10. The Basics of Swift Object-Oriented Programming

Swift provides extensive support for developing object-oriented applications. The subject area of object-oriented programming is, however, large. It is not an exaggeration to state that entire books have been dedicated to the subject. As such, a detailed overview of object-oriented software development is beyond the scope of this book. Instead, we will introduce the basic concepts involved in object-oriented programming and then move on to explaining the concept as it relates to Swift application development. Once again, while we strive to provide the basic information you need in this chapter, we recommend reading a copy of Apple’s The Swift Programming Language book for more extensive coverage of this subject area.

10.1 What is an Instance?

Objects (also referred to as class instances) are self-contained modules of functionality that can be easily used and re-used as the building blocks for a software application.

Instances consist of data variables (called properties) and functions (called methods) that can be accessed and called on the instance to perform tasks and are collectively referred to as class members.

10.2 What is a Class?

Much as a blueprint or architect’s drawing defines what an item or a building will look like once it has been constructed, a class defines what an instance will look like when it is created. It defines, for example, what the methods will do and what the properties will be.

10.3 Declaring a Swift Class

Before an instance can be created, we first need to define the class ‘blueprint’ for the instance. In this chapter we will create a bank account class to demonstrate the basic concepts of Swift object-oriented programming.

In declaring a new Swift class we specify an optional parent class from which the new class is derived and also define the properties and methods that the class will contain. The basic syntax for a new class is as follows:

class NewClassName: ParentClass {

   // Properties

   // Instance Methods

   // Type methods

}

The Properties section of the declaration defines the variables and constants that are to be contained within the class. These are declared in the same way that any other variable or constant would be declared in Swift.

The Instance methods and Type methods sections define the methods that are available to be called...

10.4 Adding Instance Properties to a Class

A key goal of object-oriented programming is a concept referred to as data encapsulation. The idea behind data encapsulation is that data should be stored within classes and accessed only through methods defined in that class. Data encapsulated in a class are referred to as properties or instance variables.

Instances of our BankAccount class will be required to store some data, specifically a bank account number and the balance currently held within the account. Properties are declared in the same way any other variables and constants are declared in Swift. We can, therefore, add these variables as follows:

class BankAccount {

    var accountBalance: Float = 0

    var accountNumber: Int = 0

}

Having defined our properties, we can now move on to defining the methods of the class that will allow us to work with our properties while staying true to the data encapsulation model...

10.5 Defining Methods

The methods of a class are essentially code routines that can be called upon to perform specific tasks within the context of that class.

Methods come in two different forms, type methods and instance methods. Type methods operate at the level of the class, such as creating a new instance of a class. Instance methods, on the other hand, operate only on the instances of a class (for example performing an arithmetic operation on two property variables and returning the result).

Instance methods are declared within the opening and closing braces of the class to which they belong and are declared using the standard Swift function declaration syntax.

Type methods are declared in the same way as instance methods with the exception that the declaration is preceded by the class keyword.

For example, the declaration of a method to display the account balance in our example might read as follows:

class BankAccount {

 

  ...

10.6 Declaring and Initializing a Class Instance

So far all we have done is define the blueprint for our class. In order to do anything with this class, we need to create instances of it. The first step in this process is to declare a variable to store a reference to the instance when it is created. We do this as follows:

var account1: BankAccount = BankAccount()

When executed, an instance of our BankAccount class will have been created and will be accessible via the account1 variable.

10.7 Initializing and De-initializing a Class Instance

A class will often need to perform some initialization tasks at the point of creation. These tasks can be implemented by placing an init method within the class. In the case of the BankAccount class, it would be useful to be able to initialize the account number and balance properties with values when a new class instance is created. To achieve this, the init method could be written in the class as follows:

class BankAccount {

 

    var accountBalance: Float = 0

    var accountNumber: Int = 0

 

    init(number: Int, balance: Float)

    {

        accountNumber = number

        accountBalance = balance

    }

 

    func displayBalance()

    {

   ...

10.8 Calling Methods and Accessing Properties

Now is probably a good time to recap what we have done so far in this chapter. We have now created a new Swift class named BankAccount. Within this new class we declared some properties to contain the bank account number and current balance together with an initializer and a method to display the current balance information. In the preceding section we covered the steps necessary to create and initialize an instance of our new class. The next step is to learn how to call the instance methods and access the properties we built into our class. This is most easily achieved using dot notation.

Dot notation involves accessing an instance variable, or calling an instance method by specifying a class instance followed by a dot followed in turn by the name of the property or method:

classInstance.propertyName

classInstance.instanceMethod()

For example, to get the current value of our accountBalance instance variable:

var...

10.9 Stored and Computed Properties

Class properties in Swift fall into two categories referred to as stored properties and computed properties. Stored properties are those values that are contained within a constant or variable. Both the account name and number properties in the BankAccount example are stored properties.

A computed property, on the other hand, is a value that is derived based on some form of calculation or logic at the point at which the property is set or retrieved. Computed properties are implemented by creating getter and optional corresponding setter methods containing the code to perform the computation. Consider, for example, that the BankAccount class might need an additional property to contain the current balance less any recent banking fees. Rather than use a stored property, it makes more sense to use a computed property which calculates this value on request. The modified BankAccount class might now read as follows:

class BankAccount {

 

...

10.10 Lazy Stored Properties

There are several different ways in which a property can be initialized, the most basic being direct assignment as follows:

var myProperty = 10

Alternatively, a property may be assigned a value within the initializer:

class MyClass {

  let title: String

 

  init(title: String) {

    self.title = title

  }

}

For more complex requirements, a property may be initialized using a closure:

class MyClass {

    

    var myProperty: String = {

        var result = resourceIntensiveTask()

        result = processData(data: result)

        return result

    }()

.

.

}

Particularly in the case of a complex closure, there is the potential for the initialization to be resource...

10.11 Using self in Swift

Programmers familiar with other object-oriented programming languages may be in the habit of prefixing references to properties and methods with self to indicate that the method or property belongs to the current class instance. The Swift programming language also provides the self property type for this purpose and it is, therefore, perfectly valid to write code which reads as follows:

class MyClass {

    var myNumber = 1

 

    func addTen() {

        self.myNumber += 10

    }

}

In this context, the self prefix indicates to the compiler that the code is referring to a property named myNumber which belongs to the MyClass class instance. When programming in Swift, however, it is no longer necessary to use self in most situations since this is now assumed to be the default for references to properties and methods. To quote The Swift...

10.12 Understanding Swift Protocols

By default, there are no specific rules to which a Swift class must conform as long as the class is syntactically correct. In some situations, however, a class will need to meet certain criteria in order to work with other classes. This is particularly common when writing classes that need to work with the various frameworks that comprise the iOS SDK. A set of rules that define the minimum requirements which a class must meet is referred to as a Protocol. A protocol is declared using the protocol keyword and simply defines the methods and properties that a class must contain in order to be in conformance. When a class adopts a protocol, but does not meet all of the protocol requirements, errors will be reported stating that the class fails to conform to the protocol.

Consider the following protocol declaration. Any classes that adopt this protocol must include both a readable String value called name and a method named buildMessage() which accepts...

10.13 Opaque Return Types

Now that protocols have been explained it is a good time to introduce the concept of opaque return types. As we have seen in previous chapters, if a function returns a result, the type of that result must be included in the function declaration. The following function, for example, is configured to return an Int result:

func doubleFunc1 (value: Int) -> Int {

    return value * 2

}

Instead of specifying a specific return type (also referred to as a concrete type), opaque return types allow a function to return any type as long as it conforms to a specified protocol. Opaque return types are declared by preceding the protocol name with the some keyword. The following changes to the doubleFunc1() function, for example, declare that a result will be returned of any type that conforms to the Equitable protocol:

func doubleFunc1(value: Int) -> some Equatable {

    value * 2

}

To conform to the...

10.14 Summary

Object-oriented programming languages such as Swift encourage the creation of classes to promote code reuse and the encapsulation of data within class instances. This chapter has covered the basic concepts of classes and instances within Swift together with an overview of stored and computed properties and both instance and type methods. The chapter also introduced the concept of protocols which serve as templates to which classes must conform and explained how they form the basis of opaque return types.