Swift 5 - Optionals

What is an optional?

• An optional is used to handle the abscence of a value.
• An optional can indicate two things:
  • There is a value, but you have to unwrap the optional to access the value
  • There isn’t a value at all
• In Swift, assigning a value to nil is not allowed, unless it’s an optional. This protects the app from crashing due to unintended side effects, such as accessing a property from a value that is nil.
• Using optionals forces the developer to be more intentional on nil handling

Can set an optional type to nil:

let possibleString: String? = "An optional string."
possibleString = nil
// possibleString now contains no value

Types of Optional Handling:

• forced unwrapping via ! operator — unsafe
• implicitly unwrapped variable declaration — unsafe in many cases
• optional binding — safe
• optional chaining — safe
• nil coalescing operator — safe
• guard statement — safe
• optional pattern — safe

Force Unwrapping

Only force unwrap an optional when you are confident that there will always be a value set, otherwise it crashes the app/ causes runtime error when the value is nil. It’s like unwrapping a present and not knowing if there’s a bomb in there or not!

let possibleString: String? = "An optional string."
let forcedString: String = possibleString! // requires an exclamation mark

Implicitly Unwrapped Optional

An optional that can also be used like a non-optional value, without the need to unwrap the optional value each time it is accessed, because it’s assumed to always have a value after that value is initially set. Try to avoid this and use optional values for safety.

let assumedString: String! = "An implicitly unwrapped optional string."
let implicitString: String = assumedString // no need for an exclamation mark

Optional Binding

Checks and unwraps an implicitly unwrapped optional

let assumedString: String! = "An implicitly unwrapped optional string."

if let definiteString = assumedString {
print(definiteString)
}
// Prints "An implicitly unwrapped optional string."

Optional Chaining

Accessing properties of an optional value. Short circuits if the optional value is nil. Fails gracefully when the value is nil.

let john = Person()

if let roomCount = john.residence?.numberOfRooms {
print("John's residence has \(roomCount) room(s).")
} else {
print("Unable to retrieve the number of rooms.")
}
// Prints "Unable to retrieve the number of rooms."

Nil Coalescing Operator

The nil-coalescing operator (a ?? b) unwraps an optional a if it contains a value, or returns a default value b if a is nil. The expression a is always of an optional type. The expression b must match the type that is stored inside a.