Build a library

百度 客观来说，主裁判可以掏红牌。

The source code for this guide can be found on GitHub

Installing Swift

To kickstart your journey, install Swift to begin using it on macOS, Linux, or Windows.

Tip: 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) that manages the distribution of Swift code. It allows easy importing of other Swift packages into your applications and libraries, making it a valuable tool for any Swift developer.

Swift is covered by the Apache License, Version 2.0.

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 2025-08-07 14:02:39.232.
Test Suite 'All tests' passed at 2025-08-07 14:02:39.233.
   Executed 0 tests, with 0 failures (0 unexpected) in 0.000 (0.001) seconds
◇ Test run started.
? Testing Library Version: 6.1 (43b6f88e2f2712e)
? Target Platform: arm64-apple-macosx
◇ Test example() started.
? Test example() passed after 0.001 seconds.
? Test run with 1 test passed after 0.001 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:

import Testing
@testable import MyLibrary

struct MyLibraryTests {
  @Test func email() throws {
    let email = try Email("john.appleseed@apple.com")
    #expect(email.description == "john.appleseed@apple.com")

    #expect(throws: (any Error).self) {
      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 2025-08-07 14:24:32.687.
Test Suite 'All tests' passed at 2025-08-07 14:24:32.689.
   Executed 0 tests, with 0 failures (0 unexpected) in 0.000 (0.001) seconds
◇ Test run started.
? Testing Library Version: 6.1 (43b6f88e2f2712e)
? Target Platform: arm64-apple-macosx
◇ Suite MyLibraryTests started.
◇ Test email() started.
? Test email() passed after 0.001 seconds.
? Suite MyLibraryTests passed after 0.001 seconds.
? Test run with 1 test passed after 0.001 seconds.

The source code for this guide can be found on GitHub.