Swift Classes – Part 2

Inheritance

A class can inherit methods, properties, and other characteristics from another class. When one class inherits from another, the inheriting class is known as a subclass, and the class it inherits from is known as its superclass. Inheritance is a fundamental behavior that differentiates classes from other types in Swift.

A simple example of inheritance:

class AClass {
    func doSomething() {
        println("Hello from AClass")
    }
}

class Subclass: AClass  {
}

let base_object = AClass()
base_object.doSomething()
//> Hello from AClass


let enhanced_object = Subclass()
enhanced_object.doSomething()
// > Hello from AClass

Overriding

You can override methods in order to provide custom behaviour. To override a method write the override keyword before the method declaration:

class AClass {
    func doSomething() {
        println("Hello from AClass")
    }
}

class Subclass: AClass  {
    override func doSomething() {
        println("Hello from Subclass")
    }
}

let base_object = AClass()
base_object.doSomething()
//> Hello from AClass


let enhanced_object = Subclass()
enhanced_object.doSomething()
//> Hello from Subclass

You can use the super keyword to call any method from the superclass.

...

class Subclass: AClass  {
    override func doSomething() {
        super.doSomething()

        println("Hello from Subclass")
    }
}

let enhanced_object = Subclass()
enhanced_object.doSomething()
//> Hello from AClass
//> Hello from Subclass

Base class

A class that does not inherit from another class is called a base class, for example:

class User {
    var name: String!
    var age: Int!

    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }
}

The iOS and Mac OS classes ussualy inherit from NSObject, either directly on indirectly. If you have a mixt codebase I would encourage you to subclass NSObject when creating a new class:

Swift classes that are subclasses of NSObject:

• are Objective-C classes themselves
• use objc_msgSend() for calls to (most of) their methods
• provide Objective-C runtime metadata for (most of) their method implementations

Swift classes that are not subclasses of NSObject:

• are Objective-C classes, but implement only a handful of methods for NSObject compatibility
• do not use objc_msgSend() for calls to their methods (by default)
• do not provide Objective-C runtime metadata for their method implementations (by default)

Subclassing NSObject in Swift gets you Objective-C runtime flexibility but also Objective-C performance. Avoiding NSObject can improve performance if you don’t need Objective-C’s flexibility.

from stackoverflow Swift native base class or NSObject

Protocols

Protocols are declared in a similar way to classes.

protocol MyFirstProtocol {
    // I do nothing

Protocols describe methods, properties and other requirements that are needed for a specific task. For example theUITableViewDelegate protocol lists all the methods that can be used to react to user events and configure a table view.

Note: you can mark a method as optional using the @optional keyword. All of the methods from UITableViewDelegate are optional. When you do not use the @optional keyword that method is required. The swift compiler will throw an error if a class conforms to a protocol and does not implement the required methods.

A class can conform to a protocol by placing its name after the type’s name separated by a colon, as part of their definition. Multiple protocols can be listed, and are separated by commas:

class AnotherSwiftClass: MyFirstProtocol, AnotherProtocol {
    ...
}

If the class inherits from another one, make sure to put the superclass name before the protocol list.

class AnotherSwiftClass: AClass, MyFirstProtocol, AnotherProtocol {
    ...
}

Note: Protocols use the same syntax as normal methods, but are not allowed to specify default values for method parameters.

One of the most overused design pattern in iOS is delegation. A class can delegate a part of it’s responsibilities to an instance of another class. This design pattern is implemented by defining a protocol that encapsulates the delegated responsibilities, such that a conforming type (known as a delegate) is guaranteed to provide the functionality that has been delegated.

Here is an example, the Player class delegates the shooting logic to the weapon:

protocol Targetable {
    var life: Int { get set }
    func takeDamage(damage: Int)
}

protocol Shootable {
    func shoot(target: Targetable)
}

class Pistol: Shootable {
    func shoot(target: Targetable) {
        target.takeDamage(1)
    }
}

class Shotgun: Shootable {
    func shoot(target: Targetable) {
        target.takeDamage(5)
    }
}

class Enemy: Targetable {
    var life: Int = 10

    func takeDamage(damage: Int) {
        life -= damage
        println("enemy lost \(damage) hit points")

        if life <= 0 {
            println("enemy is dead now")
        }
    }
}

class Player {
    var weapon: Shootable!

    init(weapon: Shootable) {
        self.weapon = weapon
    }

    func shoot(target: Targetable) {
        weapon.shoot(target)
    }
}

var terminator = Player(weapon: Pistol())

var enemy = Enemy()

terminator.shoot(enemy)
//> enemy lost 1 hit points
terminator.shoot(enemy)
//> enemy lost 1 hit points
terminator.shoot(enemy)
//> enemy lost 1 hit points
terminator.shoot(enemy)
//> enemy lost 1 hit points
terminator.shoot(enemy)
//> enemy lost 1 hit points

// changing weapon because the pistol is inefficient
terminator.weapon = Shotgun()

terminator.shoot(enemy)
//> enemy lost 5 hit points
//> enemy is dead now

Challenges

• create a Shape base class and derive from it Circle, Square and Rectangle. Shape should have two methods area() -> Float and perimeter() -> Float that both return 0. Implement the methods and add the necessary properties on Circle,Square and Rectangle.
• create the same classes from the first exercise, but this time make Shape a protocol
• add a resistance property to the Targetable protocol and reduce the damage taken by the enemy. Obs: you will need to change the life property type to Float in order to do this.

In case you missed it, check out Part 1 of this tutorial. And this is a complete guide to Object Oriented Programming in Swift.

If you found this useful please take a moment to share this with your friends