Use Optional Chaining in Swift

Written by Team Kodeco

Optional chaining is a process for querying and calling properties, methods and subscripts on an optional that might currently be nil. If the optional contains a value, the property, method, or subscript call succeeds; if the optional is nil, the property, method or subscript call returns nil.

Multiple queries can be chained together and the entire chain fails gracefully if any link in the chain is nil.

// Classes for testing
class Person {
  var residence: Residence?
}
class Residence {
  var numberOfRooms = 1
}

// Example 1: Create a person, with the residence 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."

// Example 2: Create a person, with the residence set
john.residence = Residence()
if let roomCount = john.residence?.numberOfRooms {
  print("John's residence has \(roomCount) room(s).")
} else {
  print("Unable to retrieve the number of rooms.")
}
// Prints "John's residence has 1 room(s)."

In the example above, the residence property of the john object is nil, so the call to numberOfRooms returns nil and the if-let statement’s condition is false.

Note: When using optional chaining, it’s important to keep in mind that the returned value will always be an optional, even if the property, method, or subscript you’re querying returns a non-optional value.

Set a Value if not Nil

You can also use optional chaining to set a property’s value if the optional isn’t nil:

john.residence?.numberOfRooms = 5
if let roomCount = john.residence?.numberOfRooms {
  print("John's residence has \(roomCount) room(s).")
} else {
  print("Unable to retrieve the number of rooms.")
}
// Prints "John's residence has 5 room(s)."

In the example above, numberOfRooms is only updated if residence isn’t nil.