协议只能用作通用约束 [英] Protocol can only be used as a generic constraint

问题描述

我有一个用于在地图上显示注释的MapViewController。它包含一个类型为MapPresentable的
对象。

I have a MapViewController for presenting annotations on map. It contains an object of type MapPresentable.

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

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

MapViewController也可以在 mapPresentable 符合RoutePresentable协议。

MapViewController can also present route on map in case mapPresentable conforms to RoutePresentable protocol.

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

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

：

I got this Error:

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

推荐答案

更新

对不起，我犯了错误。但是没有办法用关联类型来转换协议。

希望这会有所帮助。

因为我知道 routePresentable.getRouteLocations 与协议 MapPresentable

因此，您可以将 RoutePresentable 分成两个协议：

So you can divide RoutePresentable to two protocol:

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)
}

原始

由于 routePresentable.annotations 的类型未提供，

您可以删除 associatedtype AnnotationElement：MKAnnotation 。

或用户通用结构代替：

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)
}

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