Resolver for iOS Dependency Injection: Getting Started

Unit Testing in Action

Open AssetServiceTests.swift. Then, in testFetchAssetsSuccessfully() add:

// 1
let asset = mockAsset()
// 2
networkService.result = .success(assetList())

// 3
sut?.fetchAssets { assetList, error in
  XCTAssertEqual(assetList?.data.count, 1)
  XCTAssertEqual(assetList?.data.first, asset)
  XCTAssertNil(error)
}

Here you:

1. Create a mock asset. The helper method mockAsset() is already included in the codebase for you. You’re welcome. :]
2. Assert a success result to MockNetworkService as you test a success case.
3. Fetch the assets from MockNetworkService. Test if you receive the same asset back as you provided to MockNetworkService.

By letting Resolver handle the dependencies, you can focus on testing AssetService without any problems. As a result, you can add as many tests as needed.

Add the following code in testFetchAssetsFailure() to test a failure case:

// 1
let networkError = AppError.network(description: "Something went wrong!")
// 2
networkService.result = .failure(networkError)

// 3
sut?.fetchAssets { assetList, error in
  XCTAssertEqual(networkError, error)
  XCTAssertNil(assetList)
}

Here you:

1. Create a mock error of type AppError, as it’s the error type in this project.
2. Then create a failure result with networkError and pass it to the MockNetworkService. That’s the benefit of mocking dependencies: you have complete control over the response.
3. Fetch the assets from MockNetworkService. Test if you receive the same error back as you provided to MockNetworkService.

Good job! Run your unit test by pressing Command-U. Green is the warmest color. :]

Where to Go From Here?

You can download the completed project by clicking Download Materials at the top or bottom of the tutorial.

In this tutorial, you learned about Dependency Injection using Resolver. You can learn more about Resolver in the official GitHub repository.

You may also want to read more about using Resolver scopes.

To learn more about Dependency Injection in Swift, check out our awesome Dependency Injection Tutorial for iOS: Getting Started.

You can also read more about calling types as functions in our What’s New in Swift 5.2 article.

If you have any questions or comments, join the forum discussion below.