Speed coding – Unit Testing the MVVM in Swift

🎯 Difficulty: Intermediate to Advanced

In this speed coding session, we write unit tests for a Swift ViewModel built with the MVVM architecture and powered by Combine and @MainActor concurrency.

You’ll see how to:

• ✅ Write precise and meaningful unit tests using XCTest
• 🔍 Test async @MainActor functions with Combine’s @Published properties
• 🧪 Mock dependencies like ChinesePhraseSaver, Loader, and Deleter
• 🔄 Validate onLoad() and onTapFavorite() logic
• ⚠️ Handle error states with shouldShowErrorAlert
• 🧼 Ensure correctness using behavior-driven test naming

We aim for properly crafted unit tests that naturally lead to high code coverage, making your codebase more robust and regression-proof.

📦 Topics Covered:

• MVVM architecture in Swift
• Writing testable ViewModels
• Dependency injection
• Async/await testing with Combine
• Mocking and verifying collaborator interactions

🧠 Best for:

Intermediate to advanced iOS developers familiar with Swift, XCTest, and modern concurrency.

💡 Don’t forget to like, subscribe, and turn on notifications for more real-time iOS engineering content!

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📄 Source Code:

🔸 CellDetailViewViewModel.swift

	import Combine
	import Shared
	@MainActor
	final class CellDetailViewViewModel: ObservableObject {
	private(set) var phrase: PhraseEntity
	private let chinesePhraseSaver: any ChinesePhraseSaver
	private let chinesePhraseLoader: any ChinesePhraseLoader
	private let chinesePhraseDeleter: any ChinesePhraseDeleter
	@Published var isFavorite = false
	@Published var shouldShowErrorAlert = false
	init(
	phrase: PhraseEntity,
	chinesePhraseSaver: some ChinesePhraseSaver,
	chinesePhraseLoader: some ChinesePhraseLoader,
	chinesePhraseDeleter: some ChinesePhraseDeleter
	) {
	self.phrase = phrase
	self.chinesePhraseSaver = chinesePhraseSaver
	self.chinesePhraseLoader = chinesePhraseLoader
	self.chinesePhraseDeleter = chinesePhraseDeleter
	}
	func onLoad() async {
	isFavorite = await isFavoriteAlready(for: phrase)
	}
	private func isFavoriteAlready(for phrase: PhraseEntity) async -> Bool {
	do {
	_ = try await chinesePhraseLoader.loadSavedPhrase(phrase: phrase)
	return true
	} catch {
	return false
	}
	}
	func onTapFavorite() async {
	let isFavoriteAlready = await isFavoriteAlready(for: phrase)
	if isFavoriteAlready {
	do {
	try await chinesePhraseDeleter.delete(phrase: phrase)
	await refreshIsFavorite()
	} catch {
	shouldShowErrorAlert = true
	}
	} else {
	do {
	try await chinesePhraseSaver.save(phrase)
	await refreshIsFavorite()
	} catch {
	shouldShowErrorAlert = true
	}
	}
	}
	private func refreshIsFavorite() async {
	await onLoad()
	}
	}

view raw CellDetailViewViewModel.swift hosted with ❤ by GitHub

🔸 CellDetailViewViewModelTests.swift

	import XCTest
	@testable import HuiShuo
	import Shared
	final class CellDetailViewViewModelTests: XCTestCase {
	@MainActor
	func testInit_doesNotRequestCollaborators() {
	let (_, saver, loader, deleter) = makeSUT(phraseToLoad: .niHao)
	XCTAssertTrue(saver.invocations.isEmpty)
	XCTAssertTrue(loader.invocations.isEmpty)
	XCTAssertTrue(deleter.invocations.isEmpty)
	}
	@MainActor
	func testOnLoad_requestloadsSavedPhrase() async {
	let phraseToLoad: PhraseEntity = .niHao
	let (sut, saver, loader, deleter) = makeSUT(phraseToLoad: phraseToLoad)
	await sut.onLoad()
	XCTAssertEqual(loader.invocations, [ .loadSavedPhrase(phrase: phraseToLoad) ])
	XCTAssertTrue(saver.invocations.isEmpty)
	XCTAssertTrue(deleter.invocations.isEmpty)
	}
	@MainActor
	func testOnLoadTwice_requestloadsSavedPhraseTwice() async {
	let phraseToLoad: PhraseEntity = .niHao
	let (sut, _, loader, _) = makeSUT(phraseToLoad: phraseToLoad)
	await sut.onLoad()
	await sut.onLoad()
	XCTAssertEqual(loader.invocations, [
	.loadSavedPhrase(phrase: phraseToLoad),
	.loadSavedPhrase(phrase: phraseToLoad)
	])
	}
	@MainActor
	func testOnLoad_whenLoadedWithError_markIsFavoriteToFalse() async {
	// Arrange
	let phraseToLoad: PhraseEntity = .niHao
	let anyNSError = NSError(domain: "any-error", code: 1)
	let (sut, _, _, _) = makeSUT(
	phraseToLoad: phraseToLoad,
	loader: MockChinesePhraseLoader(loadSavedPhraseResult: .failure(anyNSError))
	)
	var receivedIsFavorite: Bool?
	let exp = expectation(description: "wait for subscription")
	let cancellable = sut.$isFavorite
	.dropFirst()
	.sink { isFavorite in
	receivedIsFavorite = isFavorite
	exp.fulfill()
	}
	XCTAssertEqual(sut.isFavorite, false)
	// Act
	await sut.onLoad()
	await fulfillment(of: [exp], timeout: 0.1)
	// Assert
	XCTAssertEqual(receivedIsFavorite, false)
	cancellable.cancel()
	}
	@MainActor
	func testOnLoad_whenLoadedSuccessfully_markIsFavoriteToTrue() async {
	// Arrange
	let phraseToLoad: PhraseEntity = .niHao
	let (sut, _, _, _) = makeSUT(
	phraseToLoad: phraseToLoad,
	loader: MockChinesePhraseLoader(loadSavedPhraseResult: .success(phraseToLoad))
	)
	var receivedIsFavorite: Bool?
	let exp = expectation(description: "wait for subscription")
	let cancellable = sut.$isFavorite
	.dropFirst()
	.sink { isFavorite in
	receivedIsFavorite = isFavorite
	exp.fulfill()
	}
	XCTAssertEqual(sut.isFavorite, false)
	// Act
	await sut.onLoad()
	await fulfillment(of: [exp], timeout: 0.1)
	// Assert
	XCTAssertEqual(receivedIsFavorite, true)
	cancellable.cancel()
	}
	@MainActor
	func testOnTapFavorite_loadsIsFavorite() async {
	// Arrange
	let phraseToLoad: PhraseEntity = .niHao
	let anyNSError = NSError(domain: "any-error", code: 1)
	let (sut, _, loader, _) = makeSUT(
	phraseToLoad: phraseToLoad,
	loader: MockChinesePhraseLoader(loadSavedPhraseResult: .failure(anyNSError))
	)
	// Act
	await sut.onTapFavorite()
	// Assert
	XCTAssertEqual(loader.invocations, [ .loadSavedPhrase(phrase: .niHao) ])
	}
	@MainActor
	func testOnTapFavorite_whenAlreadyFavorite_requestDeletion() async {
	// Arrange
	let phraseToLoad: PhraseEntity = .niHao
	let phraseToDelete = phraseToLoad
	let (sut, saver, _, deleter) = makeSUT(
	phraseToLoad: phraseToLoad,
	loader: MockChinesePhraseLoader(loadSavedPhraseResult: .success(phraseToLoad))
	)
	await sut.onTapFavorite()
	XCTAssertEqual(deleter.invocations, [ .delete(phrase: phraseToDelete) ])
	XCTAssertEqual(saver.invocations, [])
	}
	@MainActor
	func testOnTapFavorite_whenAlreadyFavorite_showsErrorWhenUnFavorite() async {
	// Arrange
	let phraseToLoad: PhraseEntity = .niHao
	let anyNSError = NSError(domain: "any-error", code: 1)
	let (sut, _, _, _) = makeSUT(
	phraseToLoad: phraseToLoad,
	loader: MockChinesePhraseLoader(loadSavedPhraseResult: .success(phraseToLoad)),
	deleter: MockChinesePhraseDeleter(deletePhraseResult: .failure(anyNSError))
	)
	var receivedShouldShowErrorAlert: Bool?
	let exp = expectation(description: "wait for subscription")
	let cancellable = sut.$shouldShowErrorAlert
	.dropFirst()
	.sink { shouldShowErrorAlert in
	receivedShouldShowErrorAlert = shouldShowErrorAlert
	exp.fulfill()
	}
	// Act
	await sut.onTapFavorite()
	await fulfillment(of: [exp], timeout: 0.1)
	// Assert
	XCTAssertEqual(receivedShouldShowErrorAlert, true)
	cancellable.cancel()
	}
	@MainActor
	func testOnTapFavorite_whenAlreadyFavorite_refreshIsFavoriteAfterDeletion() async {
	// Arrange
	let phraseToLoad: PhraseEntity = .niHao
	let phraseToDelete = phraseToLoad
	let (sut, _, loader, deleter) = makeSUT(
	phraseToLoad: phraseToLoad,
	loader: MockChinesePhraseLoader(loadSavedPhraseResult: .success(phraseToLoad)),
	deleter: MockChinesePhraseDeleter(deletePhraseResult: .success(()))
	)
	// Act
	await sut.onTapFavorite()
	// Assert
	XCTAssertEqual(deleter.invocations, [ .delete(phrase: phraseToDelete) ])
	XCTAssertEqual(loader.invocations, [
	.loadSavedPhrase(phrase: phraseToLoad), // check is favorite
	.loadSavedPhrase(phrase: phraseToLoad) // refreshIsFavorite
	])
	}
	@MainActor
	func testOnTapFavorite_whenIsNotAlreadyFavorite_requestSaveForFavorite() async {
	// Arrange
	let phraseToLoad: PhraseEntity = .niHao
	let phraseToSave = phraseToLoad
	let anyNSError = NSError(domain: "any-error", code: 1)
	let (sut, saver, _, deleter) = makeSUT(
	phraseToLoad: phraseToLoad,
	loader: MockChinesePhraseLoader(loadSavedPhraseResult: .failure(anyNSError))
	)
	await sut.onTapFavorite()
	XCTAssertEqual(deleter.invocations, [])
	XCTAssertEqual(saver.invocations, [ .save(phrase: phraseToSave) ])
	}
	@MainActor
	func testOnTapFavorite_whenIsNotAlreadyFavorite_showsErrorWhenFavorite() async {
	// Arrange
	let phraseToLoad: PhraseEntity = .niHao
	let phraseToFavorite = phraseToLoad
	let anyNSError = NSError(domain: "some error", code: 1)
	let (sut, saver, _, _) = makeSUT(
	phraseToLoad: phraseToLoad,
	saver: MockChinesePhraseSaver(savePhraseResult: .failure(anyNSError)),
	loader: MockChinesePhraseLoader(loadSavedPhraseResult: .failure(anyNSError))
	)
	var receivedShouldShowErrorAlert: Bool?
	let exp = expectation(description: "wait for subscription")
	let cancellable = sut.$shouldShowErrorAlert
	.dropFirst()
	.sink { shouldShowErrorAlert in
	receivedShouldShowErrorAlert = shouldShowErrorAlert
	exp.fulfill()
	}
	// Act
	await sut.onTapFavorite()
	await fulfillment(of: [exp], timeout: 0.1)
	// Assert
	XCTAssertEqual(saver.invocations, [ .save(phrase: phraseToFavorite) ])
	XCTAssertEqual(receivedShouldShowErrorAlert, true)
	cancellable.cancel()
	}
	@MainActor
	func testOnTapFavorite_whenIsNotAlreadyFavorite_refreshIsFavoriteAfterSaving() async {
	// Arrange
	let phraseToLoad: PhraseEntity = .niHao
	let phraseToFavorite = phraseToLoad
	let anyNSError = NSError(domain: "not favorite yet", code: 1)
	let (sut, saver, loader, _) = makeSUT(
	phraseToLoad: phraseToLoad,
	saver: MockChinesePhraseSaver(savePhraseResult: .success(phraseToFavorite)),
	loader: MockChinesePhraseLoader(loadSavedPhraseResult: .failure(anyNSError))
	)
	// Act
	await sut.onTapFavorite()
	// Assert
	XCTAssertEqual(saver.invocations, [ .save(phrase: phraseToFavorite) ])
	XCTAssertEqual(loader.invocations, [
	.loadSavedPhrase(phrase: phraseToLoad), // check is favorite
	.loadSavedPhrase(phrase: phraseToLoad) // refreshIsFavorite
	])
	}
	// MARK: – Helpers
	@MainActor
	private func makeSUT(
	phraseToLoad: PhraseEntity = .niHao,
	saver: MockChinesePhraseSaver = MockChinesePhraseSaver(),
	loader: MockChinesePhraseLoader = MockChinesePhraseLoader(),
	deleter: MockChinesePhraseDeleter = MockChinesePhraseDeleter(),
	file: StaticString = #filePath,
	line: UInt = #line
	) -> (
	sut: CellDetailViewViewModel,
	saver: MockChinesePhraseSaver,
	loader: MockChinesePhraseLoader,
	deleter: MockChinesePhraseDeleter
	) {
	let sut = CellDetailViewViewModel(
	phrase: phraseToLoad,
	chinesePhraseSaver: saver,
	chinesePhraseLoader: loader,
	chinesePhraseDeleter: deleter
	)
	trackForMemoryLeak(for: sut, file: file, line: line)
	trackForMemoryLeak(for: saver, file: file, line: line)
	trackForMemoryLeak(for: loader, file: file, line: line)
	trackForMemoryLeak(for: deleter, file: file, line: line)
	return (sut, saver, loader, deleter)
	}
	}

view raw CellDetailViewViewModelTests.swift hosted with ❤ by GitHub

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🔖 Hashtags:

#iOSDevelopment #Swift #MVVM #XCTest #UnitTesting #CombineFramework #SwiftConcurrency #TDD #SwiftUnitTests #SpeedCoding #iOSEngineer #SoftwareTesting #CleanArchitecture #iOSDev #TestDrivenDevelopment #arifinfrds