The source code for this guide can be found on GitHub

Installing Swift

If you don’t have Swift installed, install it first.

To test that you have Swift installed, run swift --version from your shell or terminal app.

Swift comes bundled with the Swift Package Manager (SwiftPM), which manages the distribution of your Swift code, and allows easy importing of other Swift packages into your apps.

Bootstrapping

Let’s write a small library with our new Swift development environment. To start, we’ll use SwiftPM to make a new project for us. In your terminal of choice run:

$ mkdir MyLibrary
$ cd MyLibrary
$ swift package init --name MyLibrary --type library

This will generate a new directory called MyLibrary with the following files:

.
├── Package.swift
├── Sources
│   └── MyLibrary
│       └── MyLibrary.swift
└── Tests
    └── MyLibraryTests
        └── MyLibraryTests.swift

Package.swift is the manifest file for Swift. It’s where you keep metadata for your project, as well as its dependencies.

Sources/MyLibrary/MyLibrary.swift is the library initial source file and where we’ll write our library code. Test/MyLibraryTests/MyLibraryTests.swift is where we can write tests for our library.

In fact, SwiftPM generated a “Hello, world!” project for us, including some unit tests! We can run the tests by running swift test in our terminal.

$ swift test
Building for debugging...
[5/5] Linking MyLibraryPackageTests
Build complete! (7.91s)
Test Suite 'All tests' started at 2023-08-29 18:59:31.328
Test Suite 'MyLibraryPackageTests.xctest' started at 2023-08-29 18:59:31.329
Test Suite 'MyLibraryTests' started at 2023-08-29 18:59:31.329
Test Case '-[MyLibraryTests.MyLibraryTests testExample]' started.
Test Case '-[MyLibraryTests.MyLibraryTests testExample]' passed (0.001 seconds).
Test Suite 'MyLibraryTests' passed at 2023-08-29 18:59:31.330.
	 Executed 1 test, with 0 failures (0 unexpected) in 0.001 (0.001) seconds
Test Suite 'MyLibraryPackageTests.xctest' passed at 2023-08-29 18:59:31.330.
	 Executed 1 test, with 0 failures (0 unexpected) in 0.001 (0.001) seconds
Test Suite 'All tests' passed at 2023-08-29 18:59:31.330.
	 Executed 1 test, with 0 failures (0 unexpected) in 0.001 (0.002) seconds

A small library

Now let’s write a small library. Replace the example content of MyLibrary.swift with the following code:

import Foundation

struct Email: CustomStringConvertible {
  var description: String

  public init(_ emailString: String) throws {
    let regex = #"[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,64}"#
    guard let _ = emailString.range(of: regex, options: .regularExpression) else {
      throw InvalidEmailError(email: emailString)
    }
    self.description = emailString
  }
}

private struct InvalidEmailError: Error {
  let email: String
}

Now let’s add a unit test for this strongly typed Email API. Replace the example content of MyLibraryTests.swift with the following code:

@testable import MyLibrary
import XCTest

final class MyLibraryTests: XCTestCase {
  func testEmail() throws {
    let email = try Email("john.appleseed@apple.com")
    XCTAssertEqual(email.description, "john.appleseed@apple.com")

    XCTAssertThrowsError(try Email("invalid"))
  }
}

Once we save that, we can run the tests again:

❯ swift test
Building for debugging...
[5/5] Linking MyLibraryPackageTests
Build complete! (3.09s)
Test Suite 'All tests' started at 2023-08-29 19:01:08.640
Test Suite 'MyLibraryPackageTests.xctest' started at 2023-08-29 19:01:08.641
Test Suite 'MyLibraryTests' started at 2023-08-29 19:01:08.641
Test Case '-[MyLibraryTests.MyLibraryTests testEmail]' started.
Test Case '-[MyLibraryTests.MyLibraryTests testEmail]' passed (0.002 seconds).
Test Suite 'MyLibraryTests' passed at 2023-08-29 19:01:08.643.
	 Executed 1 test, with 0 failures (0 unexpected) in 0.002 (0.002) seconds
Test Suite 'MyLibraryPackageTests.xctest' passed at 2023-08-29 19:01:08.643.
	 Executed 1 test, with 0 failures (0 unexpected) in 0.002 (0.002) seconds
Test Suite 'All tests' passed at 2023-08-29 19:01:08.643.
	 Executed 1 test, with 0 failures (0 unexpected) in 0.002 (0.003) seconds

The source code for this guide can be found on GitHub