FOSMVVM ViewModel Generator

Generate ViewModels following FOSMVVM architecture patterns.

Conceptual Foundation

For full architecture context, see FOSMVVMArchitecture.md

A ViewModel is the bridge in the Model-View-ViewModel architecture:

┌─────────────┐      ┌─────────────────┐      ┌─────────────┐
│    Model    │ ───► │    ViewModel    │ ───► │    View     │
│   (Data)    │      │  (The Bridge)   │      │  (SwiftUI)  │
└─────────────┘      └─────────────────┘      └─────────────┘

Key insight: In FOSMVVM, ViewModels are:

• Created by a Factory (either server-side or client-side)
• Localized during encoding (resolves all @LocalizedString references)
• Consumed by Views which just render the localized data

---

First Decision: Hosting Mode

This is a per-ViewModel decision. An app can mix both modes - for example, a standalone iPhone app with server-based sign-in.

Ask: Where does THIS ViewModel's data come from?

Data Source	Hosting Mode	Factory
Server/Database	Server-Hosted	Hand-written
Local state/preferences	Client-Hosted	Macro-generated

Server-Hosted Mode

When data comes from a server:

• Factory is hand-written on server (ViewModelFactory protocol)
• Factory queries database, builds ViewModel
• Server localizes during JSON encoding
• Client receives fully localized ViewModel

Examples: Sign-in screen, user profile from API, dashboard with server data

Client-Hosted Mode

When data is local to the device:

• Use @ViewModel(options: [.clientHostedFactory])
• Macro auto-generates factory from init parameters
• Client bundles YAML resources
• Client localizes during encoding

Examples: Settings screen, onboarding, offline-first features

Hybrid Apps

Many apps use both:

┌─────────────────────────────────────────┐
│            iPhone App                    │
├─────────────────────────────────────────┤
│ SettingsViewModel      → Client-Hosted  │
│ OnboardingViewModel    → Client-Hosted  │
│ SignInViewModel        → Server-Hosted  │
│ UserProfileViewModel   → Server-Hosted  │
└─────────────────────────────────────────┘

Same ViewModel patterns work in both modes - only the factory creation differs.

Core Responsibility: Shaping Data

A ViewModel's job is shaping data for presentation. This happens in two places:

1. Factory - what data is needed, how to transform it
2. Localization - how to present it in context (including locale-aware ordering)

The View just renders - it should never compose, format, or reorder ViewModel properties.

What a ViewModel Contains

A ViewModel answers: "What does the View need to display?"

Content Type	How It's Represented	Example
Static UI text	@LocalizedString	Page titles, button labels
Dynamic data in text	@LocalizedSubs	"Welcome, %{name}!" with substitutions
Composed text	@LocalizedCompoundString	Full name from pieces (locale-aware order)
Formatted dates	LocalizableDate	createdAt: LocalizableDate
Formatted numbers	LocalizableInt	totalCount: LocalizableInt
Dynamic data	Plain properties	content: String, count: Int
Nested components	Child ViewModels	cards: [CardViewModel]

What a ViewModel Does NOT Contain

• Database relationships (@Parent, @Siblings)
• Business logic or validation (that's in Fields protocols)
• Raw database IDs exposed to templates (use typed properties)
• Unlocalized strings that Views must look up

Anti-Pattern: Composition in Views

// ❌ WRONG - View is composing
Text(viewModel.firstName) + Text(" ") + Text(viewModel.lastName)

// ✅ RIGHT - ViewModel provides shaped result
Text(viewModel.fullName)  // via @LocalizedCompoundString

If you see + or string interpolation in a View, the shaping belongs in the ViewModel.

ViewModel Protocol Hierarchy

public protocol ViewModel: ServerRequestBody, RetrievablePropertyNames, Identifiable, Stubbable {
    var vmId: ViewModelId { get }
}

public protocol RequestableViewModel: ViewModel {
    associatedtype Request: ViewModelRequest
}

ViewModel provides:

• ServerRequestBody - Can be sent over HTTP as JSON
• RetrievablePropertyNames - Enables @LocalizedString binding (via @ViewModel macro)
• Identifiable - Has vmId for SwiftUI identity
• Stubbable - Has stub() for testing/previews

RequestableViewModel adds:

• Associated Request type for fetching from server

Two Categories of ViewModels

1. Top-Level (RequestableViewModel)

Represents a full page or screen. Has:

• An associated ViewModelRequest type
• A ViewModelFactory that builds it from database
• Child ViewModels embedded within it

@ViewModel
public struct DashboardViewModel: RequestableViewModel {
    public typealias Request = DashboardRequest

    @LocalizedString public var pageTitle
    public let cards: [CardViewModel]  // Children
    public var vmId: ViewModelId = .init()
}

2. Child (plain ViewModel)

Nested components built by their parent's factory. No Request type.

@ViewModel
public struct CardViewModel: Codable, Sendable {
    public let id: ModelIdType
    public let title: String
    public let createdAt: LocalizableDate
    public var vmId: ViewModelId = .init()
}

---

Display vs Form ViewModels

ViewModels serve two distinct purposes:

Purpose	ViewModel Type	Adopts Fields?
Display data (read-only)	Display ViewModel	No
Collect user input (editable)	Form ViewModel	Yes

Display ViewModels

For showing data - cards, rows, lists, detail views:

@ViewModel
public struct UserCardViewModel {
    public let id: ModelIdType
    public let name: String
    @LocalizedString public var roleDisplayName
    public let createdAt: LocalizableDate
    public var vmId: ViewModelId = .init()
}

Characteristics:

• Properties are let (read-only)
• No validation needed
• No FormField definitions
• Just projects Model data for display

Form ViewModels

For collecting input - create forms, edit forms, settings:

@ViewModel
public struct UserFormViewModel: UserFields {  // ← Adopts Fields!
    public var id: ModelIdType?
    public var email: String
    public var firstName: String
    public var lastName: String

    public let userValidationMessages: UserFieldsMessages
    public var vmId: ViewModelId = .init()
}

Characteristics:

• Properties are var (editable)
• Adopts a Fields protocol for validation
• Gets FormField definitions from Fields
• Gets validation logic from Fields
• Gets localized error messages from Fields

The Connection

┌─────────────────────────────────────────────────────────────────┐
│                    UserFields Protocol                          │
│        (defines editable properties + validation)               │
│                                                                 │
│  Adopted by:                                                    │
│  ┌─────────────────┐  ┌─────────────────┐  ┌─────────────────┐ │
│  │ CreateUserReq   │  │ UserFormVM      │  │ User (Model)    │ │
│  │ .RequestBody    │  │ (UI form)       │  │ (persistence)   │ │
│  └─────────────────┘  └─────────────────┘  └─────────────────┘ │
│                                                                 │
│  Same validation logic everywhere!                              │
└─────────────────────────────────────────────────────────────────┘

Quick Decision Guide

Ask: "Is the user editing data in this ViewModel?"

• No → Display ViewModel (no Fields)
• Yes → Form ViewModel (adopt Fields)

ViewModel	User Edits?	Adopt Fields?
UserCardViewModel	No	No
UserRowViewModel	No	No
UserDetailViewModel	No	No
UserFormViewModel	Yes	UserFields
CreateUserViewModel	Yes	UserFields
EditUserViewModel	Yes	UserFields
SettingsViewModel	Yes	SettingsFields

---

When to Use This Skill

• Creating a new page or screen
• Adding a new UI component (card, row, modal, etc.)
• Displaying data from the database in a View
• Following an implementation plan that requires new ViewModels

What This Skill Generates

Server-Hosted: Top-Level ViewModel (4 files)

File	Location	Purpose
{Name}ViewModel.swift	{ViewModelsTarget}/	The ViewModel struct
{Name}Request.swift	{ViewModelsTarget}/	The ViewModelRequest type
{Name}ViewModel.yml	{ResourcesPath}/	Localization strings
{Name}ViewModel+Factory.swift	{WebServerTarget}/	Factory that builds from DB

Client-Hosted: Top-Level ViewModel (2 files)

File	Location	Purpose
{Name}ViewModel.swift	{ViewModelsTarget}/	ViewModel with clientHostedFactory option
{Name}ViewModel.yml	{ResourcesPath}/	Localization strings (bundled in app)

No Request or Factory files needed - macro generates them!

Child ViewModels (1-2 files, either mode)

File	Location	Purpose
{Name}ViewModel.swift	{ViewModelsTarget}/	The ViewModel struct
{Name}ViewModel.yml	{ResourcesPath}/	Localization (if has @LocalizedString)

Project Structure Configuration

Placeholder	Description	Example
{ViewModelsTarget}	Shared ViewModels SPM target	ViewModels
{ResourcesPath}	Localization resources	Sources/Resources
{WebServerTarget}	Server-side target	WebServer, AppServer

Generation Process

Step 1: Determine Hosting Mode

Ask: Where does this ViewModel's data come from?

• Server/database → Server-Hosted (hand-written factory)
• Local state → Client-Hosted (macro-generated factory)

Step 2: Understand What the View Needs

Clarify:

1. What is this View displaying? (page, modal, card, row?)
2. What data does it need? (from database? from AppState?)
3. What static text does it have? (titles, labels, buttons)
4. Does it contain child ViewModels?
5. Is it top-level or a child?

Step 3: Design the ViewModel

Determine:

• Properties - What does the View need to render?
• Localization - Which properties are @LocalizedString?
• Identity - Singleton (vmId = .init(type: Self.self)) or instance (vmId = .init(id: id))?

Step 4: Generate Files

Server-Hosted Top-Level:

1. ViewModel struct (with RequestableViewModel)
2. Request type
3. YAML localization
4. Factory implementation

Client-Hosted Top-Level:

1. ViewModel struct (with clientHostedFactory option)
2. YAML localization

Child (either mode):

1. ViewModel struct
2. YAML localization (if needed)

Key Patterns

The @ViewModel Macro

Always use the @ViewModel macro - it generates the propertyNames() method required for localization binding.

Server-Hosted (basic macro):

@ViewModel
public struct MyViewModel: RequestableViewModel {
    public typealias Request = MyRequest
    @LocalizedString public var title
    public var vmId: ViewModelId = .init()
    public init() {}
}

Client-Hosted (with factory generation):

@ViewModel(options: [.clientHostedFactory])
public struct SettingsViewModel {
    @LocalizedString public var pageTitle
    public var vmId: ViewModelId = .init()

    public init(theme: Theme, notifications: NotificationSettings) {
        // Init parameters become AppState properties
    }
}

// Macro auto-generates:
// - typealias Request = ClientHostedRequest
// - struct AppState { let theme: Theme; let notifications: NotificationSettings }
// - class ClientHostedRequest: ViewModelRequest { ... }
// - static func model(context:) async throws -> Self { ... }

Stubbable Pattern

All ViewModels must support stub() for testing and SwiftUI previews:

public extension MyViewModel {
    static func stub() -> Self {
        .init(/* default values */)
    }
}

Identity: vmId

Every ViewModel needs a vmId for SwiftUI's identity system:

Singleton (one per page): vmId = .init(type: Self.self) Instance (multiple per page): vmId = .init(id: id) where id: ModelIdType

Localization

Static UI text uses @LocalizedString:

@LocalizedString public var pageTitle

With corresponding YAML:

en:
  MyViewModel:
    pageTitle: "Welcome"

Dates and Numbers

Never send pre-formatted strings. Use localizable types:

public let createdAt: LocalizableDate    // NOT String
public let itemCount: LocalizableInt     // NOT String

The client formats these according to user's locale and timezone.

Child ViewModels

Top-level ViewModels contain their children:

@ViewModel
public struct BoardViewModel: RequestableViewModel {
    public let columns: [ColumnViewModel]
    public let cards: [CardViewModel]
}

The Factory builds all children when building the parent.

Codable and Computed Properties

Swift's synthesized Codable only encodes stored properties. Since ViewModels are serialized (for JSON transport, Leaf rendering, etc.), computed properties won't be available.

// Computed - NOT encoded, invisible after serialization
public var hasCards: Bool { !cards.isEmpty }

// Stored - encoded, available after serialization
public let hasCards: Bool

When to pre-compute:

For Leaf templates, you can often use Leaf's built-in functions directly:

• #if(count(cards) > 0) - no need for hasCards property
• #count(cards) - no need for cardCount property

Pre-compute only when:

• Direct array subscripts needed (firstCard - array indexing not documented in Leaf)
• Complex logic that's cleaner in Swift than in template
• Performance-sensitive repeated calculations

See fosmvvm-leaf-view-generator for Leaf template patterns.

File Templates

See reference.md for complete file templates.

Naming Conventions

Concept	Convention	Example
ViewModel struct	{Name}ViewModel	DashboardViewModel
Request class	{Name}Request	DashboardRequest
Factory extension	{Name}ViewModel+Factory.swift	DashboardViewModel+Factory.swift
YAML file	{Name}ViewModel.yml	DashboardViewModel.yml

Collaboration Protocol

1. Understand what the View needs to display
2. Confirm whether it's top-level or child
3. Display or Form? - If form, use fields-generator first to create Fields protocol
4. Identify which properties need localization
5. Generate files one at a time with feedback

See Also

• FOSMVVMArchitecture.md - Full FOSMVVM architecture
• fosmvvm-fields-generator - For form validation
• fosmvvm-fluent-datamodel-generator - For Fluent persistence layer
• fosmvvm-leaf-view-generator - For Leaf templates that render ViewModels
• reference.md - Complete file templates

Version History

Version	Date	Changes
1.0	2024-12-24	Initial skill
2.0	2024-12-26	Complete rewrite from architecture; generalized from Kairos-specific
2.1	2024-12-26	Added Client-Hosted mode support; per-ViewModel hosting decision
2.2	2024-12-26	Added shaping responsibility, @LocalizedSubs/@LocalizedCompoundString, anti-pattern
2.3	2025-12-27	Added Display vs Form ViewModels section; clarified Fields adoption
2.4	2026-01-08	Added Codable/computed properties section. Clarified when to pre-compute vs use Leaf built-ins.