一个类可以继承(inherit)另一个类的方法(methods)，属性(properties)和其它特性
类可以调用和访问超类的方法，属性和下标(subscripts)，并且可以重写(override)这些方法，属性和下标来优化或修改它们的行为。
可以为类中继承来的属性添加属性观察器(property observers)

1 基类(base class)

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class Vehicle {
    var currentSpeed = 0.0
    var description: String {
        return "traveling at \(currentSpeed) miles per hour"
    }
    func makeNoise() {
        // 什么也不做-因为车辆不一定会有噪音
    }
}
let someVehicle = Vehicle()

2 子类生成(Subclassing)

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class Bicycle: Vehicle {
    var hasBasket = false
}
let bicycle = Bicycle()
bicycle.hasBasket = true
bicycle.currentSpeed = 15.0
print("Bicycle: \(bicycle.description)")
class Tandem: Bicycle {
    var currentNumberOfPassengers = 0
}

3 重写(Overriding)

任何缺少override关键字的重写都会在编译时被诊断为错误。
访问超类的方法，属性及下标。 super

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class Train: Vehicle {
    override func makeNoise() {
        print("Choo Choo")
    }
}
let train = Train()
train.makeNoise()
class Car: Vehicle {
    var gear = 1
    override var description: String {
        return super.description + " in gear \(gear)"
    }
}
class AutomaticCar: Car {
    override var currentSpeed: Double {
        didSet {
            gear = Int(currentSpeed / 10.0) + 1
        }
    }
}

4 防止重写

final var

final func

final class func

final subscript

playground文件在andyRon/LearnSwift