protocol-property-req-explain-6.swift

protocol Flyable {
    // Make speedLimit only gettable
    var speedLimit: Int { get }
    
    func fly()
}

class Bird: Flyable {
    // Make speedLimit private
    private(set) var speedLimit = 20
}

class SpeedLimitUpdater {
    
    static func reduceSpeedLimit(of flyables: [Flyable], by value: Int) {
        // Only update speedLimit of Helicopter & Airplane
        for flyable in flyables {
            if let helicopter = flyable as? Helicopter {
                helicopter.speedLimit -= value
            } else if let airplane = flyable as? Airplane {
                airplane.speedLimit -= value
            }
        }
    }
    
    static func increaseSpeedLimit(of flyables: [Flyable], by value: Int) {
        // Only update speedLimit of Helicopter & Airplane
        for flyable in flyables {
            if let helicopter = flyable as? Helicopter {
                helicopter.speedLimit += value
            } else if let airplane = flyable as? Airplane {
                airplane.speedLimit += value
            }
        }
    }
}