fosmvvm-leaf-view-generator

Name: fosmvvm-leaf-view-generator
Rating: 65
Author: foscomputerservices

by foscomputerservices

Swift libraries for supporting the Model-View-ViewModel pattern on macOS, iOS, Windows and Linux

⭐ 2🍴 1📅 Jan 24, 2026

ios ios-swift linux macos macos-swift mvvm mvvm-architecture mvvm-framework

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SKILL.md

name: fosmvvm-leaf-view-generator description: Generate Leaf templates (Views) for FOSMVVM WebApps. Use when creating HTML views that render ViewModels - both full-page templates and fragments for HTML-over-the-wire updates.

FOSMVVM Leaf View Generator

Generate Leaf templates that render ViewModels for web clients.

Architecture context: See FOSMVVMArchitecture.md

The View Layer for WebApps

In FOSMVVM, Leaf templates are the View in M-V-VM for web clients:

Model → ViewModel → Leaf Template → HTML
              ↑           ↑
        (localized)  (renders it)

Key principle: The ViewModel is already localized when it reaches the template. The template just renders what it receives.

Core Principle: View-ViewModel Alignment

The Leaf filename should match the ViewModel it renders.

Sources/
  {ViewModelsTarget}/
    ViewModels/
      {Feature}ViewModel.swift        ←──┐
      {Entity}CardViewModel.swift     ←──┼── Same names
                                          │
  {WebAppTarget}/                         │
    Resources/Views/                      │
      {Feature}/                          │
        {Feature}View.leaf            ────┤  (renders {Feature}ViewModel)
        {Entity}CardView.leaf         ────┘  (renders {Entity}CardViewModel)

This alignment provides:

Discoverability - Find the template for any ViewModel instantly
Consistency - Same naming discipline as SwiftUI
Maintainability - Changes to ViewModel are reflected in template location

Two Template Types

Full-Page Templates

Render a complete page with layout, navigation, CSS/JS includes.

{Feature}View.leaf
├── Extends base layout
├── Includes <html>, <head>, <body>
├── Renders {Feature}ViewModel
└── May embed fragment templates for components

Use for: Initial page loads, navigation destinations.

Fragment Templates

Render a single component - no layout, no page structure.

{Entity}CardView.leaf
├── NO layout extension
├── Single root element
├── Renders {Entity}CardViewModel
├── Has data-* attributes for state
└── Returned to JS for DOM swapping

Use for: Partial updates, HTML-over-the-wire responses.

The HTML-Over-The-Wire Pattern

For dynamic updates without full page reloads:

JS Event → WebApp Route → ServerRequest.processRequest() → Controller
                                                              ↓
                                                          ViewModel
                                                              ↓
HTML ← JS DOM swap ← WebApp returns ← Leaf renders ←────────┘

The WebApp route:

app.post("move-{entity}") { req async throws -> Response in
    let body = try req.content.decode(Move{Entity}Request.RequestBody.self)
    let serverRequest = Move{Entity}Request(requestBody: body)
    guard let response = try await serverRequest.processRequest(baseURL: app.serverBaseURL) else {
        throw Abort(.internalServerError)
    }

    // Render fragment template with ViewModel
    return try await req.view.render(
        "{Feature}/{Entity}CardView",
        ["card": response.viewModel]
    ).encodeResponse(for: req)
}

JS receives HTML and swaps it into the DOM - no JSON parsing, no client-side rendering.

When to Use This Skill

Creating a new page template (full-page)
Creating a new card, row, or component template (fragment)
Adding data attributes for JS event handling
Troubleshooting Localizable types not rendering correctly
Setting up templates for HTML-over-the-wire responses

Key Patterns

Pattern 1: Data Attributes for State

Fragments must embed all state that JS needs for future actions:

<div class="{entity}-card"
     data-{entity}-id="#(card.id)"
     data-status="#(card.status)"
     data-category="#(card.category)"
     draggable="true">

Rules:

data-{entity}-id for the primary identifier
data-{field} for state values (kebab-case)
Store raw values (enum cases), not localized display names
JS reads these to build ServerRequest payloads

const request = {
    {entity}Id: element.dataset.{entity}Id,
    newStatus: targetColumn.dataset.status
};

Pattern 2: Localizable Types in Leaf

FOSMVVM's LeafDataRepresentable conformance handles Localizable types automatically.

In templates, just use the property:

<span class="date">#(card.createdAt)</span>
<!-- Renders: "Dec 27, 2025" (localized) -->

If Localizable types render incorrectly (showing [ds: "2", ls: "...", v: "..."]):

Ensure FOSMVVMVapor is imported
Check Localizable+Leaf.swift exists with conformances
Clean build: swift package clean && swift build

Pattern 3: Display Values vs Identifiers

ViewModels should provide both raw values (for data attributes) and localized strings (for display). For enum localization, see the Enum Localization Pattern.

@ViewModel
public struct {Entity}CardViewModel {
    public let id: ModelIdType              // For data-{entity}-id
    public let status: {Entity}Status       // Raw enum for data-status
    public let statusDisplay: LocalizableString  // Localized (stored, not @LocalizedString)
}

<div data-status="#(card.status)">           <!-- Raw: "queued" for JS -->
    <span class="badge">#(card.statusDisplay)</span>  <!-- Localized: "In Queue" -->
</div>

Pattern 4: Fragment Structure

Fragments are minimal - just the component:

<!-- {Entity}CardView.leaf -->
<div class="{entity}-card"
     data-{entity}-id="#(card.id)"
     data-status="#(card.status)">

    <div class="card-content">
        <p class="text">#(card.contentPreview)</p>
    </div>

    <div class="card-footer">
        <span class="creator">#(card.creatorName)</span>
        <span class="date">#(card.createdAt)</span>
    </div>
</div>

Rules:

NO #extend("base") - fragments don't use layouts
Single root element - makes DOM swapping clean
All required state in data-* attributes
Localized values from ViewModel properties

Pattern 5: Full-Page Structure

Full pages extend a base layout:

<!-- {Feature}View.leaf -->
#extend("base"):
#export("content"):

<div class="{feature}-container">
    <header class="{feature}-header">
        <h1>#(viewModel.title)</h1>
    </header>

    <main class="{feature}-content">
        #for(card in viewModel.cards):
        #extend("{Feature}/{Entity}CardView")
        #endfor
    </main>
</div>

#endexport
#endextend

Pattern 6: Conditional Rendering

#if(card.isHighPriority):
<span class="priority-badge">#(card.priorityLabel)</span>
#endif

#if(card.assignee):
<div class="assignee">
    <span class="name">#(card.assignee.name)</span>
</div>
#else:
<div class="unassigned">#(card.unassignedLabel)</div>
#endif

Pattern 7: Looping with Embedded Fragments

<div class="column" data-status="#(column.status)">
    <div class="column-header">
        <h3>#(column.displayName)</h3>
        <span class="count">#(column.count)</span>
    </div>

    <div class="column-cards">
        #for(card in column.cards):
        #extend("{Feature}/{Entity}CardView")
        #endfor

        #if(column.cards.count == 0):
        <div class="empty-state">#(column.emptyMessage)</div>
        #endif
    </div>
</div>

File Organization

Sources/{WebAppTarget}/Resources/Views/
├── base.leaf                          # Base layout (all pages extend this)
├── {Feature}/
│   ├── {Feature}View.leaf             # Full page → {Feature}ViewModel
│   ├── {Entity}CardView.leaf          # Fragment → {Entity}CardViewModel
│   ├── {Entity}RowView.leaf           # Fragment → {Entity}RowViewModel
│   └── {Modal}View.leaf               # Fragment → {Modal}ViewModel
└── Shared/
    ├── HeaderView.leaf                # Shared components
    └── FooterView.leaf

Leaf Built-in Functions

Leaf provides useful functions for working with arrays:

<!-- Count items -->
#if(count(cards) > 0):
<p>You have #count(cards) cards</p>
#endif

<!-- Check if array contains value -->
#if(contains(statuses, "active")):
<span class="badge">Active</span>
#endif

Loop Variables

Inside #for loops, Leaf provides progress variables:

#for(item in items):
    #if(isFirst):<span class="first">#endif
    #(item.name)
    #if(!isLast):, #endif
#endfor

Variable	Description
`isFirst`	True on first iteration
`isLast`	True on last iteration
`index`	Current iteration (0-based)

Array Index Access

Direct array subscripts (array[0]) are not documented in Leaf. For accessing specific elements, pre-compute in the ViewModel:

public let firstCard: CardViewModel?

public init(cards: [CardViewModel]) {
    self.cards = cards
    self.firstCard = cards.first
}

Codable and Computed Properties

Swift's synthesized Codable only encodes stored properties. Since ViewModels are passed to Leaf via Codable encoding, computed properties won't be available.

// Computed property - NOT encoded by Codable, invisible in Leaf
public var hasCards: Bool { !cards.isEmpty }

// Stored property - encoded by Codable, available in Leaf
public let hasCards: Bool

If you need a derived value in a Leaf template, calculate it in init() and store it:

public let hasCards: Bool
public let cardCount: Int

public init(cards: [CardViewModel]) {
    self.cards = cards
    self.hasCards = !cards.isEmpty
    self.cardCount = cards.count
}

ViewModelId Initialization - CRITICAL

IMPORTANT: Even though Leaf templates don't use vmId directly, the ViewModels being rendered must initialize vmId correctly for SwiftUI clients.

❌ WRONG - Never use this:

public var vmId: ViewModelId = .init()  // NO! Generic identity

✅ MINIMUM - Use type-based identity:

public var vmId: ViewModelId = .init(type: Self.self)

✅ IDEAL - Use data-based identity when available:

public struct TaskCardViewModel {
    public let id: ModelIdType
    public var vmId: ViewModelId

    public init(id: ModelIdType, /* other params */) {
        self.id = id
        self.vmId = .init(id: id)  // Ties view identity to data identity
        // ...
    }
}

Why this matters for Leaf ViewModels:

ViewModels are shared between Leaf (web) and SwiftUI (native) clients
SwiftUI uses .id(vmId) to determine when to recreate vs update views
Wrong identity = SwiftUI views don't update when data changes
Data-based identity (.init(id:)) is best practice

Common Mistakes

Missing Data Attributes

<!-- BAD - JS can't identify this element -->
<div class="{entity}-card">

<!-- GOOD - JS reads data-{entity}-id -->
<div class="{entity}-card" data-{entity}-id="#(card.id)">

Storing Display Names Instead of Identifiers

<!-- BAD - localized string can't be sent to server -->
<div data-status="#(card.statusDisplayName)">

<!-- GOOD - raw enum value works for requests -->
<div data-status="#(card.status)">

Using Layout in Fragments

<!-- BAD - fragment should not extend layout -->
#extend("base"):
#export("content"):
<div class="card">...</div>
#endexport
#endextend

<!-- GOOD - fragment is just the component -->
<div class="card">...</div>

Hardcoding Text

<!-- BAD - not localizable -->
<span class="status">Queued</span>

<!-- GOOD - ViewModel provides localized value -->
<span class="status">#(card.statusDisplayName)</span>

Concatenating Localized Values

<!-- BAD - breaks RTL languages and locale-specific word order -->
#(conversation.messageCount) #(conversation.messagesLabel)

<!-- GOOD - ViewModel composes via @LocalizedSubs -->
#(conversation.messageCountDisplay)

Template-level concatenation assumes left-to-right order. Use @LocalizedSubs in the ViewModel so YAML can define locale-appropriate ordering:

en:
  ConversationViewModel:
    messageCountDisplay: "%{messageCount} %{messagesLabel}"
ar:
  ConversationViewModel:
    messageCountDisplay: "%{messagesLabel} %{messageCount}"

Formatting Dates in Templates

<!-- BAD - hardcoded format, not locale-aware, concatenation issue -->
<span>#(content.createdPrefix) #date(content.createdAt, "MMM d, yyyy")</span>

<!-- GOOD - LocalizableDate handles locale formatting, @LocalizedSubs composes -->
<span>#(content.createdDisplay)</span>

Use LocalizableDate in the ViewModel - it formats according to user locale. If combining with a prefix, use @LocalizedSubs:

public let createdAt: LocalizableDate

@LocalizedSubs(\.createdPrefix, \.createdAt)
public var createdDisplay

Mismatched Filenames

<!-- BAD - filename doesn't match ViewModel -->
ViewModel: UserProfileCardViewModel
Template:  ProfileCard.leaf

<!-- GOOD - aligned names -->
ViewModel: UserProfileCardViewModel
Template:  UserProfileCardView.leaf

Incorrect ViewModelId Initialization

// ❌ BAD - Generic identity (breaks SwiftUI clients)
public var vmId: ViewModelId = .init()

// ✅ MINIMUM - Type-based identity
public var vmId: ViewModelId = .init(type: Self.self)

// ✅ IDEAL - Data-based identity (when id available)
public init(id: ModelIdType) {
    self.id = id
    self.vmId = .init(id: id)
}

ViewModels rendered by Leaf are often shared with SwiftUI clients. Correct vmId initialization is critical for SwiftUI's view identity system.

Rendering Errors in Leaf Templates

When a WebApp route catches an error, the error type is known at compile time. You don't need generic "ErrorViewModel" patterns:

// WebApp route - you KNOW the request type, so you KNOW the error type
app.post("move-idea") { req async throws -> Response in
    let body = try req.content.decode(MoveIdeaRequest.RequestBody.self)
    let serverRequest = MoveIdeaRequest(requestBody: body)

    do {
        try await serverRequest.processRequest(mvvmEnv: req.application.mvvmEnv)
        // success path...
    } catch let error as MoveIdeaRequest.ResponseError {
        // I KNOW this is MoveIdeaRequest.ResponseError
        // I KNOW it has .code and .message
        return try await req.view.render(
            "Shared/ToastView",
            ["message": error.message.value, "type": "error"]
        ).encodeResponse(for: req)
    }
}

The anti-pattern (JavaScript brain):

// ❌ WRONG - treating errors as opaque
catch let error as ServerRequestError {
    // "How do I extract the message? The protocol doesn't guarantee it!"
    // This is wrong thinking. You catch the CONCRETE type.
}

Each route handles its own specific error type. There's no mystery about what properties are available.

How to Use This Skill

Invocation: /fosmvvm-leaf-view-generator

Prerequisites:

ViewModel structure understood from conversation context
Template type determined (full-page vs fragment)
Data attributes needed for JS interactions identified
HTML-over-the-wire pattern understood if using fragments

Workflow integration: This skill is used when creating Leaf templates for web clients. The skill references conversation context automatically—no file paths or Q&A needed. Typically follows fosmvvm-viewmodel-generator.

Pattern Implementation

This skill references conversation context to determine template structure:

ViewModel Analysis

From conversation context, the skill identifies:

ViewModel type (from prior discussion or server implementation)
Properties (what data the template will display)
Localization (which properties are Localizable types)
Nested ViewModels (child components)

Template Type Detection

From ViewModel purpose:

Page content → Full-page template (extends layout)
List item/Card → Fragment (no layout, single root)
Modal content → Fragment
Inline component → Fragment

Property Mapping

For each ViewModel property:

id: ModelIdType → data-{entity}-id="#(vm.id)" (for JS)
Raw enum → data-{field}="#(vm.field)" (for state)
LocalizableString → #(vm.displayName) (display text)
LocalizableDate → #(vm.createdAt) (formatted date)
Nested ViewModel → Embed fragment or access properties

Data Attributes Planning

Based on JS interaction needs:

Entity identifier (for operations)
State values (enum raw values for requests)
Drag/drop attributes (if interactive)
Category/grouping (for filtering/sorting)

Template Generation

Full-page:

Layout extension
Content export
Embedded fragments for components

Fragment:

Single root element
Data attributes for state
Localized text from ViewModel
No layout extension

Context Sources

Skill references information from:

Prior conversation: Template requirements, user flows discussed
ViewModel: If Claude has read ViewModel code into context
Existing templates: From codebase analysis of similar views

Version History

Version	Date	Changes
1.0	2025-12-24	Initial Kairos-specific skill
2.0	2025-12-27	Generalized for FOSMVVM, added View-ViewModel alignment principle, full-page templates, architecture connection
2.1	2026-01-08	Added Leaf Built-in Functions section (count, contains, loop variables). Clarified Codable/computed properties. Corrected earlier false claims about #count() not working.
2.2	2026-01-19	Updated Pattern 3 to use stored LocalizableString for dynamic enum displays; linked to Enum Localization Pattern. Added anti-patterns for concatenating localized values and formatting dates in templates.
2.3	2026-01-20	Added "Rendering Errors in Leaf Templates" section - error types are known at compile time, no need for generic ErrorViewModel patterns. Prevents JavaScript-brain thinking about runtime type discovery.
2.4	2026-01-24	Update to context-aware approach (remove file-parsing/Q&A). Skill references conversation context instead of asking questions or accepting file paths.

Score

Total Score

65/100

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