Optional Chaining¶

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 point (!) 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 non-optional value. You can use this optional return value to check whether the optional chaining call was successful (the returned optional contains a value), or didn’t 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:

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:

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

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 point:

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 non-optional value to the roomCount constant.

Note that this is true even though numberOfRooms is a non-optional Int. The fact that it’s 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 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:

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’s 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:

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]:

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:

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:

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:

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:

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.

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 doesn’t define a return value.

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

This method doesn’t 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 doesn’t itself define a return value. Compare the return value from the printNumberOfRooms call against nil to see if the method call was successful:

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:

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.

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: