The Swift Programming Language--Inheritance

Inheritance

• Defining a Base Class

Swift classes do not inherit from a universal base class.

  class Vehicle {
      //stored property
      var currentSpeed = 0.0
      
      //read-only computed property
      var description: String{
            return "traveling a \(currentSpeed) miles per hour." 
      }

      func makeNoise(){
          //do nothing
      }
  }

• Subclassing
  class Bycicle: Vehicle {
  var hasBasket = false
  }

  class Tandem: Bicycle {
    var numberOfPassengers = 0
  }
  
  let tandem = Tandem()
  tandem.hasBasket = true
  tandem.currentSpeed = 15
  tandem.numberOfPassengers = 2
  print("\(tandem.description)")
  

• Overriding

The override keyword also prompt the Swift compiler to check that your overriding class's superclass has a declaration that matches the one you provided.

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

You can present an inherited read-only property as a read-write property by providng both a getter and setter in your subclass property override. You cannnot, however, present an inherited read-write property as a read-only property.

If you provide a setter as part of a property override, you must also provide a getter for that override.

 class Car: Vehicle {
    var gear = 1;

    override var description: String {

        return super.description + "in gear \(gear)."
    }
 }

You cannot add property observers to inherited constant stored properties or inherited read-only computed properties. The value of these properties cannot be set.

 class Car: Vehicle {
    var gear = 1

    override var description: String {
        willSet {
            //Wrong, read-only computed property
        }
    }
 }

• Preventing Overrides

  final class
  final var
  final func
  final class func
  final subscript