> String { return name + " says Woof" }
}
class Cat: Animal {
    func meow() -> String { return name + " says Meow" }
}
let a: Animal = Cat(name: "Mia")
if let dog = a as? Dog {
    print(dog.bark())
} else {
    print("That is not a dog")
}

การใช้ร่วมกับ guard let

class Animal {
    let name: String
    init(name: String) { self.name = name }
}
class Dog: Animal {
    func bark() -> String { return name + " says Woof" }
}
class Cat: Animal {
    func meow() -> String { return name + " says Meow" }
}
func describe(_ a: Animal) {
    guard let cat = a as? Cat else {
        print("Not a cat")
        return
    }
    print(cat.meow())
}
describe(Cat(name: "Mia"))
describe(Dog(name: "Rex"))

การแคสต์แบบมีเงื่อนไขในลูป

class Animal {
    let name: String
    init(name: String) { self.name = name }
}
class Dog: Animal {
    func bark() -> String { return name + " says Woof" }
}
class Cat: Animal {
    func meow() -> String { return name + " says Meow" }
}
let zoo: [Animal] = [Dog(name: "Rex"), Cat(name: "Mia"), Dog(name: "Buddy")]
for a in zoo {
    if let dog = a as? Dog {
        print(dog.bark())
    }
}

การแคสต์เป็นโพรโทคอล

protocol Greeter { func greet() -> String }
class Person: Greeter { func greet() -> String { return "Hi" } }
class Rock {}
let items: [Any] = [Person(), Rock()]
for item in items {
    if let g = item as? Greeter { print(g.greet()) }
}

การใช้ค่าทดแทนหลังการแคสต์

class Animal {
    let name: String
    init(name: String) { self.name = name }
}
class Dog: Animal {
    func bark() -> String { return name + " says Woof" }
}
class Cat: Animal {
    func meow() -> String { return name + " says Meow" }
}
let a: Animal = Cat(name: "Mia")
let sound = (a as? Dog)?.bark() ?? "unknown sound"
print(sound)

การเรียกแคสต์แบบต่อค่าทางเลือก

class Animal {
    let name: String
    init(name: String) { self.name = name }
}
class Dog: Animal {
    func bark() -> String { return name + " says Woof" }
}
class Cat: Animal {
    func meow() -> String { return name + " says Meow" }
}
let a: Animal = Dog(name: "Rex")
let bark = (a as? Dog)?.bark()
print(bark as Any)

เหตุผลที่ควรเลือกการแคสต์แบบมีเงื่อนไข

class Animal {
    let name: String
    init(name: String) { self.name = name }
}
class Dog: Animal {
    func bark() -> String { return name + " says Woof" }
}
class Cat: Animal {
    func meow() -> String { return name + " says Meow" }
}
let unknown: Animal = Cat(name: "Mia")
let result = unknown as? Dog
print("Crashes? No. Value is nil:", result == nil)

ตัวอย่างสรุปการแคสต์แบบมีเงื่อนไข

class Animal {
    let name: String
    init(name: String) { self.name = name }
}
class Dog: Animal {
    func bark() -> String { return name + " says Woof" }
}
class Cat: Animal {
    func meow() -> String { return name + " says Meow" }
}
func sound(of a: Animal) -> String {
    if let d = a as? Dog { return d.bark() }
    if let c = a as? Cat { return c.meow() }
    return "silence"
}
print(sound(of: Dog(name: "Rex")))
print(sound(of: Cat(name: "Mia")))

ตรวจสอบอย่างรวดเร็ว

สรุป: การแคสต์แบบมีเงื่อนไขด้วย as?

class Animal {
    let name: String
    init(name: String) { self.name = name }
}
class Dog: Animal {
    func bark() -> String { return name + " says Woof" }
}
class Cat: Animal {
    func meow() -> String { return name + " says Meow" }
}
let pets: [Animal] = [Dog(name: "Rex"), Cat(name: "Mia")]
for p in pets {
    if let d = p as? Dog { print(d.bark()) } else if let c = p as? Cat { print(c.meow()) }
}