在Swift中重写方法或属性时的多态性(Polymorphism when overriding methods or properties in Swift)

 我想通过编译器确保CarViewController仅在车辆属性中接收Car 。 
 
 鉴于以下快速示例代码： 
 
class Vehicle {
    func doSomething(){}
}

class Car: Vehicle {
    func doCarThings(){}
}

class VehicleViewController : UIViewController {
    var vehicle : Vehicle!;

    override func viewDidLoad() {
        super.viewDidLoad();
        vehicle.doSomething();
    }
}

class CarViewController:VehicleViewController {
    var vehicle: Car!

    override func viewDidLoad() {
        super.viewDidLoad();
        vehicle.doCarThings();
    }
}
 
 我收到以下错误： Cannot override mutable property 'vehicle' of type 'Vehicle!' with covariant type 'Car!' Cannot override mutable property 'vehicle' of type 'Vehicle!' with covariant type 'Car!' 
 
 我尝试了一种基于泛型的方法： 
 
class Vehicle {
    func doSomething(){}
}

class Car: Vehicle {
    func doCarThings(){}
}

class VehicleViewController<T:Vehicle> : UIViewController {
    var vehicle : T!;

    override func viewDidLoad() {
        super.viewDidLoad();
        vehicle.doSomething();
    }
}

class CarViewController:VehicleViewController<Car> {
    override func viewDidLoad() {
        super.viewDidLoad();
        vehicle.doCarThings();
    }
}
 
 这是正确的，但在故事板类中使用泛型导致错误（因为它们被编译为objective-c）。 
 
 我怎样才能做到这一点，而不使用泛型？ 
 
 谢谢！ 

I want to ensure by compiler that CarViewController only receives a Car in the vehicle property.
 
Given the following swift example code:
 
class Vehicle {
    func doSomething(){}
}

class Car: Vehicle {
    func doCarThings(){}
}

class VehicleViewController : UIViewController {
    var vehicle : Vehicle!;

    override func viewDidLoad() {
        super.viewDidLoad();
        vehicle.doSomething();
    }
}

class CarViewController:VehicleViewController {
    var vehicle: Car!

    override func viewDidLoad() {
        super.viewDidLoad();
        vehicle.doCarThings();
    }
}
 
I get the following error: Cannot override mutable property 'vehicle' of type 'Vehicle!' with covariant type 'Car!'
 
I tried with a generics-based approach:
 
class Vehicle {
    func doSomething(){}
}

class Car: Vehicle {
    func doCarThings(){}
}

class VehicleViewController<T:Vehicle> : UIViewController {
    var vehicle : T!;

    override func viewDidLoad() {
        super.viewDidLoad();
        vehicle.doSomething();
    }
}

class CarViewController:VehicleViewController<Car> {
    override func viewDidLoad() {
        super.viewDidLoad();
        vehicle.doCarThings();
    }
}
 
It is correct but using generics in storyboard classes results in errors (since they get compiled to objective-c).
 
How can I do this without using generics?
 
Thanks!

原文：https://stackoverflow.com/questions/43937762