协议只能用作通用约束

Question

协议只能用作通用约束

4

我有一个MapViewController用于在地图上展示标注。它包含一个类型为MapPresentable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也可以在地图上呈现路线。

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

但是，当在MapViewController内进行检查时

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

我得到了这个错误：

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

- GeRyCh

1个回答

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- beeth0ven · Accepted Answer

已更新

抱歉，我犯了错误。但是没有办法使用关联类型转换协议。

希望这能有所帮助。

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

因此，您可以将RoutePresentable分为两个协议：

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)
}

翻译

因为routePresentable.annotations的类型未提供，

您可以直接删除associatedtype AnnotationElement: MKAnnotation。

或者使用通用结构体：

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)
}