fosmvvm-leaf-view-generator
# FOSMVVM Leaf View Generator
Generate Leaf templates that render ViewModels for web clients.
> **Architecture context:** See [FOSMVVMArchitecture.md](../../docs/FOSMVVMArchitecture.md) | [OpenClaw reference]({baseDir}/references/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:**
```swift
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:
```html
<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
```javascript
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:**
```html
<span class="date">#(card.createdAt)</span>
<!-- Renders: "Dec 27, 2025" (localized) -->
```
**If Localizable types render incorrectly** (showing `[ds: "2", ls: "...", v: "..."]`):
1. Ensure FOSMVVMVapor is imported
2. Check `Localizable+Leaf.swift` exists with conformances
3. 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](../fosmvvm-viewmodel-generator/SKILL.md#enum-localization-pattern).
```swift
@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)
}
```
```html
<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:
```html
<!-- {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:**
1. NO `#extend("base")` - fragments don't use layouts
2. **Single root element** - makes DOM swapping clean
3. All required state in data-* attributes
4. Localized values from ViewModel properties
### Pattern 5: Full-Page Structure
Full pages extend a base layout:
```html
<!-- {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
```html
#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
```html
<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:
```html
<!-- 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:
```html
#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:
```swift
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.
```swift
// 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:
```swift
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:**
```swift
public var vmId: ViewModelId = .init() // NO! Generic identity
```
**✅ MINIMUM - Use type-based identity:**
```swift
public var vmId: ViewModelId = .init(type: Self.self)
```
**✅ IDEAL - Use data-based identity when available:**
```swift
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
```html
<!-- 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
```html
<!-- 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
```html
<!-- 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
```html
<!-- BAD - not localizable -->
<span class="status">Queued</span>
<!-- GOOD - ViewModel provides localized value -->
<span class="status">#(card.statusDisplayName)</span>
```
### Concatenating Localized Values
```html
<!-- 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:
```yaml
en:
ConversationViewModel:
messageCountDisplay: "%{messageCount} %{messagesLabel}"
ar:
ConversationViewModel:
messageCountDisplay: "%{messagesLabel} %{messageCount}"
```
### Formatting Dates in Templates
```html
<!-- 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`:
```swift
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
```swift
// ❌ 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:
```swift
// 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):**
```swift
// ❌ 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:**
1. Layout extension
2. Content export
3. Embedded fragments for components
**Fragment:**
1. Single root element
2. Data attributes for state
3. Localized text from ViewModel
4. 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
---
## See Also
- [Architecture Patterns](../shared/architecture-patterns.md) - Mental models (errors are data, type safety, etc.)
- [FOSMVVMArchitecture.md](../../docs/FOSMVVMArchitecture.md) - Full architecture
- [fosmvvm-viewmodel-generator](../fosmvvm-viewmodel-generator/SKILL.md) - Creates the ViewModels this skill renders
- [fosmvvm-serverrequest-generator](../fosmvvm-serverrequest-generator/SKILL.md) - Creates requests that return ViewModels
- [reference.md](reference.md) - Complete template examples
---
## 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. |
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