协议只能用作通用约束

Question

我有一个 MapViewController 用于在地图上显示注释。它包含一个 MapPresentable 类型的对象。

protocol MapPresentable {
    associatedtype AnnotationElement: MKAnnotation
    var annotations: [AnnotationElement] { get }
}

class MapViewController<M: MapPresentable>: UIViewController {
    var mapPresentable: M!
}

如果mapPresentable 符合 RoutePresentable 协议，MapViewController 也可以在地图上显示路线。

protocol RoutePresentable: MapPresentable {
    var getRouteLocations: [CLLocation] { get }
}

但是在MapViewController内部进行检查时

if let routePresentable = mapPresentable as? RoutePresentable {
    showRoute(routePresentable.getRouteLocations)
}

我收到此错误：

Protocol 'RoutePresentable' can only be used as a generic constraint because it has Self or associated type requirements

Answer 1

更新

对不起，我犯了错误。但是没有办法用associated type 强制转换协议。

希望这会有所帮助。

据我所知，routePresentable.getRouteLocations 与协议 MapPresentable 无关。

所以你可以把RoutePresentable分成两个协议：

protocol MapPresentable {
    associatedtype AnnotationElement: MKAnnotation
    var annotations: [AnnotationElement] { get }
}

class MapViewController<M: MapPresentable>: UIViewController {
    var mapPresentable: M!

}

protocol RoutePresentable: MapPresentable, CanGetRouteLocations {}

protocol CanGetRouteLocations {
    var getRouteLocations: [CLLocation] { get }
}


if let routePresentable = mapPresentable as? CanGetRouteLocations {
    showRoute(routePresentable.getRouteLocations)
}

原创

因为routePresentable.annotations的Type没有提供，

你可以删除associatedtype AnnotationElement: MKAnnotation。

或者改为用户通用结构：

struct MapPresentable<AnnotationElement: MKAnnotation> {
    var annotations: [AnnotationElement] = []
}

struct RoutePresentable<AnnotationElement: MKAnnotation> {
    var mapPresentable: MapPresentable<AnnotationElement>
    var getRouteLocations: [CLLocation] = []
}

class MapViewController<AnnotationElement: MKAnnotation>: UIViewController {

    var mapPresentable: MapPresentable<AnnotationElement>!

}

if let routePresentable = mapPresentable as? RoutePresentable<MKAnnotation> {
    showRoute(routePresentable.getRouteLocations)
}