CircleCI で iOS ビルドのパイプラインを調べる必要が出て、Swift を触りはじめました。普段は TypeScript が中心なので、まずテスト環境がどうなっているか確認することにしました。swift pac […]

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CircleCI で iOS ビルドのパイプラインを調べる必要が出て、Swift を触りはじめました。普段は TypeScript が中心なので、まずテスト環境がどうなっているか確認することにしました。swift package init を叩くと、テスト環境がすでに整っていました。手順をメモしておきます。

swift package init で何が生まれるか

ターミナルでパッケージを作成します。

mkdir first-swift-app\ncd first-swift-app\nswift package init --type executable\n

以下のファイルが自動生成されます。

.gitignore\nPackage.swift\nSources/first-swift-app/first_swift_app.swift\nTests/first-swift-appTests/first_swift_appTests.swift\n

swift run を実行すると、Hello, world! が出力されます。

swift run\n# Hello, world!\n

次に swift test を打ちます。

swift test\n

最初から1テストが通ります（Green）。生成されたテストファイルを見ると import Testing と書かれており、Swift Testing が使われています。これは Swift 6 / Xcode 16 以降の標準スタイルです。環境によっては import XCTest の従来スタイルが生成される場合もあります。

FizzBuzz で TDD サイクルを1回回す

TDD の流れを確認するために、FizzBuzz 関数を作ります。

Red: テストを書いて落とす

Sources/first-swift-app/first_swift_app.swift を以下に書き換えます。関数の中身は空にして、意図的にテストを落とします。

func fizzBuzz(_ number: Int) -> String {\n    return \"\"\n}\n

Tests/first-swift-appTests/first_swift_appTests.swift にテストを書きます。

import Testing\n@testable import first_swift_app\n\n@Test func testFizzBuzz() {\n    #expect(fizzBuzz(3) == \"Fizz\")\n}\n

swift test を実行すると、狙い通り失敗します。

✘ Test testFizzBuzz() recorded an issue at first_swift_appTests.swift:6:5: Expectation failed: (fizzBuzz(3) → \"\") == \"Fizz\"\n✘ Test testFizzBuzz() failed after 0.001 seconds with 1 issue.\n

エラーメッセージが (fizzBuzz(3) → \"\") == \"Fizz\" と、実際の返り値まで表示されるので原因がすぐわかります。

Green: 最短で通す

まずテストを通すだけのコードを書きます。

func fizzBuzz(_ number: Int) -> String {\n    if number == 3 {\n        return \"Fizz\"\n    }\n    return \"\"\n}\n

swift test を実行すると通ります。

✔ Test testFizzBuzz() passed after 0.001 seconds.\n

Refactor: テストを増やして本実装にする

5の倍数と15の倍数のテストを追加して、一度落とします。

@Test func testFizzBuzz() {\n    #expect(fizzBuzz(3) == \"Fizz\")\n    #expect(fizzBuzz(5) == \"Buzz\")\n    #expect(fizzBuzz(15) == \"FizzBuzz\")\n    #expect(fizzBuzz(1) == \"1\")\n}\n

本実装に書き換えます。15の倍数を先に判定するのは、3と5の両方の倍数になるためです。

func fizzBuzz(_ number: Int) -> String {\n    if number % 15 == 0 { return \"FizzBuzz\" }\n    if number % 3 == 0 { return \"Fizz\" }\n    if number % 5 == 0 { return \"Buzz\" }\n    return String(number)\n}\n

swift test を実行して全テストが通れば完了です。

TypeScript との対比メモ

TypeScript に慣れていると、Swift の記法で引っかかる部分がいくつかあります。

@Test と #expect() は、Vitest の test() と expect() に対応します。テスト関数の前に @Test をつけるだけで認識されるので、クラスを継承する必要がありません。

_ を引数名の前につけると、呼び出し側でラベルを省略できます。fizzBuzz(_ number: Int) と書くことで、fizzBuzz(3) のように呼べます。書かない場合は fizzBuzz(number: 3) という形になります。

String(number) は TypeScript の String(number) と同じ書き方です。

コマンドラインだけで swift test が動き、Red/Green/Refactor のサイクルを回せる環境が最初から整っています。TypeScript + Vitest に慣れていれば、違和感は少ないと思います。

まとめ

swift package init --type executable → swift run → swift test の3コマンドで、テストが動く環境が整います。あとは FizzBuzz を題材に Red → Green → Refactor を1サイクル回せば、流れは掴めます。

次は CircleCI での iOS ビルドパイプラインの設定に戻る予定です。

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Tag: TDD

swift test で TDD をはじめる — FizzBuzz で Red/Green/Refactor を回す