Can't use concrete subclass to implement a property in protocol in Swift

Question

I\'m doing something similar to the Bridge Pattern in JAVA, DriverType is a protocol requires a property named vehicle to be a Drivable

Answer 1

I think the compiler error is misleading. DriverType states that any class adopting it must expose a vehicle property with Drivable type, not a property with a class type adopting the Drivable type.

I would solve this issue by defining both the DriverType protocol and the Car class using generics:

protocol Drivable {
    var speed: Double { get }
    init()
}

protocol DriverType {
    typealias T: Drivable
    var vehicle: T { get }
}

class Car: Drivable {
    var speed = 80.0;
    var brand = "BMW"
    required init() {}
}

class Driver: DriverType {   
    var vehicle: T = T() 
}

This explicitly states that classes adopting DriverType must expose a vehicle property whose type is any class adopting the Drivable protocol.