Chapter 6: Optionals

大綱

• Introducing nil

  • Sentinel values

• Introducing optionals

• Unwrapping optionals

  • Force unwrapping

  • Optional binding

• Introducing guard

• Nil coalescing

• Key points

Introducing nil

• it’s useful to represent the absence of a value

Sentinel values

• it’s useful to represent the absence of a value

  • 0 is a sentinel value.

• Nil is the name given to the absence of a value, and you’re about to see how Swift incorporates this concept directly into the language in a rather elegant way.

var errorCode = 0

Introducing optionals

• Optionals are Swift’s solution to the problem of representing both a value and the absence of a value

var errorCode: Int?

Unwrapping optionals

Force unwrapping

• you get this error at runtime rather than compile time

• if this code were inside an app, the runtime error would cause the app to crash

authorName = nil
print("Author is \(authorName!)") // Fatal error: Unexpectedly found nil while unwrapping an Optional value

Optional binding

• You can combine unwrapping multiple optionals with additional Boolean checks

if let authorName = authorName,
   let authorAge = authorAge,
   authorAge >= 40 {
  print("The author is \(authorName) who is \(authorAge) years old.")
} else {
  print("No author or no age or age less than 40.")
}

Introducing guard

func maybePrintSides(shape: String) {
  guard let sides = calculateNumberOfSides(shape: shape) else {
    print("I don't know the number of sides for \(shape).")
    return
  }

  print("A \(shape) has \(sides) sides.")
}

Nil coalescing

var optionalInt: Int? = 10
var mustHaveResult = optionalInt ?? 0

Key points

• nil represents the absence of a value.

• Non-optional variables and constants must always have a non-nil value.

• Optional variables and constants are like boxes that can contain a value or be empty (nil).

• To work with the value inside an optional, you must first unwrap it from the optional.

• The safest ways to unwrap an optional’s value is by using optional binding or nil coalescing.

• Use forced unwrapping only when appropriate, as it could produce a runtime error.