Peasy 1.2.0

Peasy 1.2.0

Maintained by Kane Cheshire.

Peasy 1.2.0


Actions Status Version License Platform

Closed Issues Open Issues

A lightweight mock server written purely in Swift, that you can run directly in your UI tests, with no need for any external process to be spun up as part of the tests. 🎉

Quick start

Simply create and start a server in your tests, then tell it what to respond with for requests:

import XCTest
import Peasy

class MyUITest: XCTestCase {

  let server = Server()

  override func setUp() {
    server.start() // Default port is 8880
    let ok = Response(status: .ok)
    server.respond(with: ok, when: .path(matches: "/"))

  func test_someStuff() {
    // Run your tests that cause the app to call http://localhost:8880/


Starting and stopping the server

To start a Peasy server, just call start:

let server = Server()

By default, Peasy starts the server on whatever port is available on the system, but you can override this and choose any specific port you want:

server.start(port: 8080)

NOTE: Starting two servers on the same port is not supported and will fail.

NOTE: iOS simulators share the same network as your Mac, so you can communicate directly with your Peasy server from Terminal, Safari or Postman on your Mac.

To stop a server (i.e. when tests finish), just call stop:


Configuring responses

By default Peasy doesn't know how to respond to any requests made to it.

You'll need to tell Peasy what to respond with, which you do by building up a set of rules that must be valid for an incoming request:

let response = Response(status: .ok)
server.respond(with: response, when: .path(matches: "/"))

You can provide multiple rules to filter responses further, for example, only matching paths "/" for GET requests:

server.respond(with: response, when: .path(matches: "/"), .method(matches: .get))

Now, whenever Peasy receives a GET request matching the root path of "/", it will respond with an empty response of 200 OK.

If none of Peasy's built in rules work for you, you can always provide a custom handler:

let customRule: Rule = .custom { request in
  return request.path.contains("/common/path")
server.respond(with: response, when: customRule)

Default and override responses

It's common to want to set a "default" response to a request, but sometimes override it.

For example, in a UI test, you might want to set the default response as happy path, but then test that if the request is made again what happens if you get a different response.

Peasy supports this by allowing you to indicate whether a set of rules are removed after they're matched with removeAfterResponding:

server.respond(with: happyPathResponse, when: .path(matches: "/api")) // removeAfterResponding defaults to false, so this will persist

server.respond(with: unhappyPathResponse, when: .path(matches: "/api"), removeAfterResponding: true) // This will match before the happy path response and will be removed after responding

In the case of multiple configurations matching a request (as above), Peasy will use the last set one (the unhappy path one above). Since we're also telling the unhappy path response to be removed after responding, Peasy will then carry on matching the first configuration (the happy path) until a new override response is set.

Delaying requests

You might want to slow responses down on your Peasy servers, for example to write a UI test to test what happens when a request times out.

Peasy makes this really easy, just provide a TimeInterval when configuring your responses:

server.respond(with: response, when: .path(matches: "/api"), delay: 60)

By default, Peasy will respond immediately to requests you've told it to respond to. You must provide an explicit delay if you want a delay.

Intercepting requests

There might be times when you want to know when Peasy has received a request so you you know how to respond or take some other action, like track certain requests (i.e. analytics).

Peasy supports this by allowing you to provide a handler to return a response where the request is provided to you as an argument:

var analytics: [Request] = []
server.respond(with { request in
  return response
}, when: .path(matches: "/analytics-event"))

Wildcards and variables in paths

It's common to use wildcards and variables in paths that may be dynamic. Peasy supports this by allowing you to indicate which parts of the path can be dynamic with the :variable syntax:

server.respond(with: response, when: .path(matches: "/constant/:variable"))

The name after : can be anything you like, but that path component must exist otherwise the rule will fail to match (i.e. "/constant/" is not valid, but "/constant/value" is).

If you want to get the value of a variable you can do so using a key-value subscript on the request:

server.respond(with { request in
  print("The value is", request["variable"])
  return response
}, when: .path(matches: "/constant/:variable"))


Q: Does Peasy run on a real device?

A: Yes! Peasy uses low level Darwin APIs built into the open-source bits of iOS and macOS.

Q: Can this ever break?

A: Technically yeah, but these APIs have been around forever and haven't been deprecated.

Q: Can I use this in my app for the App Store?

A: Peasy is designed for UI tests but also works in regular apps. Peasy doesn't use any private APIs, so shouldn't be rejected, but it will also depend on what you're using Peasy for.


Peasy was hugely inspired by Envoy's Embassy server, but it does too much and is far too complicated for what I want to achieve. Peasy's interface is designed to be the simplest and most accessible it can be.

Without Envoy's hard work, Peasy would not exist.