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!
}

MapViewController还可以在地图上显示路由,以防mapPresentable符合RoutePresentable协议.

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

但在里面检查时 MapViewController

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

我有这个错误:

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.但是没有办法施放协议associated type.

希望这会有所帮助.

据我所知routePresentable.getRouteLocations,这与协议无关MapPresentable.

所以你可以分为RoutePresentable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)
}

原版的

因为routePresentable.annotations没有提供类型,

你可以删除associatedtype AnnotationElement: MKAnnotation.

或者用户通用结构代替:

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)
}