Swift - 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.

Defining a Base Class

Any class that doesn’t inherit from another class is known as a base class.

Note:
Swift classes don’t inherit from a universal base class. Classes you define without specifying a superclass automatically become base classes for you to build upon.

class Vehicle {
    var currentSpeed = 0.0
    var description: String {
        return "traveling at \(currentSpeed) miles per hour"
    }
    func makeNoise() {
        // do nothing - an arbitrary vehicle doesn't necessarily make a noise
    }
}

The Vehicle base class also defines a method called makeNoise. This method doesn’t actually do anything for a base Vehicle instance, but will be customised by subclasses of Vehicle later on:

Subclassing

Subclassing is the act of basing a new class on an existing class. The subclass inherits characteristics from the existing class, which you can then refine. You can also add new characteristics to the subclass.

class Bicycle: Vehicle {
    var hasBasket = false
}

Overriding

A subclass can provide its own custom implementation of an instance method, type method, instance property, type property, or subscript that it would otherwise inherit from a superclass. This is known as overriding.

To override a characteristic that would otherwise be inherited, you prefix your overriding definition with the override keyword.

class Train: Vehicle {
    override func makeNoise() {
        print("Choo Choo")
    }
}

Accessing Superclass Methods, Properties, and Subscripts

When you provide a method, property, or subscript override for a subclass, it’s sometimes useful to use the existing superclass implementation as part of your override. For example, you can refine the behavior of that existing implementation, or store a modified value in an existing inherited variable.

Where this is appropriate, you access the superclass version of a method, property, or subscript by using the super prefix:

• An overridden method named someMethod() can call the superclass version of someMethod() by calling super.someMethod() within the overriding method implementation.
• An overridden property called someProperty can access the superclass version of someProperty as super.someProperty within the overriding getter or setter implementation.
• An overridden subscript for someIndex can access the superclass version of the same subscript as super[someIndex] from within the overriding subscript implementation.

Overriding Properties

You can override an inherited instance or type property to provide your own custom getter and setter for that property, or to add property observers to enable the overriding property to observe when the underlying property value changes.

Overriding Property Getters and Setters

You can provide a custom getter (and setter, if appropriate) to override any inherited property, regardless of whether the inherited property is implemented as a stored or computed property at source.

Note:
If you provide a setter as part of a property override, you must also provide a getter for that override. If you don’t want to modify the inherited property’s value within the overriding getter, you can simply pass through the inherited value by returning super.someProperty from the getter, where someProperty is the name of the property you are overriding.

Overriding Property Observers

You can use property overriding to add property observers to an inherited property.

Note:
You can’t add property observers to inherited constant stored properties or inherited read-only computed properties. The value of these properties can’t be set, and so it isn’t appropriate to provide a willSet or didSet implementation as part of an override.

class AutomaticCar: Car {
    override var currentSpeed: Double {
        didSet {
            gear = Int(currentSpeed / 10.0) + 1
        }
    }
}

let automatic = AutomaticCar()
automatic.currentSpeed = 35.0
print("AutomaticCar: \(automatic.description)")
// AutomaticCar: traveling at 35.0 miles per hour in gear 4

Preventing Overrides

You can prevent a method, property, or subscript from being overridden by marking it as final. Do this by writing the final modifier before the method, property, or subscript’s introducer keyword (such as final var, final func, final class func, and final subscript).