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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
 2:
 3: class myClass: myParent, myProtocol {
 4:
 5:     var myString: String = ""
 6:     var myOtherString: String?
 7:     var yetAnotherVariable: Float!
 8:     let myAge: Int = 29
 9:
10:     @IBOutlet weak var userOutput: UILabel!
11:     @IBOutlet var anotherUserOutput: UILabel!
12:
13:     class func myTypeMethod(aString: String) -> String {
14:         // Implement a Type method here
15:     }
16:
17:     func myInstanceMethod(myString: String, myURL: NSURL) -> NSDate? {
18:         // Implement the Instance method here
19:     }
20:
21:     override func myOverriddenInstanceMethod() {
22:         // Override the parent's method here
23:     }
24:
25:     @IBAction func myActionMethod(sender: AnyObject) {
26:         // React to an action in the user interface
27:     }
28:
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:     }
16:
17:     func myInstanceMethod(myString: String, myURL: NSURL) -> NSDate? {
18:         // Implement the Instance method here
19:     }
20:
21:     override func myOverriddenInstanceMethod() {
22:         // Override the parent's method here
23:     }
24:
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.

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