Optional Chaining

Optional Chaining

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.

Note

Optional chaining in Swift is similar to messaging nil in Objective-C, but in a way that works for any type, and that can be checked for success or failure.

Optional Chaining as an Alternative to Forced Unwrapping

You specify optional chaining by placing a question mark (?) after the optional value on which you wish to call a property, method or subscript if the optional is non-nil. This is very similar to placing an exclamation mark (!) after an optional value to force the unwrapping of its value. The main difference is that optional chaining fails gracefully when the optional is nil, whereas forced unwrapping triggers a runtime error when the optional is nil.

To reflect the fact that optional chaining can be called on a nil value, the result of an optional chaining call is always an optional value, even if the property, method, or subscript you are querying returns a nonoptional value. You can use this optional return value to check whether the optional chaining call was successful (the returned optional contains a value), or did not succeed due to a nil value in the chain (the returned optional value is nil).

Specifically, the result of an optional chaining call is of the same type as the expected return value, but wrapped in an optional. A property that normally returns an Int will return an Int? when accessed through optional chaining.

The next several code snippets demonstrate how optional chaining differs from forced unwrapping and enables you to check for success.

First, two classes called Person and Residence are defined:

class Person {
    var residence: Residence?
}
class Residence {
    var numberOfRooms = 1
}

Residence instances have a single Int property called numberOfRooms, with a default value of 1. Person instances have an optional residence property of type Residence?.

If you create a new Person instance, its residence property is default initialized to nil, by virtue of being optional. In the code below, john has a residence property value of nil:

let john = Person()

If you try to access the numberOfRooms property of this person’s residence, by placing an exclamation mark after residence to force the unwrapping of its value, you trigger a runtime error, because there is no residence value to unwrap:

let roomCount = john.residence!.numberOfRooms
// this triggers a runtime error

The code above succeeds when john.residence has a non-nil value and will set roomCount to an Int value containing the appropriate number of rooms. However, this code always triggers a runtime error when residence is nil, as illustrated above.

Optional chaining provides an alternative way to access the value of numberOfRooms. To use optional chaining, use a question mark in place of the exclamation mark:

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."

This tells Swift to “chain” on the optional residence property and to retrieve the value of numberOfRooms if residence exists.

Because the attempt to access numberOfRooms has the potential to fail, the optional chaining attempt returns a value of type Int?, or “optional Int”. When residence is nil, as in the example above, this optional Int will also be nil, to reflect the fact that it was not possible to access numberOfRooms. The optional Int is accessed through optional binding to unwrap the integer and assign the nonoptional value to the roomCount variable.

Note that this is true even though numberOfRooms is a nonoptional Int. The fact that it is queried through an optional chain means that the call to numberOfRooms will always return an Int? instead of an Int.

You can assign a Residence instance to john.residence, so that it no longer has a nil value:

john.residence = Residence()

john.residence now contains an actual Residence instance, rather than nil. If you try to access numberOfRooms with the same optional chaining as before, it will now return an Int? that contains the default numberOfRooms value of 1:

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)."

Defining Model Classes for Optional Chaining

You can use optional chaining with calls to properties, methods, and subscripts that are more than one level deep. This enables you to drill down into subproperties within complex models of interrelated types, and to check whether it is possible to access properties, methods, and subscripts on those subproperties.

The code snippets below define four model classes for use in several subsequent examples, including examples of multilevel optional chaining. These classes expand upon the Person and Residence model from above by adding a Room and Address class, with associated properties, methods, and subscripts.

The Person class is defined in the same way as before:

class Person {
    var residence: Residence?
}

The Residence class is more complex than before. This time, the Residence class defines a variable property called rooms, which is initialized with an empty array of type [Room]:

class Residence {
    var rooms = [Room]()
    var numberOfRooms: Int {
        return rooms.count
    }
    subscript(i: Int) -> Room {
        get {
            return rooms[i]
        }
        set {
            rooms[i] = newValue
        }
    }
    func printNumberOfRooms() {
        print("The number of rooms is \(numberOfRooms)")
    }
    var address: Address?
}

Because this version of Residence stores an array of Room instances, its numberOfRooms property is implemented as a computed property, not a stored property. The computed numberOfRooms property simply returns the value of the count property from the rooms array.

As a shortcut to accessing its rooms array, this version of Residence provides a read-write subscript that provides access to the room at the requested index in the rooms array.

This version of Residence also provides a method called printNumberOfRooms, which simply prints the number of rooms in the residence.

Finally, Residence defines an optional property called address, with a type of Address?. The Address class type for this property is defined below.

The Room class used for the rooms array is a simple class with one property called name, and an initializer to set that property to a suitable room name:

class Room {
    let name: String
    init(name: String) { self.name = name }
}

The final class in this model is called Address. This class has three optional properties of type String?. The first two properties, buildingName and buildingNumber, are alternative ways to identify a particular building as part of an address. The third property, street, is used to name the street for that address:

class Address {
    var buildingName: String?
    var buildingNumber: String?
    var street: String?
    func buildingIdentifier() -> String? {
        if let buildingNumber = buildingNumber, let street = street {
            return "\(buildingNumber) \(street)"
        } else if buildingName != nil {
            return buildingName
        } else {
            return nil
        }
    }
}

The Address class also provides a method called buildingIdentifier(), which has a return type of String?. This method checks the properties of the address and returns buildingName if it has a value, or buildingNumber concatenated with street if both have values, or nil otherwise.

Accessing Properties Through Optional Chaining

As demonstrated in Optional Chaining as an Alternative to Forced Unwrapping, you can use optional chaining to access a property on an optional value, and to check if that property access is successful.

Use the classes defined above to create a new Person instance, and try to access its numberOfRooms property as before:

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."

Because john.residence is nil, this optional chaining call fails in the same way as before.

You can also attempt to set a property’s value through optional chaining:

let someAddress = Address()
someAddress.buildingNumber = "29"
someAddress.street = "Acacia Road"
john.residence?.address = someAddress

In this example, the attempt to set the address property of john.residence will fail, because john.residence is currently nil.

The assignment is part of the optional chaining, which means none of the code on the right-hand side of the = operator is evaluated. In the previous example, it’s not easy to see that someAddress is never evaluated, because accessing a constant doesn’t have any side effects. The listing below does the same assignment, but it uses a function to create the address. The function prints “Function was called” before returning a value, which lets you see whether the right-hand side of the = operator was evaluated.

func createAddress() -> Address {
    print("Function was called.")
    let someAddress = Address()
    someAddress.buildingNumber = "29"
    someAddress.street = "Acacia Road"
    return someAddress
}
john.residence?.address = createAddress()

You can tell that the createAddress() function isn’t called, because nothing is printed.

Calling Methods Through Optional Chaining

You can use optional chaining to call a method on an optional value, and to check whether that method call is successful. You can do this even if that method does not define a return value.

The printNumberOfRooms() method on the Residence class prints the current value of numberOfRooms. Here’s how the method looks:

func printNumberOfRooms() {
    print("The number of rooms is \(numberOfRooms)")
}

This method does not specify a return type. However, functions and methods with no return type have an implicit return type of Void, as described in Functions Without Return Values. This means that they return a value of (), or an empty tuple.

If you call this method on an optional value with optional chaining, the method’s return type will be Void?, not Void, because return values are always of an optional type when called through optional chaining. This enables you to use an if statement to check whether it was possible to call the printNumberOfRooms() method, even though the method does not itself define a return value. Compare the return value from the printNumberOfRooms call against nil to see if the method call was successful:

if john.residence?.printNumberOfRooms() != nil {
    print("It was possible to print the number of rooms.")
} else {
    print("It was not possible to print the number of rooms.")
}
// Prints "It was not possible to print the number of rooms."

The same is true if you attempt to set a property through optional chaining. The example above in Accessing Properties Through Optional Chaining attempts to set an address value for john.residence, even though the residence property is nil. Any attempt to set a property through optional chaining returns a value of type Void?, which enables you to compare against nil to see if the property was set successfully:

if (john.residence?.address = someAddress) != nil {
    print("It was possible to set the address.")
} else {
    print("It was not possible to set the address.")
}
// Prints "It was not possible to set the address."

Accessing Subscripts Through Optional Chaining

You can use optional chaining to try to retrieve and set a value from a subscript on an optional value, and to check whether that subscript call is successful.

Note

When you access a subscript on an optional value through optional chaining, you place the question mark before the subscript’s brackets, not after. The optional chaining question mark always follows immediately after the part of the expression that is optional.

The example below tries to retrieve the name of the first room in the rooms array of the john.residence property using the subscript defined on the Residence class. Because john.residence is currently nil, the subscript call fails:

if let firstRoomName = john.residence?[0].name {
    print("The first room name is \(firstRoomName).")
} else {
    print("Unable to retrieve the first room name.")
}
// Prints "Unable to retrieve the first room name."

The optional chaining question mark in this subscript call is placed immediately after john.residence, before the subscript brackets, because john.residence is the optional value on which optional chaining is being attempted.

Similarly, you can try to set a new value through a subscript with optional chaining:

john.residence?[0] = Room(name: "Bathroom")

This subscript setting attempt also fails, because residence is currently nil.

If you create and assign an actual Residence instance to john.residence, with one or more Room instances in its rooms array, you can use the Residence subscript to access the actual items in the rooms array through optional chaining:

let johnsHouse = Residence()
johnsHouse.rooms.append(Room(name: "Living Room"))
johnsHouse.rooms.append(Room(name: "Kitchen"))
john.residence = johnsHouse
if let firstRoomName = john.residence?[0].name {
    print("The first room name is \(firstRoomName).")
} else {
    print("Unable to retrieve the first room name.")
}
// Prints "The first room name is Living Room."

Accessing Subscripts of Optional Type

If a subscript returns a value of optional type—such as the key subscript of Swift’s Dictionary type—place a question mark after the subscript’s closing bracket to chain on its optional return value:

var testScores = ["Dave": [86, 82, 84], "Bev": [79, 94, 81]]
testScores["Dave"]?[0] = 91
testScores["Bev"]?[0] += 1
testScores["Brian"]?[0] = 72
// the "Dave" array is now [91, 82, 84] and the "Bev" array is now [80, 94, 81]

The example above defines a dictionary called testScores, which contains two key-value pairs that map a String key to an array of Int values. The example uses optional chaining to set the first item in the "Dave" array to 91; to increment the first item in the "Bev" array by 1; and to try to set the first item in an array for a key of "Brian". The first two calls succeed, because the testScores dictionary contains keys for "Dave" and "Bev". The third call fails, because the testScores dictionary does not contain a key for "Brian".

Linking Multiple Levels of Chaining

You can link together multiple levels of optional chaining to drill down to properties, methods, and subscripts deeper within a model. However, multiple levels of optional chaining do not add more levels of optionality to the returned value.

To put it another way:

• If the type you are trying to retrieve is not optional, it will become optional because of the optional chaining.

• If the type you are trying to retrieve is already optional, it will not become more optional because of the chaining.

Therefore:

• If you try to retrieve an Int value through optional chaining, an Int? is always returned, no matter how many levels of chaining are used.

• Similarly, if you try to retrieve an Int? value through optional chaining, an Int? is always returned, no matter how many levels of chaining are used.

The example below tries to access the street property of the address property of the residence property of john. There are two levels of optional chaining in use here, to chain through the residence and address properties, both of which are of optional type:

if let johnsStreet = john.residence?.address?.street {
    print("John's street name is \(johnsStreet).")
} else {
    print("Unable to retrieve the address.")
}
// Prints "Unable to retrieve the address."

The value of john.residence currently contains a valid Residence instance. However, the value of john.residence.address is currently nil. Because of this, the call to john.residence?.address?.street fails.

Note that in the example above, you are trying to retrieve the value of the street property. The type of this property is String?. The return value of john.residence?.address?.street is therefore also String?, even though two levels of optional chaining are applied in addition to the underlying optional type of the property.

If you set an actual Address instance as the value for john.residence.address, and set an actual value for the address’s street property, you can access the value of the street property through multilevel optional chaining:

let johnsAddress = Address()
johnsAddress.buildingName = "The Larches"
johnsAddress.street = "Laurel Street"
john.residence?.address = johnsAddress
if let johnsStreet = john.residence?.address?.street {
    print("John's street name is \(johnsStreet).")
} else {
    print("Unable to retrieve the address.")
}
// Prints "John's street name is Laurel Street."

In this example, the attempt to set the address property of john.residence will succeed, because the value of john.residence currently contains a valid Residence instance.

Chaining on Methods with Optional Return Values

The previous example shows how to retrieve the value of a property of optional type through optional chaining. You can also use optional chaining to call a method that returns a value of optional type, and to chain on that method’s return value if needed.

The example below calls the Address class’s buildingIdentifier() method through optional chaining. This method returns a value of type String?. As described above, the ultimate return type of this method call after optional chaining is also String?:

if let buildingIdentifier = john.residence?.address?.buildingIdentifier() {
    print("John's building identifier is \(buildingIdentifier).")
}
// Prints "John's building identifier is The Larches."

If you want to perform further optional chaining on this method’s return value, place the optional chaining question mark after the method’s parentheses:

if let beginsWithThe =
    john.residence?.address?.buildingIdentifier()?.hasPrefix("The") {
    if beginsWithThe {
        print("John's building identifier begins with \"The\".")
    } else {
        print("John's building identifier does not begin with \"The\".")
    }
}
// Prints "John's building identifier begins with "The"."

Note

In the example above, you place the optional chaining question mark after the parentheses, because the optional value you are chaining on is the buildingIdentifier() method’s return value, and not the buildingIdentifier() method itself.