finally adjust
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梁杰
@ -1,6 +1,10 @@
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> 1.0
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> 翻译：[Hawstein](https://github.com/Hawstein)
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> 校对：[menlongsheng](https://github.com/menlongsheng)
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> 2.0
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> 翻译+校对：[shanksyang](https://github.com/shanksyang)
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# 继承（Inheritance）
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-------------------
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@ -25,34 +29,34 @@
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> 注意：
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Swift 中的类并不是从一个通用的基类继承而来。如果你不为你定义的类指定一个超类的话，这个类就自动成为基类。
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下面的例子定义了一个叫`Vehicle`的基类。这个基类声明了一个名为`currentSpeed `，默认值是0.0的存储属性(属性类型推断为`Double `)。`currentSpeed `属性的值被一个`String` 类型的只读计算型属性`description`使用，用来创建车辆的描述。
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下面的例子定义了一个叫`Vehicle`的基类。这个基类声明了一个名为`currentSpeed `，默认值是0.0的存储属性(属性类型推断为`Double `)。`currentSpeed `属性的值被一个`String` 类型的只读计算型属性`description`使用，用来创建车辆的描述。
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`Vehicle`基类也定义了一个名为`makeNoise`的方法。这个方法实际上不为`Vehicle`实例做任何事，但之后将会被`Vehicle`的子类定制：
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```swift
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class Vehicle {
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var currentSpeed = 0.0
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var description: String {
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return "traveling at \(currentSpeed) miles per hour"
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}
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func makeNoise() {
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// 什么也不做-因为车辆不一定会有噪音
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}
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class Vehicle {
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var currentSpeed = 0.0
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var description: String {
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return "traveling at \(currentSpeed) miles per hour"
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}
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func makeNoise() {
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// 什么也不做-因为车辆不一定会有噪音
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}
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}
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```
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您可以用初始化语法创建一个`Vehicle `的新实例，即类名后面跟一个空括号：
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```swift
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let someVehicle = Vehicle()
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```
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现在已经创建了一个`Vehicle`的新实例，你可以访问它的`description`属性来打印车辆的当前速度。
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```swift
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print("Vehicle: \(someVehicle.description)")
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// Vehicle: traveling at 0.0 miles per hour
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```
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您可以用初始化语法创建一个`Vehicle `的新实例，即类名后面跟一个空括号：
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```swift
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let someVehicle = Vehicle()
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```
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现在已经创建了一个`Vehicle`的新实例，你可以访问它的`description`属性来打印车辆的当前速度。
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```swift
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print("Vehicle: \(someVehicle.description)")
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// Vehicle: traveling at 0.0 miles per hour
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```
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`Vehicle`类定义了一个通用特性的车辆类，实际上没什么用处。为了让它变得更加有用，需要改进它能够描述一个更加具体的车辆类。
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<a name="subclassing"></a>
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@ -68,51 +72,51 @@ class SomeClass: SomeSuperclass {
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}
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```
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下一个例子，定义一个叫`Bicycle`的子类，继承成父类`Vehicle`
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```swift
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class Bicycle: Vehicle {
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var hasBasket = false
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}
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```
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下一个例子，定义一个叫`Bicycle`的子类，继承成父类`Vehicle`
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新的`Bicycle`类自动获得`Vehicle`类的所有特性，比如 `currentSpeed `和`description`属性，还有它的`makeNoise`方法。
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除了它所继承的特性，`Bicycle`类还定义了一个默认值为`false`的存储型属性`hasBasket`（属性推断为`Bool`）。
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默认情况下，你创建任何新的`Bicycle`实例将不会有一个篮子，创建该实例之后，你可以为特定的`Bicycle`实例设置`hasBasket `属性为`ture`：
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```swift
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let bicycle = Bicycle()
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bicycle.hasBasket = true
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```
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你还可以修改`Bicycle `实例所继承的`currentSpeed `属性，和查询实例所继承的`description `属性：
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```swift
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bicycle.currentSpeed = 15.0
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print("Bicycle: \(bicycle.description)")
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// Bicycle: traveling at 15.0 miles per hour
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```swift
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class Bicycle: Vehicle {
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var hasBasket = false
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}
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```
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新的`Bicycle`类自动获得`Vehicle`类的所有特性，比如 `currentSpeed `和`description`属性，还有它的`makeNoise`方法。
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除了它所继承的特性，`Bicycle`类还定义了一个默认值为`false`的存储型属性`hasBasket`（属性推断为`Bool`）。
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默认情况下，你创建任何新的`Bicycle`实例将不会有一个篮子，创建该实例之后，你可以为特定的`Bicycle`实例设置`hasBasket `属性为`ture`：
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```swift
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let bicycle = Bicycle()
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bicycle.hasBasket = true
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```
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你还可以修改`Bicycle `实例所继承的`currentSpeed `属性，和查询实例所继承的`description `属性：
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```swift
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bicycle.currentSpeed = 15.0
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print("Bicycle: \(bicycle.description)")
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// Bicycle: traveling at 15.0 miles per hour
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```
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子类还可以继续被其它类继承，下面的示例为`Bicycle `创建了一个名为`Tandem `（双人自行车）的子类：
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```swift
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class Tandem: Bicycle {
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var currentNumberOfPassengers = 0
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class Tandem: Bicycle {
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var currentNumberOfPassengers = 0
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}
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```
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`Tandem`从`Bicycle`继承了所有的属性与方法，这又使它同时继承了`Vehicle`的所有属性与方法。`Tandem`也增加了一个新的叫做`currentNumberOfPassengers`的存储型属性，默认值为0。
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`Tandem`从`Bicycle`继承了所有的属性与方法，这又使它同时继承了`Vehicle`的所有属性与方法。`Tandem`也增加了一个新的叫做`currentNumberOfPassengers`的存储型属性，默认值为0。
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如果你创建了一个`Tandem`的实例，你可以使用它所有的新属性和继承的属性，还能查询从`Vehicle`继承来的只读属性`description `：
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```swift
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let tandem = Tandem()
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tandem.hasBasket = true
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tandem.currentNumberOfPassengers = 2
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tandem.currentSpeed = 22.0
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print("Tandem: \(tandem.description)")
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let tandem = Tandem()
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tandem.hasBasket = true
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tandem.currentNumberOfPassengers = 2
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tandem.currentSpeed = 22.0
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print("Tandem: \(tandem.description)")
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// Tandem: traveling at 22.0 miles per hour
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```
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@ -142,18 +146,18 @@ print("Tandem: \(tandem.description)")
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下面的例子定义了`Vehicle`的一个新的子类，叫`Train `，它重写了从`Vehicle`类继承来的`makeNoise `方法：
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```swift
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class Train: Vehicle {
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override func makeNoise() {
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print("Choo Choo")
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}
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class Train: Vehicle {
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override func makeNoise() {
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print("Choo Choo")
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}
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}
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```
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如果你创建一个`Train `的新实例，并调用了它的`makeNoise `方法，你就会发现`Train `版本的方法被调用：
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```swift
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let train = Train()
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train.makeNoise()
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let train = Train()
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train.makeNoise()
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// prints "Choo Choo"
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```
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@ -173,23 +177,23 @@ train.makeNoise()
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以下的例子定义了一个新类，叫`Car`，它是`Vehicle `的子类。这个类引入了一个新的存储型属性叫做`gear `，默认为整数1。`Car`类重写了继承自`Vehicle `的description属性，提供自定义的，包含当前档位的描述：
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```swift
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class Car: Vehicle {
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var gear = 1
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override var description: String {
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return super.description + " in gear \(gear)"
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}
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class Car: Vehicle {
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var gear = 1
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override var description: String {
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return super.description + " in gear \(gear)"
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}
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}
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```
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重写的`description `属性，首先要调用`super.description`返回`Vehicle`类的`description`属性。之后，`Car `类版本的`description`在末尾增加了一些额外的文本来提供关于当前档位的信息。
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```
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重写的`description `属性，首先要调用`super.description`返回`Vehicle`类的`description`属性。之后，`Car `类版本的`description`在末尾增加了一些额外的文本来提供关于当前档位的信息。
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如果你创建了`Car `的实例并且设置了它的`gear`和`currentSpeed`属性，你可以看到它的`description`返回了`Car`中定义的`description`：
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```swift
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let car = Car()
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car.currentSpeed = 25.0
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car.gear = 3
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print("Car: \(car.description)")
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let car = Car()
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car.currentSpeed = 25.0
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car.gear = 3
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print("Car: \(car.description)")
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// Car: traveling at 25.0 miles per hour in gear 3
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```
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@ -203,21 +207,21 @@ print("Car: \(car.description)")
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下面的例子定义了一个新类叫`AutomaticCar`，它是`Car`的子类。`AutomaticCar`表示自动挡汽车，它可以根据当前的速度自动选择合适的挡位:
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```swift
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class AutomaticCar: Car {
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override var currentSpeed: Double {
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didSet {
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gear = Int(currentSpeed / 10.0) + 1
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}
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}
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class AutomaticCar: Car {
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override var currentSpeed: Double {
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didSet {
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gear = Int(currentSpeed / 10.0) + 1
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}
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}
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}
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```
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当你设置`AutomaticCar`的`currentSpeed `属性，属性的`didSet`观察器就会自动地设置`gear`属性，为新的速度选择一个合适的挡位。具体来说就是，属性观察器将新的速度值除以10，然后向下取得最接近的整数值，最后加1来得到档位`gear`的值。例如，速度为10.0时，挡位为1；速度为35.0时，挡位为4：
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```swift
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let automatic = AutomaticCar()
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automatic.currentSpeed = 35.0
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print("AutomaticCar: \(automatic.description)")
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let automatic = AutomaticCar()
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automatic.currentSpeed = 35.0
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print("AutomaticCar: \(automatic.description)")
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// AutomaticCar: traveling at 35.0 miles per hour in gear 4
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```
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