fosmvvm-viewmodel-generator

FOSMVVM ViewModel Generator

Generate ViewModels following FOSMVVM architecture patterns.

Conceptual Foundation

For full architecture context, see FOSMVVMArchitecture.md | OpenClaw reference

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.

The key question: Where does THIS ViewModel's data come from?

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, error display

Error Display Pattern

Error display is a classic client-hosted scenario. You already have the data from ResponseError - just wrap it in a specific ViewModel for that error:

// Specific ViewModel for MoveIdeaRequest errors
@ViewModel(options: [.clientHostedFactory])
struct MoveIdeaErrorViewModel {
    let message: LocalizableString
    let errorCode: String

    public var vmId = ViewModelId()

    // Takes the specific ResponseError
    init(responseError: MoveIdeaRequest.ResponseError) {
        self.message = responseError.message
        self.errorCode = responseError.code.rawValue
    }
}

Usage:

catch let error as MoveIdeaRequest.ResponseError {
    let vm = MoveIdeaErrorViewModel(responseError: error)
    return try await req.view.render("Shared/ToastView", vm)
}

Each error scenario gets its own ViewModel:

• MoveIdeaErrorViewModel for MoveIdeaRequest.ResponseError
• CreateIdeaErrorViewModel for CreateIdeaRequest.ResponseError
• SettingsValidationErrorViewModel for settings form errors

Don't create a generic "ToastViewModel" or "ErrorViewModel" - that's unified error architecture, which we avoid.

Key insights:

• No server request needed - you already caught the error
• The LocalizableString properties in ResponseError are already localized (server did it)
• Standard ViewModel → View encoding chain handles this correctly; already-localized strings pass through unchanged
• Client-hosted ViewModel wraps existing data; the macro generates the factory

Hybrid Apps

Many apps use both:

┌───────────────────────────────────────────────┐
│               iPhone App                       │
├───────────────────────────────────────────────┤
│ SettingsViewModel           → Client-Hosted   │
│ OnboardingViewModel         → Client-Hosted   │
│ MoveIdeaErrorViewModel      → Client-Hosted   │  ← Error display
│ 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?"

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:

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

The key question: "Is the user editing data in this ViewModel?"

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

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