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A web server based on SwiftNIO designed to be used as a mock server in UI automation tests.

When running UI tests against real server several issues can come along: network can be unstable, content on the remote server can change, some of the test scenarios may require actions in some external system, and in general making network calls slows down all the tests.

SwiftNIOMock aims to address these issues by providing a mock web server implementation that runs on the localhost and which app should access instead of a real server when running under UI test. Unlike other solutions like registering custom URLProtocol using mock server requires only switching the app to the localhost and gives you much more flexibility and control as the server is controlled by the test and you can change its state from within the test scenarios.

SwiftNIOMock supports three common scenarios:

  • redirect requests to the real service using redirect middleware. This can be used for logging purposes which can make debugging easier as all the network logs will be in the console log of the test runner process

  • mock endpoints using router middleware. This is useful when you need to have more control over the state of the mock server and allows you to completely mock out all network requests the app makes (although it can be used along with redirect middleware). This is usually needed when tests scenarios require actions in some external systems. When you control the state of the server completely you can easily fake such actions

  • record & replay network calls using redirect middleware provided with the implementation of URLSession that can record and replay requests (using Vinyl or any other similar implementation). This is useful to ensure that tests receive the same data between runs and aims to guard against changes on the remote server that are out of your control


In the test create an instance of the server and start it in the setUp method and provide it with a middleware to handle requests. In the tearDown method stop the server.

override func setUp() {
    server = Server(port: 8080, handler: <#Middleware#>)
    try! server.start()

override func tearDown() {
    try! server.stop()
    server = nil

A middleware is a function that is based on an incoming request can modify a response.

typealias Middleware = (
    _ request: Server.HTTPHandler.Request,
    _ response: Server.HTTPHandler.Response,
    _ next: @escaping () -> Void
) -> Void

When middleware function is done with response it should call the next closure (it's fine to call it asynchronously) that is passed to it to return control to the server. You can write your own middleware or use those provided by SwiftNIOMock, which are redirect, router and delay. Here is an example of a simple middleware that echoes back any incoming request:

func echo(
    request: Server.HTTPHandler.Request,
    response: Server.HTTPHandler.Response,
    next: @escaping () -> Void
) {
    response.statusCode = .ok
    response.body = request.body

redirect middleware allows you to redirect incoming requests and intercept responses. It also accepts an instance of URLSession (URLSession.shared session by default) that it will use to perform a request, which allows it to record & replay all requests. Check out SwiftNIOMockExampleUITests.swift to see the example of how to use SwiftNIOMock in record & replay mode powered by Vinyl.

func redirect(
    session: URLSession = URLSession.shared,
    request override: @escaping (Server.HTTPHandler.Request) -> Server.HTTPHandler.Request,
    response intercept: @escaping (Server.HTTPHandler.Response) -> Void = { _ in }
) -> Middleware

router middleware allows you to return arbitrary middleware based on a request, i.e. its method and path. If request can't be handled a fallback middleware passed as notFound parameter will be used.

func router(
    route: @escaping (Server.HTTPHandler.Request) -> Middleware?,
    notFound: @escaping Middleware
) -> Middleware

To see an example of usage SwiftNIOMock in UI tests check SwiftNIOMockExample.


While you can use the router function to create a routing middleware for the mock server on the practice it is a good idea to break it down into smaller services. For that you can use Service type:

let fooService = Service {
    route({ $0.head.uri == "/foo" }) { (request, response, next) in
        response.sendString(.ok, value: "Foo")
let barService = Service {
    route({ $0.head.uri == "/bar" }) { (request, response, next) in
        response.sendString(.ok, value: "Bar")

let router = SwiftNIOMock.router(services: [

You can as well implement your own service by subclassing Service:

class HelloService: Service {
    override init() {
        routes {
            route({ $0.head.uri == "/helloworld" }) { (request, response, next) in
                response.sendString(.ok, value: "Hello World!")
let helloService = HelloService()
let router = SwiftNIOMock.router(services: [helloService])

You can register routes using closure predicates as in above examples or using URLFormat:

import URLFormat

routes {
    route(GET/.helloworld) { (request, response, next) in
        response.sendString(.ok, value: "Hello World!")

You can also use a shorthand syntax to bind routes to a service KeyPath or a function with Encodable return value type:

class HelloService: Service {
    let users: [String]
    func user(byName name: String) -> String? {
        users.first { $0 == name }
    init(users: [String]) {
        self.users = users
        routes {
            GET/.users == \HelloService.users
            GET/.users/.string == HelloService.user(byName:)

As routes are bound to key paths or functions you can mutate the state of your service and these changes will be reflected in the later responses.

When binding a route to a function its parameters types must match URLFormat type, i.e. URLFormat<((String, String), Int)> can be only bound to a function (String, String, Int) -> T where T: Encodable. Additionally you can add a Server.HTTPHandler.Request parameter as the last parameter of the function to access a request body data.


import PackageDescription

let package = Package(
    dependencies: [
        .package(url: "", .branch("master")),


A web server based on SwiftNIO designed to be used as a mock server in UI automation tests




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