Mastering Swift 4 - Fourth Edition

3.9 (12 reviews total)
By Jon Hoffman
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  1. Taking the First Steps with Swift

About this book

Swift is the definitive language for Apple development today. It's a vital part of any iOS and macOS 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 4.0, the Swift team has 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 4.0 and quickly learn how to use the newest updates to your development advantage. From Objective-C interoperability and ARC to closures and concurrency, this advanced Swift guide will develop your expertise and help you become fluent in this vital programming language.

We'll give you an in-depth knowledge of some of the most sophisticated elements of Swift development, including protocol extensions, error-handling, design patterns, and concurrency. We'll guide you on how to use and apply them in your own projects. You'll see how to leverage the power of protocol-oriented programming to write flexible and easier-to-manage code.

Publication date:
September 2017


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 I became a professional programmer, programming remained more of a passion than a job, but in the years preceding the first release of Swift, that passion had waned. I was unsure why I was losing that passion. I attempted to recapture that passion with some of my side projects, but nothing really brought back the excitement that I used to have. Then Apple announced Swift in 2014. Swift is such an exciting and progressive language that it has brought a lot of that passion back and made programming fun again. Now that official versions of Swift are available for the Linux platform, and un-official versions for Windows and the ARM platform, learning and using Swift is becoming available to people outside the Apple ecosystem. This is really an exciting time to be learning the Swift language.

In this chapter, you will learn:

  • What is Swift?
  • What are some of the features of Swift?
  • What are Playgrounds?
  • How to use Playgrounds?
  • What the basic syntaxes of the Swift language are?

What is Swift?

Swift is a programming language that was introduced, by Apple, at the World Wide Developers Conference (WWDC) in 2014. 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 could 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 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 direct feedback that Apple received from the development community. It was also announced that Swift would become an open source project. In my opinion, this was the most exciting announcement of the WWDC 2015.

In December of 2015, Apple officially released Swift as open source 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. A list of which proposals were accepted and which were rejected can be found in the evolution repository.

Swift 3, which was released in 2016, was a major enhancement to the Swift language that was not source-compatible with previous releases of the Swift language. It contained fundamental changes to the language itself and to the Swift standard library. One of the main goals of Swift 3 was to be source-compatible across all platforms so the code that was written for one platform would be compatible with all other platforms. This means that the code we develop for macOS should work on Linux.

Now, Apple has released Swift 4. One of the primary goals of the Swift 4 compiler is to be source-compatible with Swift 3. This will allow us to compile both Swift 3 and Swift 4 projects with the Swift 4 compiler. Apple has established a community-owned source compatibility test suite that will be used to regression-test changes to the compiler. Projects that are added to the test suite will be periodically built against the latest development version of Swift to help understand the impact of the changes being made to Swift. You can find the Swift source compatibility page here:

One of the original goals of Swift 4 was to stabilize the Swift ABI (Application Binary Interface). The main benefit of a stable ABI is to allow us to distribute frameworks in a binary format across multiple versions of Swift. If a stable ABI were in place, we would be able to build a framework with the Swift 4 compiler and have it work with applications that were written in future versions of Swift. This feature ended up being deferred for now. Hopefully, Apple will be able to stabilize the ABI in future versions of Swift.

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 contribute to Swift. In July of 2013, it became a major focus of the Apple Developer Tools group.

Chris started working at Apple in the summer of 2005. He has held several positions in the Developer Tools group, and was the director and architect of that group when he left Apple in 2017. On his home page (, he notes that Xcode's Playground (read more on Playgrounds a little later in this chapter) became a personal passion of his because it makes programming more interactive and approachable. If you are using Swift on the Apple platform, you will be using Playgrounds a lot as a test and experimentation platform. You can also use Swift Playgrounds on the iPad.

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 for 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 the goto fail, in Apple's SSL library.

Swift features

When Apple first introduced Swift, it said that Swift is Objective-C without the C. This really only tells 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 has still maintained compatibility with the C language.

Since Swift does not need to maintain the same C compatibility as Objective-C, Apple was free to add any feature/enhancement 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 offers as compared to the Objective-C language:

Swift feature


Type inference

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


Generics allow us to write code only once to perform identical tasks for different types of object.

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. This is one of my favorite improvements.


Functions can have multiple return types using tuples.

Operator overloading

Classes can provide their own implementation of existing operators.

Enumerations with associated values

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

Protocols and Protocol-oriented Design

Apple introduced the Protocol-oriented Programming paradigm with Swift version 2. This is a new way of not only writing applications but also changes how we think about programming.


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 is also a feature that is specific to the Apple platform with Xcode. This feature is Mix and Match, which allows us to create applications that contain both Objective-C and Swift files. This allows us to systematically update our existing Objective-C applications with Swift classes and 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. 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 Playground as fun as the playgrounds of my youth. While Apple's Playgrounds might not be as fun as playing kickball when I was 9-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. Now that we can use Swift Playgrounds on the iPad, we do not even need to have a computer in front of us to experiment with Swift.


If you are using Swift on the Linux platform you will not have Playgrounds available, but you can use the REPL (Read-Evaluate-Print-Loop) shell to experiment with Swift without compiling your code. If you are using Swift on something other than a macOS computer or an iPad, you can safely skip this section.

Playgrounds also make it incredibly easy to try out new APIs, prototype new algorithms, and demonstrate how 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 proceed through the book, the code we use in the following examples will begin to make sense. We are simply trying to get a feel for Playgrounds right now.

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

  • 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 to 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. This screen lets us name our Playground and select whether the Playground is an iOS, tvOS, or macOS Playground. For most of the examples in this chapter, it is safe to assume that you can select any of the OS options unless it is otherwise noted. You can also select a template to use. For the examples in this book, we will be using the Blank template for all of our code:

Finally, we are asked for the location in which to save our Playground. After we select the location, the Playground will open 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 an 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 Cocoa framework since it is a macOS playground. If it was an iOS playground it would import the UIKit framework instead.

If your new Playground does not open the debug area, you can open it manually by pressing the shift + command + Y keys together. You can also close the debug area by pressing shift + command + Y again. Later in the chapter, we will see why the debug area is so useful. Another way to open or close the debug area is to click on the button that looks like an upside-down triangle in a box that is in the border between the debug area and the coding area.

iOS, tvOS, and macOS Playgrounds

When you start a new iOS or tvOS Playground, the Playground imports the UIKit framework. This gives us access to the UIKit framework that provides the core infrastructure for iOS and tvOS applications. When we start a new macOS Playground, the Playground imports the Cocoa framework.

What the last paragraph means is that, if we want to experiment with 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 a macOS 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; however; they can also do a lot more than just work with text. We can display other items such as images and graphs. Let's look at how we would show an image in a Playground. The first thing we need to do is to 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:
  1. 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 here:
  1. Now, we can access the image that is in our Resources folder within our code. The following screenshot shows how we would do this. At this time, the code used to access the image is not as important as knowing how to access resources within a Playground:
  1. 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:
  1. We can press either of the symbols to show the image. The one that looks like a box within a box 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 looks like if we display the image within the playground:

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 a variable throughout the course of the calculations. To see how graphing works, 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 should 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 better 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 project.
  • Playgrounds do not support on-device execution. You cannot run the code that is present in a Playground as an external application or on an external device.

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 look at some of the basic syntax of the Swift language.


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 the /* and */ for multiline comments; however, if we want to use the comments to also document our code, we need to use the triple slash ///.


Xcode will also auto-generate a comment template based on your signature of the method/function by highlighting it and pushing command + option + / together.

To document our code we generally use fields that Xcode recognizes. These fields are:

  • Parameter: When we start a line with - parameter {param name}: Xcode recognizes this as the description of a parameter
  • Return: When we start a line with - return: Xcode recognizes this as the description of the return value
  • Throws: When we start a line with - throws: Xcode recognizes this as the description of any 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 would recommend using single-line comments within a function to give quick one-line explanations of your code. We then use multiline comments outside functions and classes to explain what the function and class do. The preceding Playground shows a good way to use 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 pop-up with the description of the function.

This next screenshot shows what that pop-up would look like:

We can see that the documentation contains six fields. These fields are:

  • 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

There are significantly more fields that we can add to our comments. You can find the complete list on Apple's site here:



If you are developing for the Linux platform, I would still recommend using Apple's documentation guidelines because as other Swift IDEs are developed, I believe they will support the same guidelines.


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

For style purposes, it is strongly recommended that you do not use semicolons in your Swift code. If you are really set on using semicolons, then be consistent and use them on every line of code; however, there is no warning 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:

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 the individual conditional statements that are on the same line.

Curly brackets

In Swift, unlike most other languages, the curly bracket is required after conditional or loop statements. This is one of the safety features that are 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 the error you get if you forget to include 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) {} 

In Swift, this statement is not valid. Using an assignment operator (=) in a conditional statement (if, while, and guard) 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 the white spaces as the j block shows (for readability purposes) but, as long as you pick one style and are 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 first Hello World application will be a 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.

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 by using string interpolation or by separating the values within the print() function with commas. String interpolation uses a special sequence of characters, \( ), to include the value of variables and/or constants in the string. 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/constants 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 to: 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) 

Previously, 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.



We began this chapter with a discussion on the Swift language and gave a brief history about the language. We also mentioned some of the changes that will be present in Swift 4 and noted that ABI stabilization will be pushed out to a future version of Swift. We then showed 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.

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Latest Reviews

(12 reviews total)
It's well explained book about Swift.
Author does not comply with Apple styleguide. On pointing this out, the author (kind of) denies. I was very annoyed by this stubborn "I know better" approach. The book should adopt the Swift 4 style guidelines instead of sticking to old Swift 2 that are not preferred any more by apple. This made me feel like the book is full of other mistakes (which I have not proven yet).
It's good. It's good. It's good. It's good. It's good. It's good.