Downcasting, “as!”. Someone please, pretend I’m 5 years old and explain downcasting, and why should we use it ?

class Animal { //Animal gives each one a name
    var name: String
    init(name: String) {
        self.name = name;
    }
}

class Bird : Animal { //Bird has properties that Animal and Dog do not but also has property of Animal
    var wings = 2
    var legs = 2
    var featherColor: String
    var flightSpeedMPH: Int
    init(name: String, flightSpeedMPH: Int, featherColor: String) {
        self.featherColor = featherColor
        self.flightSpeedMPH = flightSpeedMPH
        super.init(name: name)
    }
}

class Dog : Animal { //Dog has properties that Animal and Bird do not but also has property of Animal
    var legs = 4
    var furColor:String
    init(name: String, furColor: String) {
        self.furColor = furColor
        super.init(name: name)
    }
}

var animal: [Animal] = [ //Array of our animals
    Dog(name: "Fido", furColor: "Black"), //Instance of Dog
    Bird(name: "Tweety", flightSpeedMPH: 30, featherColor: "Yellow")] //Instance of Bird

if let myDog = animal[0] as? Dog { //Downcasted Animal class to its subclass Dog
    println("My dog has \(myDog.furColor) fur") //Can now access furColor
}

if let myBird = animal[1] as? Bird { //Downcasted Animal class to its subclass Bird
    println("My bird can fly at \(myBird.flightSpeedMPH) MPH") //Can now access flightSpeed
}
var dog = animal[0] as! Dog //Forced unwrapped only use when sure it will work
dog.furColor //No need to unwrap

var dog1 = animal[0] as? Dog //Optional unwrapped in case of nil however you must chain optionals later
dog1?.furColor //This is the preffered method if it receives nil it will fail gracefully
dog1!.furColor //This work however if furColor were to be nil it would give a runtime error

var dog2 = dog as Animal //Upcasting dog to animal class
dog2.name //Can only access name, try to access legs of furColor on your own it will fail