App Store Rating Lookup for Vapor 3

1_AppleAppLookup.swift

import Foundation
import Vapor

struct AppInfo: Codable {
    let averageUserRating: Double
    let userRatingCount: Int
}

enum AppleAppLookupError: Error {
    case notFound
}

final class AppleAppLookup: ServiceType {
    private let client: Client

    init(client: Client) {
        self.client = client
    }

    static func makeService(for container: Container) throws -> AppleAppLookup {
        let client = try container.client()
        return AppleAppLookup(client: client)
    }

    func getAppInfo(_ itunesID: String) -> Future<AppInfo> {
        var components = URLComponents(string: "https://itunes.apple.com/gb/lookup")!
        components.queryItems = [URLQueryItem(name: "id", value: itunesID)]
        return client.get(components.url!).flatMap { resp -> Future<AppInfoResponse> in
            // Override Content-Type returned from the server with application/json
            // so Vapor's Content decoding does not throw up.
            resp.http.contentType = .json
            return try resp.content.decode(AppInfoResponse.self)
        }.map {
            guard let info = $0.results.first else {
                throw AppleAppLookupError.notFound
            }

            return info
        }
    }
}

private struct AppInfoResponse: Decodable {
    let results: [AppInfo]
}

2_configure.swift

import Vapor

public func configure(_ config: inout Config, _ env: inout Environment, _ services: inout Services) throws {
    // ... other configuration omitted for brevity ...

    // Register AppleAppLookup service
    services.register(AppleAppLookup.self)
}

3_routes.swift

import Vapor

public func routes(_ router: Router) throws {
    // Example route that uses AppleAppLookup
    router.get("apple", String.parameter) { req -> Future<String> in
        let itunesID: String = try req.parameters.next()
        let lookup = try req.make(AppleAppLookup.self)

        return lookup.getAppInfo(itunesID)
            .map { "Rating: \($0.averageUserRating) out of \($0.userRatingCount) review\($0.userRatingCount == 1 ? "" : "s")" }
    }
}

4_example_call.sh

curl localhost:8080/apple/1229016807
Rating: 5.0 out of 34953 reviews