Vapor4+Parameter.swift

import Vapor

public protocol FutureParameter {
    associatedtype ResolvedParameter

    static var routingSlug: String { get }
    static var parameter: PathComponent { get }

    static func resolveParameter(_ parameter: String, on app: Application) -> EventLoopFuture<ResolvedParameter>
}

public protocol Parameter {
    associatedtype ResolvedParameter

    static var routingSlug: String { get }
    static var parameter: PathComponent { get }

    static func resolveParameter(_ parameter: String) throws -> ResolvedParameter
}

extension FutureParameter {
    static var parameter: PathComponent { .parameter(routingSlug.lowercased()) }
}
extension Parameter {
    public static var parameter: PathComponent { .parameter(routingSlug.lowercased()) }
}

extension Parameters {
    public func grab<T>(_ type: T.Type = T.self, at routingSlug: String = T.routingSlug, on app: Application) -> EventLoopFuture<T.ResolvedParameter> where T: FutureParameter {
        app.eventLoopGroup.future().flatMapThrowing {
            guard let parameter = self.get(routingSlug.lowercased())
            else { throw Abort(.badRequest) }
            return parameter
        }.flatMap {
            T.resolveParameter($0, on: app)
        }
    }
    
    public func grab<T>(_ type: T.Type = T.self, at routingSlug: String = T.routingSlug, on req: Request) -> EventLoopFuture<T.ResolvedParameter> where T: FutureParameter {
        grab(type, at: routingSlug.lowercased(), on: req.application)
    }
    
    public func grab<T>(_ type: T.Type = T.self, at routingSlug: String = T.routingSlug) throws -> T.ResolvedParameter where T: Parameter {
        guard let parameter = self.get(routingSlug.lowercased())
            else { throw Abort(.badRequest, reason: "Unable to find \(routingSlug) in URL params") }
        return try T.resolveParameter(parameter)
    }
}

extension UUID: Parameter {
    public static var routingSlug: String { "id" }
    
    public static func resolveParameter(_ parameter: String) throws -> UUID {
        guard let id = UUID(uuidString: parameter) else { throw Abort(.badRequest) }
        return id
    }
}