Home > Articles > Programming > General Programming/Other Languages

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

Exploring the Swift File Structure

In the preceding hour, you learned how to use Xcode to create projects and navigate their files. As mentioned then, the vast majority of your time will be spent in the project group of Xcode, which is shown for the MyNewApp project in Figure 3.1. You’ll be adding methods to class files that Xcode creates for you when you start a project or, occasionally, creating your own class files to implement entirely new functionality in your application.

Figure 3.1

FIGURE 3.1 Most of your coding will occur within the files in your project group.

Okay, sounds simple enough, but where will the coding take place? If you create a new project look, you’ll see quite a few different files staring back at you.

Class Files

In Swift, a class is implemented within a single file with the .swift extension. This file contains all of the variable/constant definitions, and all of the methods containing the application logic. Other classes in your project will automatically be able to access the methods in this file, if needed.

Let’s review the structure of an entirely made-up class file in Listing 3.1.

LISTING 3.1 A Sample Interface File

 1: import UIKit
 3: class myClass: myParent, myProtocol {
 5:     var myString: String = ""
 6:     var myOtherString: String?
 7:     var yetAnotherVariable: Float!
 8:     let myAge: Int = 29
10:     @IBOutlet weak var userOutput: UILabel!
11:     @IBOutlet var anotherUserOutput: UILabel!
13:     class func myTypeMethod(aString: String) -> String {
14:         // Implement a Type method here
15:     }
17:     func myInstanceMethod(myString: String, myURL: NSURL) -> NSDate? {
18:         // Implement the Instance method here
19:     }
21:     override func myOverriddenInstanceMethod() {
22:         // Override the parent's method here
23:     }
25:     @IBAction func myActionMethod(sender: AnyObject) {
26:         // React to an action in the user interface
27:     }
29: }

The import Declaration

1: import UIKit

First, in line 1, the interface file uses the import declaration to include any other files that our application will need to access. The string UIKit designates a system framework that gives us access to a vast majority of the classes.

Whenever we need to import something, I explain how and why in the text. The UIKit example is included by default when Xcode sets up your classes and covers most of what you need for this book’s examples.

The class Declaration

The class declaration, shown in line 3, tells Xcode what class the file is going to be implementing. In this case, the file should contain the code to implement myClass:

3: class myClass: myParent, myProtocol {

Notice that line 3 includes a few additional items as well: that is, myParent, myProtocol. The class name (myClass) is always followed by a colon (:) and a list of the classes that this class is inheriting from (that is, the parent classes) and any protocols it will be implementing. In this example, myClass is going to by inheriting from myParent and will be implementing the myProtocol protocol.

The class line ends with an opening curly brace {. All blocks of code are contained in curly braces. The rest of the class code will follow this brace and eventually by terminated with a closing brace } (line 29).

Variable Properties Declarations

Lines 5–6 declare three different variable properties. A variable property is just a variable that can be accessed from any method in the class, or from code within other classes.

5:     var myString: String = ""
6:     var myOtherString: String?
7:     var yetAnotherVariable: Float!

In this example, a variable named myString that contains a String is declared and initialized with an empty string (""). A second String (myOtherString) is also declared, but designated as “optional” with the ? modifier. A third variable property, yetAnotherVariable, is declared as a floating-point number and set to be “implicitly unwrapped” by including the ! modifier. We’ll get to the point of these modifiers in a little bit. (They look confusing, but they have an important role to play.)

A Constant Declaration

Just below the variable properties is a constant declaration:

8:     let myAge: Int = 29

This creates a constant (myAge) and sets it to the integer value 29. Constants declared alongside variable properties (that is, outside of a method) are nearly identical to variable properties in how they are used—but with one important difference—they can’t be changed or reassigned. In other words, I’m 29 forever.

IBOutlet Declarations

Lines 9–10 are, yet again, variable property declarations, but they include the keyword IBOutlet at the start. This indicates that they are going to be connected to objects defined within an application’s user interface:

10:    @IBOutlet weak var userOutput: UILabel!
11:    @IBOutlet var anotherUserOutput: UILabel!

You learn more about IBOutlet in Hour 5, “Exploring Interface Builder.”

Declaring Methods

The final pieces of the class file are the method declarations. Lines 13, 17, 21, and 25 declare four methods that will be implemented in the class:

13:     class func myTypeMethod(aString: String) -> String {
14:         // Implement a Type method here
15:     }
17:     func myInstanceMethod(myString: String, myURL: NSURL) -> NSDate? {
18:         // Implement the Instance method here
19:     }
21:     override func myOverriddenInstanceMethod() {
22:         // Override the parent's method here
23:     }
25:     @IBAction func myActionMethod(sender: AnyObject) {
26:         // React to an action in the user interface
27:     }

Method declarations follow a simple structure. They begin with the word func, but can include the prefix modifiers class and override. A method that begins with class func is a Type method (often also referred to as a Class method). A method starting with override func is one that is redefining a method already provided in a parent class. This indicates that rather than inheriting functionality from a higher class, we’re going to write our own logic.

In the example file, line 13 defines a Type method named myTypeMethod that returns a String and accepts a String as a parameter. The input parameter is made available in a variable called aString.

Line 14 defines an instance method named myInstanceMethod that returns an optional NSDate object, taking a String and an NSURL as parameters. These are made available to the code in the method via the variables myString and myURL. I’ll rant about what optional values are and how to deal with them later in the hour. For the moment, just understand that by saying this method has an optional return type, it may return an NSDate object, or nothing (nil).

Line 21 declares an instance method, myOverriddenInstanceMethod, that takes no parameters and returns no results. What makes this interesting is that it uses the keyword override to indicate that it will be replacing a method provided by the parent class (myParent). When you start defining methods in your classes, Xcode knows what methods are inherited, so the moment you go to define a method provided by a parent class, it will automatically add the override keyword for you.

The fourth instance method, myActionMethod, declared in line 25 differs from the others because it defines an action, as indicated by the @IBAction keyword. Methods that begin with @IBAction are called when the user touches a button, presses a switch, or otherwise interacts with your application’s user interface (UI). These methods take a single parameter, usually denoted as sender that is set to whatever object the user interacted with. Just as with the @IBOutlet mentioned earlier, you’ll be learning much more about IBAction in Hour 5.

Ending the Class File

To end the class file, you just need a closing brace: }. You can see this on line 29 of the example file:

29: }

Although this might seem like quite a bit to digest, it covers almost everything you’ll see in a Swift class file.

Structure for Free

Even though we’ve just spent quite a bit of time going through the structure of a Swift class file, you’re rarely (if ever) going to need to type it all out by hand. Whenever you add a new class to your Xcode project, the structure of the file will be set up for you. What’s more, much of the work of declaring variable properties and methods can be done visually. Of course, you still need to know how to write code manually, but Xcode goes a long way toward making sure that you don’t have to sweat the details.

  • + 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