Mastering Swift 3

4.8 (4 reviews total)
By Jon Hoffman
    Advance your knowledge in tech with a Packt subscription

  • Instant online access to over 7,500+ books and videos
  • Constantly updated with 100+ new titles each month
  • Breadth and depth in over 1,000+ technologies
  1. Taking the First Steps with Swift

About this book

Swift is the definitive language of Apple development today. It’s a vital part of any iOS and OS X developer’s skillset, helping them to build the most impressive and popular apps on the App Store—the sort of apps that are essential to iPhone and iPad users every day. With version 3.0, the Swift team have added new features to improve the development experience—making it easier to get the results you want and customers expect.

Inside, you’ll find the key features of Swift 3.0 and quickly learn how to use the newest updates to your development advantage. From Objective-C interoperability to ARC, to closures and concurrency, this advanced Swift guide will develop your expertise and make you more fluent in this vital programming language.

We give you in-depth knowledge of some of the most sophisticated elements of Swift development including protocol extensions, error-handling, design patterns, and concurrency, and guide you on how to use and apply them in your own projects. You'll see how even the most challenging design patterns and programming techniques can be used to write cleaner code and to build more performant iOS and OS X applications.

By the end of this book, you’ll have a handle on effective design patterns and techniques, which means you’ll soon be writing better iOS and OS X applications with a new level of sophistication and control.

Publication date:
October 2016


Chapter 1. Taking the First Steps with Swift

Ever since I was 12 years old and wrote my first program in the BASIC programming language, programming has been a passion for me. Even as programming became my career, it always remained more of a passion than a job, but over the past few years, that passion waned. I was unsure why I was losing that passion. I attempted to recapture it with some of my side projects, but nothing really brought back the excitement that I used to have. Then, something wonderful happened! Apple announced Swift. Swift is such an exciting and progressive language that it has brought a lot of that passion back and made programming fun for me again.

In this chapter, you will learn:

  • What Swift is?

  • What are some of the features of Swift?

  • What Playgrounds are?

  • How to use Playgrounds?

  • What the basic syntaxes of the Swift language are?


What is Swift?

Swift is Apple's new programming language, introduced at the Worldwide Developers Conference (WWDC) in 2014 alongside the integrated development environment Xcode 6 and iOS 8. Swift was arguably the most significant announcement at WWDC 2014, and very few people, including Apple insiders, were aware of the project's existence prior to it being announced.

It was amazing, even by Apple's standards, that they were able to keep Swift a secret for as long as they did and that no one suspected they were going to announce a new development language. At WWDC 2015, Apple made another big splash when they announced Xcode 7 and Swift 2. Swift 2 was a major enhancement to the Swift language. During that conference, Chris Lattner said that a lot of the enhancements were based on the direct feedback that Apple received from the developer community.

On December of 2015, Apple officially released Swift as an open source project, with the site dedicated to the open source Swift community. The Swift repository is located on Apple's GitHub page ( The Swift evolution repository ( tracks the evolution of Swift by documenting the proposed changes for Swift. You can also find a list of which proposals were accepted and which were rejected in the evolution repository. If you are interested in understanding where Swift is heading, you should check out this repository. It is interesting to note that Swift 3 contains several enhancements that were proposed by the community.

Swift 3 is a major enhancement to the Swift language that is NOT source compatible with previous releases of the Swift language. It contains fundamental changes to the language itself and to the Swift standard library. One of the main goals of Swift 3 is to be source compatible across all platforms, so the code that we write for the one platform will be compatible with all other platforms. This means that the code we develop for Mac OS will work on Linux, although certain frameworks, such as UIKit, may not be compatible across platforms.

The development of Swift was started in 2010 by Chris Lattner. He implemented much of the basic language structure, with only a few people being aware of its existence. It wasn't until late 2011 that other developers began to really contribute to Swift and in July of 2013, it became a major focus of the Apple Developer Tools group.

Chris Lattner started working at Apple in the summer of 2005. He has held several positions in the Developer Tools group and is currently the director and architect of that group. On his home page (, he notes that Xcode's Playgrounds (you can read more on Playgrounds a little later in this chapter) became a personal passion of his because it makes programming more interactive and approachable. We will be using Playgrounds a lot in this book as a test and experimentation platform. Starting with iOS 10, we will be able to use Swift Playgrounds on the iPad.


To me, being able to use Swift Playgrounds on the iPad is very exciting because it will make it easier for people getting started with programming to learn the Swift language. I am really looking forward to showing my daughters how to use Playgrounds on their iPads.

There are a lot of similarities between Swift and Objective-C. Swift adopts the readability of Objective-C's named parameters and dynamic object model. When we refer to Swift as having a dynamic object model, we are referring to the ability of types to change at runtime. This includes adding new (custom) types and changing/extending the existing types.

While there are a lot of similarities between Swift and Objective-C, there are significant differences between them as well. Swift's syntax and formatting are a lot closer to Python than Objective-C, but Apple did keep the curly braces. I know Python people would disagree with me, and that is all right because we all have different opinions, but I like the curly braces. Swift actually makes the curly braces required for control statements, such as if and while, which eliminates bugs, such as goto fail in Apple's SSL library.

Swift was also built to be fast. At WWDC 2014, Apple showed a number of benchmarks, which proved that Swift significantly outperformed Objective-C. Swift uses the LLVM compiler, which is included with Xcode 7 to transform Swift code into highly optimized native code that is tuned to get the most out of Apple's modern hardware.

Swift features

When Apple said that Swift is Objective-C without the C, they were really only telling us half of the story. Objective-C is a superset of C and provides object-oriented capabilities and a dynamic runtime to the C language. This meant that with Objective-C, Apple needed to maintain compatibility with C, which limited the enhancements it could make to the Objective-C language. As an example, Apple could not change how the switch statement functioned and still maintain the C compatibility.

Since Swift does not need to maintain the same C compatibility as Objective-C, Apple was free to add any features/enhancements to the language. This allowed Apple to include the best features from many of today's most popular and modern languages, such as Objective-C, Python, Java, Ruby, C#, Haskell, and many others.

The following chart shows a list of some of the most exciting enhancements that Swift includes:

Swift feature


Type inference

Swift can automatically deduce the type of the variable or constant based on the initial value.


Generics allow us to write code once to perform identical tasks for different types of objects while still maintaining type safety.

Collection mutability

Swift does not have separate objects for mutable or non-mutable containers. Instead, you define mutability by defining the container as a constant or variable.

Closure syntax

Closures are self-contained blocks of functionality that can be passed around and used in our code.


Optionals define a variable that might not have a value.

Switch statement

The switch statement has been drastically improved with features such as pattern matching, guard conditions, and no automatic fall-through. This is one of my favorite improvements.

Multiple return types

Functions can have multiple return types using tuples.

Operator overloading

Classes can provide their own implementation of the existing operators.

Enumerations with Associated values

In Swift, we can do a lot more than just defining a group of related values with enumerations.

There is one feature that I did not mention in the preceding chart because it is technically not a feature of Swift; it is a feature of Xcode and the compiler. This feature is Mix and match. Mix and match allow us to create applications that contain both Objective-C and Swift files. This allows us to systematically update our current Objective-C applications with Swift classes and also use Objective-C libraries/frameworks in our Swift applications.

Before we begin our journey into the wonderful world of Swift development, let's take a detour and visit a place that I have loved ever since I was a kid—the playground.



When I was a kid, the best part of the school day was going to the playground. It really did not matter what we were playing; as long as we were on the playground, I knew it would be fun. When Apple introduced Playgrounds as part of Xcode 6, I was excited just by the name, but I wondered if Apple would be able to make its Playgrounds as fun as the playgrounds of my youth. While Apple's Playgrounds might not be as fun as playing kickball when I was nine years old, it definitely brings a lot of fun back to experimenting and playing with code.

Getting started with Playgrounds

Playgrounds are interactive work environments that let us write code and see the results immediately as changes are made to the code. This means that Playgrounds are a great way to learn and experiment with Swift.

Playgrounds also make it incredibly easy to try out the new APIs, prototype new algorithms, and demonstrate how the code works. We will be using Playgrounds throughout this book to show how our sample code works. Therefore, before we really get into Swift development, let's spend some time learning about and getting comfortable with Playgrounds.

Do not worry if the Swift code does not make a lot of sense right now; as we go through the book, this code will begin to make sense. We are simply trying to get a feel of Playgrounds right now.

A Playground can have several sections, but the three that we will be using extensively in this book are as follows:

  • Coding Area: This is where you enter your Swift code.

  • Results Sidebar: This is where the results of your code are shown. Each time you type in a new line of code, the results are re-evaluated and the results' sidebar is updated with the new results.

  • Debug Area: This area displays the output of the code, and it can be very useful for debugging.

The following screenshot shows how the sections are arranged in a Playground:

Let's start a new Playground. The first thing we need to do is start Xcode. Once Xcode has started, we can select the Get started with a playground option, as shown in the following screenshot:

Alternatively, we can navigate to the Playground by going to File | New from the top menu bar, as shown in the following screenshot:

Next, we should see a screen similar to the following screenshot; it lets us name our Playground and select whether the Playground is an iOS or OS X Playground. For most of the examples in this chapter, it is safe to assume that you can select either iOS or OS X unless otherwise noted:

Finally, we are asked for the location to save our Playground. After we select the location, the Playground will open up and look similar to the following screenshot:

In the preceding screenshot, we can see that the coding area of the Playground looks similar to the coding area for a Xcode project. What is different here is the sidebar on the right-hand side. This sidebar is where the results of our code are shown. The code in the previous screenshot imports the iOS UIKit framework and sets a variable named str to the Hello, playground string. You can see the content of the str string in the sidebar to the right of the code. We also create a for loop that prints the numbers 0 to 5 to the console.

By default, a new Playground does not open the debug area. You can open it manually by pressing the shift + command + Y keys together. Later in the chapter, we will see why the debug area is so useful.

iOS and OS X Playgrounds

When you start a new iOS Playground, the Playground imports UIKit (Cocoa Touch). This gives us access to the UIKit framework, which provides the core infrastructure for iOS applications. When we start a new OS X Playground, the Playground imports Cocoa. This gives us access to the OS X Cocoa framework.

What the last paragraph means is if we want to experiment with the specific features of either UIKit or Cocoa, we will need to open the correct Playground. As an example, if we have an iOS Playground open and we want to create an object that represents a color, we would use a UIColor object. If we had an OS X playground open, we would use an NSColor object to represent a color.

Showing images in a Playground

Playgrounds are great at showing the results of code as text in the results sidebar, but they can also do a lot more than that. We can display other items, such as images and graphs. Let's take a look at how we would show an image in a Playground. The first thing we need to do is load the image into the resource directory of our Playground.

The following steps show how to load an image into the resource directory:

  1. Let's begin by showing the Project Navigator sidebar. To do this, in the top menu bar, navigate to View | Navigators | Show Project Navigator or use the command + 1 keyboard shortcut. The Project Navigator looks similar to this:

  2. Once we have the Project Navigator open, we can drag the image into the Resources folder so that we can access it from our code. Once we drag the image file over it and drop it, it will appear in the Resources folder, as shown in the following screenshot:

  3. Now we can access the image that is in our Resources folder within our code. The following screenshot shows how we can do this. The actual code that we use to access the image is not as important at this time as knowing how to access resources within a playground:

  4. To view the image, we need to hover our cursor in the results sidebar over the section that shows the width and height of the image. In our example, the width and height section shows w 256 h 256. Once we hover the mouse pointer over the width and height, we should see two symbols, as shown in the following screenshot:

  5. We can press either of the symbols to show the image. The one that is shaped like a circle with a plus sign in it will display the image within the playground's code section, while the one that looks like an eye will pop the image up outside the playground. The following screenshot shows what it shows if we press the circle with a plus sign in it:

Having the ability to create and display graphs can be very useful when we want to see the progression of our code. Let's look at how we can create and display graphs in a Playground.

Creating and displaying graphs in Playgrounds

Creating and displaying graphs is really useful when we are prototyping new algorithms because it allows us to see the value of the variable throughout the course of the calculations. To see how graphing works, take a look at the following Playground:

In this playground, we set the variable j to 1. Next, we create a for loop that assigns numbers 1 through 5 to the variable i. At each step in the for loop, we set the value of the variable j to the current value of j multiplied by i. The graph shows the values of the variable j at each step of the for loop. We will be covering for loops in detail later in this book.

To bring up the graph, click on the symbol that is shaped like a circle with a dot in it. We can then move the timeline slider to see the values of variable j at each step of the for loop. The following playground shows what the graph will look like:

What Playgrounds are not

There is a lot more that we can do with Playgrounds, and we have only scratched the surface in our quick introduction here. Before we leave this brief introduction, let's take a look at what Playgrounds are not so that we can understand when not to use Playgrounds:

  • Playgrounds should not be used for performance testing: The performance you see from any code that is run in a Playground is not representative of how fast the code will run when it is in your projects

  • Playgrounds do not support user interaction: Users cannot interact with code that is run in a Playground

  • Playgrounds do not support on-device execution: You cannot run code on an external device from a Playground

Swift language syntax

If you are an Objective-C developer and you are not familiar with modern languages such as Python or Ruby, the code in the previous screenshots may have looked pretty strange. The Swift language syntax is a huge departure from Objective-C, which was based largely on Smalltalk and C.

The Swift language uses modern concepts and syntax to create very concise and readable code. There is also a heavy emphasis on eliminating common programming mistakes. Before we get into the Swift language itself, let's take a look at some of the basic syntax of it.


Writing comments in Swift code is a little different from writing comments in Objective-C code. We can still use the double slash // for single-line comments and /* and */ for multiline comments. However, if we want to use the comments to also document our code, we need to use the triple slash, ///. To document our code, we use fields that Xcode recognizes. These fields are as follows:

  • Parameter: When we start a line with - parameter {param name}:, Xcode recognizes this as the description for a parameter

  • Returns: When we start a line with - returns:, Xcode recognizes this as the description for the return value

  • Throws: When we start a line with - throws:, Xcode recognizes this as the description for the errors that this method may throw

The following Playground shows examples of both single-line and multiline comments and how to use the comment fields:

To write good comments, I recommend using single-line comments within a function to give quick one-line explanations of your code. We will then use multiline comments outside of functions and classes to explain what the function and class does. The preceding Playground shows a good use of comments. By using proper documentation, as we did in the preceding screenshot, we can use the documentation feature within Xcode. If we hold down the option key and then click on the function name anywhere in our code, Xcode will display a popup with the description of the function.

This next screenshot shows what that popup  may look like:

This screenshot shows the documentation feature of Xcode if we hold down the option key and then click on the myAdd() method. We can see that the documentation contains six fields. These fields are as follows:

  • Declaration: This is the function's declaration

  • Description: This is the description of the function as it appears in the comments

  • Parameters: The parameter descriptions are prefixed with the - Parameters: tag in the comment section

  • Throws: The throws description is prefixed with the - throws: tag and describes what errors are thrown by the methods

  • Returns: The return description is prefixed with the - returns: tag in the comment section

  • Declared In: This is the file that the function is declared in so that we can easily find it


You may have probably noticed from the code samples so far that we are not using semicolons at the end of the lines. The semicolons are optional in Swift; therefore, both lines in the following Playground are valid in Swift. You can see the results of the code in the results sidebar, as shown in the following screenshot:

The only time semicolons are required is when we put consecutive statements on one line. For example, if we had a line such as this:

print("Hello from Swift"); return 42 

Then the semicolon between the print and return statements would be required. It is recommended that you do not put multiple statements on the same line, but if you want too, then remember the semicolon is required.

For style purposes, it is strongly recommended that you do not use semicolons in your Swift code. If you are really intent on using semicolons in your code, then be consistent and use them on every line of code; however, Swift will not warn you if you forget them. I will stress again that it is recommended that you do not use semicolons in Swift.


In Swift, parentheses around conditional statements are optional; for example, both if statements in the following Playground are valid. You can see the results of the code in the sidebar.

For style purposes, it is recommended that you do not include parentheses in your code unless you have multiple conditional statements on the same line. For readability purposes, it is good practice to put parentheses around individual conditional statements that are on the same line.

See the following Playground for samples:

Curly brackets for control statements

In Swift, unlike most other languages, the curly bracket is required after conditional or loop statements. This is one of the safety features that is built into Swift. Arguably, there have been numerous security bugs that may have been prevented if the developer would have used curly braces. These bugs could also have been prevented by other means such as unit testing and code reviews, but requiring developers to use curly braces, in my opinion, is a good security standard.

The following Playground shows you what error you get if you forget to include the curly braces:

An assignment operator does not return a value

In most other languages, the following line of code is valid, but it probably is not what the developer meant to do:

if (x = 1) {} 


Downloading the example code

You can download the example code files from your account at for all the Packt Publishing books you have purchased. If you purchased this book elsewhere, you can visit and register to have the files e-mailed directly to you.

In Swift, this statement is not valid. Using an assignment operator (=) in a conditional statement (if and while) will throw an error. This is another safety feature built into Swift. It prevents the developer from forgetting the second equals sign (=) in a comparison statement. This error is shown in the following Playground:

Spaces are optional in conditional and assignment statements

For both conditional (if and while) and assignment (=) statements, the white spaces are optional. Therefore, in the following Playground, both The i block and The j block of code are valid:


For style purposes, I would recommend adding white spaces (such as The j block for readability purposes), but as long as you pick one style and be consistent, either style should be acceptable.


Hello World

All good computer books that are written to teach a computer language have a section that shows a user how to write a Hello World application. This book is no exception. In this section, we will show you how to write two different Hello World applications.

Our fist Hello World application will be the traditional Hello World application that simply prints Hello World to the console. Let's begin by creating a new Playground and naming it Chapter_1_Hello_World. The Playground can be either an iOS or an OS X Playground.

In Swift, to print a message to the console, we use the print() function. In its most basic form, we would use the print function to print out a single message, as shown in the following code:

print("Hello World") 

Usually, when we use the print() function, we want to print more than just static text. We can include the value of variables and/or constants using a special sequence of characters, \( ), or by separating the values within the print() function with commas. The following code shows how to do this:

var name = "Jon" 
var language = "Swift" 
var message1 = " Welcome to the wonderful world of " 
var message2 = "\(name) Welcome to the wonderful world of \(language)!" 
print(name, message1, language, "!") 

We can also define two parameters in the print() function that change how the message is displayed in the console. These parameters are the separator and terminator parameters. The separator parameter defines a string that is used to separate the values of the variables/constant in the print() function. By default, the print() function separates each variable/constant with a space. The terminator parameter defines what character is put at the end of the line. By default, the newline character is added at the end of the line.

The following code shows how we would create a comma-separated list that does not have a newline character at the end:

var name1 = "Jon" 
var name2 = "Kim" 
var name3 = "Kailey" 
var name4 = "Kara" 
print(name1, name2, name3, name4, separator:", ", terminator:"") 

There is one other parameter that we can add to our print() function. This is the toStream parameter. This parameter will let us redirect the output of the print() function. In the following example, we redirect the output to a variable named line:

var name1 = "Jon" 
var name2 = "Kim" 
var name3 = "Kailey" 
var name4 = "Kara" 
var line = "" 
print(name1, name2, name3, name4, separator:", ", terminator:"", to:&line) 

The print() function was simply a useful tool for basic debugging, but now with the new enhanced print() function, we can use it for a lot more.



In this chapter, we showed you how to start and use Playgrounds to experiment with Swift programming. We also covered the basic Swift language syntax and discussed proper language styles. The chapter concluded with two Hello World examples.

In the next chapter, we will see how to use variables and constants in Swift. We will also look at the various data types and how to use operators in Swift.

About the Author

  • Jon Hoffman

    Jon Hoffman has over 20 years' experience in the field of Information Technology. Over those 20 years, Jon has worked in the system administration, network administration, network security, application development, and architecture arenas. Currently, he works as an Enterprise Software Manager at Syntech Systems. He has developed extensively for the iOS platform since 2008. This includes several apps that he has published in the App Store, apps that he has written for third parties, and numerous enterprise applications. Some of Jon's other interests are playing basketball, kayaking, and working out with his daughters. Jon also really enjoys Tae Kwon Do, where he and his oldest daughter earned their black belts together early in 2014 and are currently 3rd-degree Black Belts.

    Browse publications by this author

Latest Reviews

(4 reviews total)
Excellent source of information and exceptional prices. Great seller!!!
Excellent coverage of Swift 3. Wish I had more time before Swift 4 lands.
Mastering Swift 3
Unlock this book and the full library for FREE
Start free trial