快速的非零值可选链

Question

I have the following program which works fine -

class Person {
    var pet: Pet?
}

class Pet {
    var name: String

    var favoriteToy: Toy?

    init(name: String) {
        self.name = name
    }
}

class Toy {
    var name: String

    init(name: String) {
        self.name = name
    }
}

let q = Person()
// Pet(name:"goofy")
// Toy(name:"teddy")

if let someToy = q.pet?.favoriteToy?.name {
    print("This person's pet likes \(someToy).")
} else {
    print("This person's pet does not have a favorite toy")
}

Prints:

This person's pet does not have a favorite toy

How to use the above code modified to print "This person's pet likes teddy."?

I know I will have not be able to use if let as there will be nothing to unwrap. So I have to write something like this instead of if let :

let someToy = q.pet.favoriteToy.name
print("This person's pet likes \(someToy).")

Should print "This person's pet likes the teddy."

I also know I have to put non-nil value something like this:

class Person {
    var pet = Pet ()
}

Still I am having initialization problems. How to go about it?

Answer 1

This should solve your immediate needs...

let man = Person()
let goofy = Pet(name: "goofy")
let squeaky = Toy(name: "squeaky toy")

goofy.favoriteToy = squeaky
man.pet = goofy

But if a person is usually going to be initialized with a pet and a toy, and both of those classes are initialized with a string, then you might want to define a convenience initializer:

class Pet {
  var name: String
  var favoriteToy: Toy?
  init(name: String) {
    self.name = name
  }
}

class Toy {
  var name: String
  init(name: String) {
    self.name = name
  }
}

class Person {
  var pet: Pet?
  convenience init(petName: String, toyName: String) {
    self.init()
    let toy = Toy(name: toyName)
    let pet = Pet(name: petName)
    pet.favoriteToy = toy
    self.pet = pet
  }
}

func test() {
  let bill = Person(petName: "goofy", toyName: "squeaky ball")
  guard let toyName = bill.pet?.favoriteToy?.name else { return }
  print(toyName)
}

test()

One more thing: optional chaining can be combined with a guard statement to great effect in Swift.

Answer 2

Use ? = nil ? = nil in your initializer to pass in some optional parameters.

class Person {
    var pet: Pet?

    init(pet: Pet? = nil) {
        self.pet = pet
    }
}

class Pet {
    var name: String

    var favoriteToy: Toy?

    init(name: String, toy: Toy? = nil) {
        self.name = name
        self.favoriteToy = toy
    }
}

class Toy {
    var name: String

    init(name: String) {
        self.name = name
    }
}

let q = Person(pet: Pet(name: "goofy", toy: Toy(name: "teddy")))
// Pet(name:"goofy")
// Toy(name:"teddy")

if let someToy = q.pet?.favoriteToy?.name {
    print("This person's pet likes \(someToy).")
} else {
    print("This person's pet does not have a favorite toy")
}