public class MockLoader: HTTPLoading {
    public func load(request: HTTPRequest, completion: @escaping (HTTPResult) -> Void) {
        let urlResponse = HTTPURLResponse(url: request.url!, statusCode: HTTPStatus(rawValue: 200), httpVersion: "1.1", headerFields: nil)!
        let response = HTTPResponse(request: request, response: urlResponse, body: nil)
        completion(.success(response))
    } 
}

public class StarWarsAPI {
    private let loader: HTTPLoading
    public init(loader: HTTPLoading = URLSession.shared) {
        self.loader = loader
    }
    public func requestPeople(completion: @escaping (...) -> Void) {
        var r = HTTPRequest()
        r.host = "swapi.dev"
        r.path = "/api/people"
        loader.load(request: r) { result in
            // TODO: interpret the result
            completion(...)
        }
    }
}

public class MockLoader: HTTPLoading {
    // typealiases help make method signatures simpler
    public typealias HTTPHandler = (HTTPResult) -> Void
    public typealias MockHandler = (HTTPRequest, HTTPHandler) -> Void
    private var nextHandlers = Array<MockHandler>()
    public override func load(request: HTTPRequest, completion: @escaping HTTPHandler) {
        if nextHandlers.isEmpty == false {
            let next = nextHandlers.removeFirst()
            next(request, completion)
        } else {
            let error = HTTPError(code: .cannotConnect, request: request)
            completion(.failure(error))
        }
    }
    @discardableResult
    public func then(_ handler: @escaping MockHandler) -> Mock {
        nextHandlers.append(handler)
        return self
    }
}

func test_sequentialExecutions() {
    let mock = MockLoader()
    for i in 0 ..< 5 {
        mock.then { request, handler in
            XCTAssert(request.path, "/(i)")
            handler(.success(...))
        }
    }
    for i in 0 ..< 5 {
        var r = HTTPRequest()
        r.path = "/(i)"
        mock.load(r) { result in
            XCTAssertEqual(result.response?.statusCode, .ok)
        }
    }
}

class StarWarsAPITests: XCTestCase {
    let mock = MockLoader()
    lazy var api: StarWarsAPI = { StarWarsAPI(loader: mock) }()
    func test_200_OK_WithValidBody() {
        mock.then { request, handler in
            XCTAssertEqual(request.path, "/api/people")
            handler(.success(/* 200 OK with some valid JSON */))
        }
        api.requestPeople { ...
            // assert that "StarWarsAPI" correctly decoded the response
        }
    }
    func test_200_OK_WithInvalidBody() {
        mock.then { request, handler in
            XCTAssertEqual(request.path, "/api/people")
            handler(.success(/* 200 OK but some mangled JSON */))
        }
        api.requestPeople { ... 
            // assert that "StarWarsAPI" correctly realized the response was bad JSON
        }
    }
    func test_404() {
        mock.then { request, handler in
            XCTAssertEqual(request.path, "/api/people")
            handler(.success(/* 404 Not Found */))
        }
        api.requestPeople { ... 
            // assert that "StarWarsAPI" correctly produced an error
        }
    }
    func test_DroppedConnection() {
        mock.then { request, handler in
            XCTAssertEqual(request.path, "/api/people")
            handler(.failure(/* HTTPError of some kind */))
        }
        api.requestPeople { ... 
            // assert that "StarWarsAPI" correctly produced an error
        }
    }
    ...
}