FOSMVVM ViewModel Test Generator

Generate test files for ViewModels following FOSMVVM testing patterns.

Conceptual Foundation

For full architecture context, see FOSMVVMArchitecture.md

ViewModel testing in FOSMVVM verifies three critical aspects:

1. Codable round-trip - ViewModel encodes and decodes without data loss
2. Versioning stability - Structure hasn't changed unexpectedly
3. Multi-locale translations - All @LocalizedString properties have values in all supported locales

The LocalizableTestCase protocol provides infrastructure that tests all three in a single call.

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When to Use This Skill

• Creating tests for new ViewModels
• Adding test coverage to existing ViewModels
• Verifying localization completeness across locales
• Testing ViewModels with embedded/nested child ViewModels
• Verifying @LocalizedSubs substitution behavior

What This Skill Generates

File	Location	Purpose
{Name}ViewModelTests.swift	Tests/{Target}Tests/Localization/	Test suite conforming to LocalizableTestCase
{Name}ViewModel.yml	Tests/{Target}Tests/TestYAML/	YAML translations for test (if needed)

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The Testing Pattern

Standard Pattern (Most Tests)

For most ViewModels, a single line provides complete coverage:

@Test func dashboardViewModel() throws {
    try expectFullViewModelTests(DashboardViewModel.self)
}

This verifies:

• Codable encoding/decoding
• Versioned ViewModel stability
• Translations exist for all locales (en, es by default)

This is sufficient for the vast majority of ViewModel tests.

Extended Pattern (Specific Formatting Verification)

When testing specific formatting behavior (substitutions, compound strings), add locale-specific assertions:

@Test func greetingWithSubstitution() throws {
    try expectFullViewModelTests(GreetingViewModel.self)

    // Verify specific substitution behavior
    let vm: GreetingViewModel = try .stub()
        .toJSON(encoder: encoder(locale: en))
        .fromJSON()

    #expect(try vm.welcomeMessage.localizedString == "Welcome, John!")
}

This is optional - use only when verifying specific formatting techniques.

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LocalizableTestCase Protocol

Test suites conform to LocalizableTestCase to access testing infrastructure:

import FOSFoundation
@testable import FOSMVVM
import FOSTesting
import Foundation
import Testing

@Suite("My ViewModel Tests")
struct MyViewModelTests: LocalizableTestCase {
    let locStore: LocalizationStore

    init() throws {
        self.locStore = try Self.loadLocalizationStore(
            bundle: Bundle.module,
            resourceDirectoryName: "TestYAML"
        )
    }
}

What LocalizableTestCase Provides

Property/Method	Purpose
locStore	Required - the localization store
locales	Optional - locales to test (default: en, es)
encoder(locale:)	Creates a localizing JSONEncoder
en, es, enGB, enUS	Locale constants

Testing Methods

Method	Use When
expectFullViewModelTests(_:)	Primary - complete ViewModel testing
expectTranslations(_:)	Translation-only verification
expectFullFieldValidationModelTests(_:)	Testing FieldValidationModel types
expectFullFormFieldTests(_:)	Testing FormField instances
expectCodable(_:encoder:)	Codable round-trip only
expectVersionedViewModel(_:encoder:)	Versioning stability only

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YAML Requirements

ViewModels with @LocalizedString

Every ViewModel with @LocalizedString properties needs YAML entries:

@ViewModel
public struct DashboardViewModel: RequestableViewModel {
    @LocalizedString public var pageTitle      // Needs YAML entry
    @LocalizedString public var emptyMessage   // Needs YAML entry
    public let itemCount: Int                   // No YAML needed
}

# DashboardViewModel.yml
en:
  DashboardViewModel:
    pageTitle: "Dashboard"
    emptyMessage: "No items yet"

es:
  DashboardViewModel:
    pageTitle: "Tablero"
    emptyMessage: "No hay elementos todavía"

Embedded ViewModels

When a ViewModel contains child ViewModels, all types in the hierarchy need YAML entries:

@ViewModel
public struct BoardViewModel: RequestableViewModel {
    @LocalizedString public var title
    public let cards: [CardViewModel]  // Child ViewModel
}

@ViewModel
public struct CardViewModel {
    @LocalizedString public var cardTitle
}

Both BoardViewModel and CardViewModel need YAML entries (can be in same or separate files).

Private Test ViewModels

When tests define private ViewModel structs for testing specific scenarios, those also need YAML:

// In test file
private struct TestParentViewModel: ViewModel {
    @LocalizedString var title
    let children: [TestChildViewModel]
}

private struct TestChildViewModel: ViewModel {
    @LocalizedString var label
}

Add entries to a test YAML file for these private types.

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Generation Process

Step 1: Identify ViewModels to Test

Determine which ViewModels need test coverage:

• New ViewModels being created
• Existing ViewModels without tests
• ViewModels with localization properties

Step 2: Check YAML Coverage

Verify YAML entries exist for:

• The ViewModel itself
• Any embedded/child ViewModels
• All supported locales (typically en, es)

Step 3: Generate Test File

Create test suite conforming to LocalizableTestCase:

• One @Test function per ViewModel (or logical grouping)
• Use expectFullViewModelTests() as the primary assertion
• Add specific formatting tests only when needed

Step 4: Run Tests

swift test --filter {TestSuiteName}

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File Templates

See reference.md for complete file templates.

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Common Scenarios

Testing a Single Top-Level ViewModel

@Test func dashboardViewModel() throws {
    try expectFullViewModelTests(DashboardViewModel.self)
}

Testing Multiple Related ViewModels

@Test func boardViewModels() throws {
    try expectFullViewModelTests(BoardViewModel.self)
    try expectFullViewModelTests(ColumnViewModel.self)
    try expectFullViewModelTests(CardViewModel.self)
}

Testing with Custom Locales

var locales: Set<Locale> { [en, es, enGB] }  // Override default

@Test func multiLocaleViewModel() throws {
    try expectFullViewModelTests(MyViewModel.self)
    // Tests en, es, AND en-GB
}

Testing Substitution Behavior

@Test func greetingSubstitutions() throws {
    try expectFullViewModelTests(GreetingViewModel.self)

    let vm: GreetingViewModel = try .stub(userName: "Alice")
        .toJSON(encoder: encoder(locale: en))
        .fromJSON()

    #expect(try vm.welcomeMessage.localizedString == "Welcome, Alice!")
}

Testing Embedded ViewModels

@Test func parentWithChildren() throws {
    // Tests parent AND verifies children can be encoded/decoded
    try expectFullViewModelTests(ParentViewModel.self)

    // Optionally verify specific child values
    let vm: ParentViewModel = try .stub()
        .toJSON(encoder: encoder(locale: en))
        .fromJSON()

    #expect(try vm.children[0].label.localizedString == "Child 1")
}

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Troubleshooting

"Missing Translation" Error

FOSLocalizableError: _pageTitle -- Missing Translation -- en

Cause: YAML entry missing for a @LocalizedString property.

Fix: Add the property to the YAML file:

en:
  MyViewModel:
    pageTitle: "Page Title"  # Add this

"Is pending localization" Error

Cause: The ViewModel wasn't encoded with a localizing encoder.

Fix: Ensure using encoder(locale:) or expectFullViewModelTests().

Test Passes But Translations Seem Wrong

Cause: YAML values exist but may have typos or wrong content.

Fix: Add specific assertions to verify exact values:

let vm = try .stub().toJSON(encoder: encoder(locale: en)).fromJSON()
#expect(try vm.title.localizedString == "Expected Value")

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Naming Conventions

Concept	Convention	Example
Test suite	{Feature}ViewModelTests	DashboardViewModelTests
Test file	{Feature}ViewModelTests.swift	DashboardViewModelTests.swift
YAML file	{ViewModelName}.yml	DashboardViewModel.yml
Test method	{viewModelName}() or descriptive	dashboardViewModel()

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See Also

• FOSMVVMArchitecture.md - Testing Support - Architecture overview
• fosmvvm-viewmodel-generator - For creating ViewModels
• fosmvvm-fields-generator - For form validation testing
• reference.md - Complete file templates

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Version History

Version	Date	Changes
1.0	2025-01-02	Initial skill