David Mark - Beginning iPhone development with Swift : exploring the iOS SDK

Appendix A

A Swift Introduction to Swift

Until recently, writing an iPhone or iPad application meant working with Objective-C. Because of its unusual (some would say elegant) syntax, Objective-C. With that understanding, lets get started right away!

Swift Basics

One of the best new features of Xcode 6 is the playground. As the name suggests, a playground is a place where you can go to play with code without having to create an environment in which to run itjust open a playground, type in some code, and see the results. Playgrounds are a great place to prototype something new and they are also an ideal place to start learning a new language, so were going to use them throughout this appendix.

Lets begin by creating a new playground. Start Xcode and go to FileNewPlayground. In the dialog that opens, choose a name for your playground (something like SwiftBasics) and make sure that the Platform is iOS, and then press Next. Choose the folder in which your playground will be saved and then press Create. Xcode creates the playground and opens it in a new window, as shown in . As you read through the examples in this appendix, feel free to experiment by adding code of your own to the playground or by modifying the examples to see what happens.

. Our newly created playground

Playgrounds, Comments, Variables, and Constants

Lets take a moment to look at what weve got in our playground. As you can see, its divided into two areascode on the left and results on the right. As you type code, the Swift compiler compiles and executes it, and shows you the result almost immediately. The code in declares a new variable called str and initializes it with the string "Hello, playground". You can see this string in the results column on the right. Try changing the value and notice that the result updates to match as soon as you stop typing.

The code on line 1 in is a comment. Anything following the character sequence // up to the end of the line is ignored by the compiler. Here, the comment occupies the whole line, but thats not necessary. We could add a comment to the end of a line of code too:

var str = "Hello, world" // K&R would be proud

To write a comment thats longer than one line, start it with /* and end it with */ , like this:

/*
This is a comment that occupies
More than one line.
*/

There are various different ways to write a multiline comment. Some people like to make it clear that a line is part of a comment by starting each line with a * character:

/*
* This is a comment that occupies
* More than one line.
*/

Other people like to write single-line comments like this:

/* This is another way to write a single-line comment. */

The import statement on line 3 makes Apples UIKit framework available for use in the playground:

import UIKit

IOS has many frameworks, some of which youll read about in this book. UIKit is the user interface framework, which well use in all of our code examples. Another framework that youll frequently make use of is Foundation, which contains classes that provide basic functionality like date and time handling, collections, file management, networking, and much more. To get access to this framework, you need to import it, like this:

import Foundation

However, UIKit automatically imports Foundation, so any playground that imports UIKit gets access to Foundation for free, without having to add an explicit import for it.

Line 5 is the first (and only) line of executable code in this playground:

var str = "Hello, playground"

The var keyword declares a new variable with a given name. Here, the variable is called str , which is appropriate because its a string. Swift is very liberal with variable names: you can use almost any character you like in the name, with the exception of the first character, which is somewhat restricted. You can find the precise rules in Apples documentation at https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language .

Following the declaration of the variable comes an expression that assigns its initial value. You dont have to initialize a variable when you declare it, but you must do so before you use it (i.e., before you execute any code that reads its value). However, if you do, then Swift can infer the variables type, saving you the trouble of stating it explicitly. In this example, Swift infers that str is a string variable, because you have initialized it with a string literal. If you choose not to provide an initializer (perhaps because there is no fixed initial value), you must declare the variables type by appending it to the name and separated from it by a colon, like this:

var str: String

Try changing the code on line 5 of the playground to this:

var str: String
str = "Hello, playground"

This is completely equivalent to the original code, but now youve had to explicitly state that str is a string variable ( String is the Swift type that represents a string). In most cases, its easier to combine the declaration and initialization, and allow the compiler to infer the variables type.

Heres another example that makes use of Swifts type inference feature:

var count = 2

Here, the compiler infers that the count variable is an integer. Its actual type is Int (well cover the numeric types.

. Getting the inferred type of a variable

The fact that you dont declare the type of a variable doesnt mean that it doesnt have one, or that you can play fast and loose with it. Swift assigns a type when the variable is declared and then you have to stick with that type. Unlike dynamic languages like JavaScript, you cant change the type of a variable simply by assigning a new value to it. Try doing this:

var count = 2
count = "Two"

Attempting to assign a string value to an integer variable is an error; youll see a red marker in the margin on the left of the playground. Click it, and Swift displays a message explaining the problem (see ).

. Swift is not a dynamic language. You cant change the type of a variable

This error message is not particularly clear. In fact, at the time of writing, Swift error messages can sometimes be a little hard to understand, but you can expect this to improve with later releases of Xcode.

Youve probably noticed that we arent bothering to add semicolons at the end of our statements. Thats one of the many nice little features of Swift: its almost never necessary to end a statement with a semicolon. If youre used to writing in C, C++, Objective-C or Java, that will probably feel a bit strange at first, but after a while, youll get used to it. Of course, you can type the semicolon if you want to, but most likely youll wind up not doing so. The only time you must use a semicolon is if you want to write two statements on the same line. This code is not valid:

var count = 2 count = 3

Add a semicolon, and the compiler is happy again:

var count = 2; count = 3

As the preceding line of code shows, you can change the value of a variable. Thats why its called a variable, after all. What if you just want to give a name to a fixed value? In other words, you want to create a constant and give it a name. For that, Swift provides the let statement. Syntactically, let is just like var , except that you must provide an initial value:

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