Home > Articles > Programming > General Programming/Other Languages

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

Optional Chaining

In Swift, you chain methods and properties by appending period-delimited selectors. Each function in the chain returns an intermediate value. This allows calls to be joined into a single statement without requiring variables that store intermediate results:


This approach creates a fluent interface, which is ideally a parsimonious and more readable expression of a set of operations you want to consider as a single unit. A danger, of course, lies in over-chaining. If you’re producing enormous lines of code that are difficult to debug and hard to read and that cannot be easily commented or differentiated on updates, you’re probably doing this wrong. Ask yourself, “Would I ever need to put a breakpoint in this statement or step through it?” If the answer is yes, you are over-chaining.

Swift introduces a powerful feature called optional chaining. Swift method calls may return optionals, and you must take this into account when forming chains. Swift provides a way that an entire chain can fail gracefully on encountering nil.

Optional chaining involves adding question marks after optional values. For example, you might look up an animal in the sound dictionary and use optional chaining to return a capitalized version of the sound:

soundDictionary[animal]?.capitalizedString // Moo or nil

Even though capitalizedString normally returns a non-optional, this chain returns String?. It may succeed or fail, depending on the lookup.

Add question marks to any chain participants that return optionals:

soundDictionary[animal]?.characters.first?.hashValue // returns Int?

You can add forced unwrapping to any chain item by replacing the question mark with an exclamation point. This usage comes with the same forced unwrapping dangers discussed earlier in this chapter:

soundDictionary[animal]!.capitalizedString // Moo or Runtime Error

Here’s a real-world example of where you might use optional chaining to simplify an if-let pattern. This code extends Array to return the index of a possible maximum element. Swift’s standard library maxElement() function returns an optional based on whether a sequence has values to compare (Apple writes in the standard library, “Returns the maximum element in `self` or `nil` if the sequence is empty.”):

extension Array where Element:Comparable {
    var maxIndex: Int? {
        if let e =
            self.enumerate().maxElement({$1.element > $0.element}) {
            return e.index
        return nil

Introducing optional chaining greatly simplifies this code, enabling you to shortcut the index lookup and returning nil if the maxElement call fails. Recipe 3-2 returns the index of an array’s maximum value.

Recipe 3-2 Using Optional Chaining to Shortcut Evaluations

extension Array where Element:Comparable {
    var maxIndex: Int? {
        return self.enumerate().maxElement(
            {$1.element > $0.element})?.index

Extend Recipe 3-2’s functionality to all collection types with the following snippet:

extension CollectionType where Generator.Element: Comparable {
    var maxIndex: Index? {
        return self.indices.maxElement({self[$1] > self[$0]})

Selector Testing and Optional Chaining

Optional chaining isn’t just about transforming your code into bite-sized lines. It also acts as shorthand to test whether an item responds to a method or property selector. Optional chaining offers a rough equivalent to Objective-C’s respondsToSelector: method, enabling you to determine whether it’s safe to execute calls on particular instances.

Commonly, you work with subclasses that are directly related to each other but that implement distinct method sets. For example, you might retrieve a collection of SpriteKit nodes from a scene and then adjust the line widths of the shape nodes. This snippet uses a failable cast followed by an optionally chained property assignment:

for node in nodes {(node as? SKShapeNode)?.lineWidth = 4.0}

This selector-testing approach also works in pure Swift, as in the following example:

// Root class put two subclasses
class Root {func rootFunc() {}}
class Sub1: Root {func sub1Func() {print("sub1")}}
class Sub2: Root {func sub2Func() {print("sub2")}}

// Create heterogeneous array of items
var items: [Root] = [Sub1(), Sub2()]

// Conditionally test and run selectors
(items[0] as? Sub1)?.sub1Func() // runs
(items[0] as? Sub2)?.sub2Func() // no-op, nil

This snippet constructs a heterogeneous array of Root subclasses. It then performs conditional casts and uses selector tests before calling class-specific methods.

Selector testing enables you to test whether a method exists before constructing a new instance. Adding the question mark ensures that the NSString call won’t fail with a runtime “unrecognized selector” exception:

let colorClass: AnyClass = UIColor.self
let noncolorClass: AnyClass = NSString.self
colorClass.blueColor?() // returns a blue color instance
noncolorClass.blueColor?() // returns nil

This is a special behavior of AnyClass and AnyObject that works only with Objective-C methods, for compatibility with Class and id. These are special cases because these types return functions as implicitly unwrapped optionals. Other types don’t do that.


Contrary to expectations, optional chaining with subscripts doesn’t introduce safe lookups. This is an important factor that you should internalize as soon as possible and recognize in your code. In the following example, if you try to access index 8 (aka the ninth element of this six-element array), your code dies with a fatal Array index out of range error:

let array: Array? = [0, 1, 2, 3, 4, 5]
array?[0] // 0
// array?[8] // still fails

In this example, the question mark does not qualify the lookup for safety. It is required for subscripting after array, which is optional. With subscripts, you add chain annotations in-line after the optional value and before the subscript brackets.

Optional chaining is meant solely to set and retrieve values for subscripts used with optional values. It does not and cannot short-circuit failed subscripts unless you build a failable subscripting extension, as in the following example:

extension Array {
    subscript (safe index: UInt) -> Element? {
        return Int(index) < count ? self[Int(index)] : nil

Once you add a simple array safe-index extension, you can optionally chain the safe version of the subscript. In the following calls, the Element? results of the safe: subscript are now optional and can be chained:

print(array?[safe: 0]?.dynamicType) // nil
print(array?[safe: 8]?.dynamicType) // Optional(Swift.Int)
  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020