XCTest Dynamic Overlay

Define XCTest assertion helpers directly in your application and library code.

It is very common to write test support code for libraries and applications. This often comes in the form of little domain-specific functions or helpers that make it easier for users of your code to formulate assertions on behavior.

Currently there are only two options for writing test support code:

• Put it in a test target, but then you can't access it from multiple other test targets. For whatever reason test targets cannot be imported, and so the test support code will only be available in that one single test target.
• Create a dedicated test support module that ships just the test-specific code. Then you can import this module into as many test targets as you want, while never letting the module interact with your regular, production code.

Neither of these options is ideal. In the first case you cannot share your test support, and the second case will lead you to a proliferation of modules. For each feature you potentially need 3 modules: MyFeature, MyFeatureTests and MyFeatureTestSupport. SPM makes managing this quite easy, but it's still a burden.

It would be far better if we could ship the test support code right along side or actual library or application code. After all, they are intimately related. You can even fence off the test support code in #if DEBUG ... #endif if you are worried about leaking test code into production.

However, as soon as you add import XCTest to a source file in your application or a library it loads, the target becomes unbuildable:

import XCTest

🛑 ld: warning: Could not find or use auto-linked library 'XCTestSwiftSupport'

🛑 ld: warning: Could not find or use auto-linked framework 'XCTest'

This is due to a confluence of problems, including test header search paths, linker issues, and more. XCTest just doesn't seem to be built to be loaded alongside your application or library code.

So, the XCTest Dynamic Overlay library is a microlibrary that dynamically loads the XCTFail symbol at runtime and exposes it publicly so that it can be used from anywhere. This means you can import this library instead of XCTest:

import XCTestDynamicOverlay // ✅

…and your application or library will continue to compile just fine.

A real world example of using this is in our library, the Composable Architecture. That library vends a TestStore type whose purpose is to make it easy to write tests for your application's logic. The TestStore uses XCTFail internally, and so that forces us to move the code to a dedicated test support module. However, due to how SPM works you cannot currently have that module in the same package as the main module, and so we would be forced to extract it to a separate repo. By loading XCTFail dynamically we can keep the code where it belongs.

As another example, let's say you have an analytics dependency that is used all over your application:

struct AnalyticsClient {
  var track: (Event) -> Void

  struct Event: Equatable {
    var name: String
    var properties: [String: String]
  }
}

If you are disciplined about injecting dependencies, you probably have a lot of objects that take an analytics client as an argument (or maybe some other fancy form of DI):

class LoginViewModel: ObservableObject {
  // ...

  init(analytics: AnalyticsClient) {
    // ...
  }

  // ...
}

When testing this view model you will need to provide an analytics client. Typically this means you will construct some kind of "test" analytics client that buffers events into an array, rather than sending live events to a server, so that you can assert on what events were tracked during a test:

func testLogin() {
  var events: [AnalyticsClient.Event] = []
  let viewModel = LoginViewModel(
    analytics: .test { events.append($0) }
  )

  // ...

  XCTAssertEqual(events, [.init(name: "Login Success")])
}

This works really well, and it's a great way to get test coverage on something that is notoriously difficult to test.

However, some tests may not use analytics at all. It would make the test suite stronger if the tests that don't use the client could prove that it's never used. This would mean when new events are tracked you could be instantly notified of which test cases need to be updated.

One way to do this is to create an instance of the AnalyticsClient type that simply performs an XCTFail inside the track endpoint:

import XCTest

extension AnalyticsClient {
  static let testValue = Self(
    track: { _ in XCTFail("\(Self.self).track is unimplemented.") }
  )
}

With this you can write a test that proves analytics are never tracked, and even better you don't have to worry about buffering events into an array anymore:

func testValidation() {
  let viewModel = LoginViewModel(
    analytics: .testValue
  )

  // ...
}

However, you cannot ship this code with the target that defines AnalyticsClient. You either need to extract it out to a test support module (which means AnalyticsClient must also be extracted), or the code must be confined to a test target and thus not shareable.

With XCTest Dynamic Overlay we can have our cake and eat it too 😋. We can define both the client type and the unimplemented test instance right next to each in application code without needing to extract out needless modules or targets:

struct AnalyticsClient {
  var track: (Event) -> Void

  struct Event: Equatable {
    var name: String
    var properties: [String: String]
  }
}

import XCTestDynamicOverlay

extension AnalyticsClient {
  static let testValue = Self(
    track: { _ in XCTFail("\(Self.self).track is unimplemented.") }
  )
}

XCTest Dynamic Overlay also comes with a helper that simplifies this exact pattern: unimplemented. It creates failing closures for you:

extension AnalyticsClient {
  static let testValue = Self(
    track: unimplemented("\(Self.self).track")
  )
}

And it can simplify the work of more complex dependency endpoints, which can throw or need to return a value:

struct AppDependencies {
  var date: () -> Date = Date.init,
  var fetchUser: (User.ID) async throws -> User,
  var uuid: () -> UUID = UUID.init
}

extension AppDependencies {
  static let testValue = Self(
    date: unimplemented("\(Self.self).date", placeholder: Date()),
    fetchUser: unimplemented("\(Self.self).fetchUser"),
    uuid: unimplemented("\(Self.self).uuid", placeholder: UUID())
  )
}

The above placeholder parameters can be left off, but will fatal error when the endpoint is called.

Full documentation can be found here.

This library is released under the MIT license. See LICENSE for details.