SwiftUI のプレビュー上でユニットテストを実行する Proof of Concept (ContentView のテストをしたい場合)

ContentView.swift

import SwiftUI

struct ContentView: View {
    @State var hoge = 0
    @Environment(\.testCaseNumber) private var testCaseNumber

    func change() {
        hoge = 3
    }

    var body: some View {
        VStack {
            Image(systemName: "globe")
                .imageScale(.large)
                .foregroundColor(.accentColor)
            Text("Hello, world!")
        }
        .modifier(TestModifier(testCase: allTestCases[testCaseNumber]))
    }
}

extension ContentView {
    var allTestCases: [TestCase] {
        [
            test1,
            test2,
        ]
    }

    var test1: TestCase {
        .init(label: "hoge == 0") {
            .init(success: hoge == 0)
        }
    }

    var test2: TestCase {
        .init(label: "hoge == 3") {
            change()
            return .init(success: hoge == 3)
        }
    }
}

#Preview {
    VStack {
        // FIXME: 個数はどこから持ってくる？
        ForEach(0..<2, id: \.self) { i in
            ContentView()
                .environment(\.testCaseNumber, i)
        }
    }
    .environment(\.isTesting, true)
}

struct TestCase: Identifiable {
    var id: UUID = .init()
    var label: String
    var run: () -> TestResult
}

struct TestResult {
    var success: Bool
}

struct IsTestingKey: EnvironmentKey {
    static let defaultValue: Bool = false
}

extension EnvironmentValues {
    var isTesting: Bool {
        get { self[IsTestingKey.self] }
        set { self[IsTestingKey.self] = newValue }
    }
}

struct TestCaseNumberKey: EnvironmentKey {
    static let defaultValue: Int = 0
}

extension EnvironmentValues {
    var testCaseNumber: Int {
        get { self[TestCaseNumberKey.self] }
        set { self[TestCaseNumberKey.self] = newValue }
    }
}

struct TestModifier: ViewModifier {
    let testCase: TestCase
    @Environment(\.isTesting) private var isTesting
    @State private var result: TestResult?

    func body(content: Content) -> some View {
        if isTesting {
            VStack {
                if let result {
                    Text("\(testCase.label): \(result.success ? "passed" : "failed")")
                } else {
                    Text("Running")
                }
            }
            .onAppear {
                result = testCase.run()
            }
        } else {
            content
        }
    }
}

テストケース記述の自由度を上げようと思ったらもうちょっと考える必要あり