# Milestone: Local-first Multiplayer Kanban

A board with drag-and-drop cards, 60fps pointer-follow, optimistic updates,
offline queue, conflict reconciliation, live presence, and SSR-first rendering —
all without React/Vue/VDOM, in a single typed Rust codebase.

This is the north-star integration test for slhx + hemplate + slhx-sync.

---

## 1. Template: `board.heml`

```html
<section data-slhx-slot="board" data-slhx-atom="board">
    <header>
        <h1>{self.title}</h1>

        <form data-slhx-handle="create_card">
            <input name="title" type="text" required>
            <select name="column">
                @for column in self.columns key column.id {
                    <option value="{column.id}">{column.title}</option>
                }
            </select>
            <button>Add card</button>
        </form>
    </header>

    <div class="columns" data-slhx-slot="columns">
        @for column in self.columns key column.id {
            <section
                class="column"
                data-slhx-slot="column"
                data-column-id="{column.id}"
            >
                <h2>{column.title}</h2>

                <div
                    class="cards"
                    data-slhx-dropzone="column"
                    data-column-id="{column.id}"
                >
                    @for card in column.cards key card.id {
                        <article
                            class="card"
                            data-slhx-slot="card"
                            data-slhx-handle="drag_card"
                            data-card-id="{card.id}"
                            draggable="true"
                        >
                            <strong>{card.title}</strong>
                            <small>{card.assignee}</small>
                        </article>
                    }
                </div>
            </section>
        }
    </div>

    <aside data-slhx-slot="presence">
        @for user in self.online_users key user.id {
            <span>{user.name}</span>
        }
    </aside>
</section>
```

Notes on keyed scopes:

- `@for column key column.id` — **required** for slhx-addressable nodes inside
- `@for card key card.id` — **required**
- A slot inside a keyed loop is addressed as `(SlotId, KeyValue)`, never `querySelector(".card:nth-child(3)")`
- Without `key`, slhx rejects the build — no runtime selector fallback

---

## 2. What hemplate exports

hemplate does **not** interpret `data-slhx-*`. It records raw facts:

```rust
Node {
    id: NodeId(12),
    element: "article",
    attrs: [
        ("class", "card"),
        ("data-slhx-slot", "card"),
        ("data-slhx-handle", "drag_card"),
        ("data-card-id", "{card.id}"),
        ("draggable", "true"),
    ],
    scope: ScopeId(For { binding: "card", key_expr: "card.id" }),
}

FormSurface {
    handle_attr: Some("create_card"),
    controls: [
        Control { name: "title", kind: Text, required: true },
        Control { name: "column", kind: Select, required: true },
    ],
}
```

slhx reads this from `hemplate.surface.postcard` and generates typed constants:

```rust
pub const CARD: KeyedSlot<CardId, CardView> = KeyedSlot::new(3);
pub const CREATE_CARD: Handle<CreateCardForm> = Handle::new(0);
```

No string desync. No runtime mapping.

---

## 3. App State

```rust
#[slhx::app]
pub struct BoardApp {
    pub board: Atom<BoardState>,
    pub drag: Atom<Option<DragState>>,
    pub online_users: Atom<Vec<UserPresence>>,
}
```

The same struct runs on server (SSR) and in WASM (client-local effects).

---

## 4. Normal Form: Server-first

```rust
#[derive(SlhxForm)]
pub struct CreateCardForm {
    pub title: String,
    pub column: ColumnId,
}

#[slhx::handler]
pub fn create_card(
    form: Form<CreateCardForm>,
    app: &mut BoardApp,
) -> impl IntoEffect {
    let card = Card { id: CardId::new(), title: form.title, assignee: "Thomas".into() };

    app.board.update(|board| board.insert_card(form.column, card.clone()));

    Effect::batch((
        Effect::append_keyed(slots::CARD, card.id, CardView::from(card)),
        // slhx-sync: queue atomic board state diff for sync
        SyncEffect::send_patch(atoms::BOARD, Patch::insert_card(form.column, card)),
    ))
}
```

HTML submits as usual. Server returns `EffectBatch`. Browser applies DOM ops.

---

## 5. Drag: 60fps client-local WASM

```rust
#[slhx::handler(client)]
pub fn drag_card(
    event: DragEvent,
    app: &mut BoardApp,
) -> impl IntoEffect {
    app.drag.set(Some(DragState {
        card_id: event.card_id,
        from_column: event.column_id,
        pointer_x: event.x,
        pointer_y: event.y,
    }));

    Effect::batch((
        Effect::class_keyed(slots::CARD, event.card_id, "dragging", true),
        Effect::transform_keyed(
            slots::CARD, event.card_id,
            Transform::translate(event.x, event.y),
        ),
    ))
}
```

Zero round-trip. Zero custom JS. Pure Rust → EffectBatch → DOM.

---

## 6. Drop: optimistic update + sync

```rust
#[slhx::handler(client)]
pub fn drop_card(
    event: DropEvent,
    app: &mut BoardApp,
) -> impl IntoEffect {
    let patch = app.board.update(|board| {
        board.move_card(event.card_id, event.to_column, event.before_card)
    });

    app.drag.set(None);

    Effect::batch((
        Effect::move_keyed(
            slots::CARD, event.card_id,
            slots::COLUMN, event.to_column,
            InsertBefore(event.before_card),
        ),
        Effect::class_keyed(slots::CARD, event.card_id, "dragging", false),
        // slhx-sync: queue patch, send when online
        SyncEffect::send_patch(atoms::BOARD, patch),
    ))
}
```

A pure htmx+SSR app cannot model this: 60fps pointer → local transient drag → optimistic update → offline queue → reconciliation. You'd need custom JS or a parallel React/Vue layer.

slhx models it in one type graph.

---

## 7. Server reconciliation

```rust
#[slhx_sync::handler]
pub fn apply_board_patch(
    patch: BoardPatch,
    app: &mut BoardApp,
    user: UserId,
) -> impl IntoEffect {
    let result = app.board.update(|board| board.apply_patch_from(user, patch));

    match result {
        PatchResult::Accepted { changed_cards } => Effect::batch((
            Effect::ack(atoms::BOARD),
            Effect::broadcast(
                Channel::Board(app.board.id()),
                Effect::batch(changed_cards.into_iter().map(|c|
                    Effect::replace_keyed(slots::CARD, c.id, CardView::from(c))
                )),
            ),
        )),

        PatchResult::Conflict { canonical_board } => Effect::batch((
            Effect::set(atoms::BOARD, canonical_board.clone()),
            Effect::render(slots::BOARD, BoardView::from(canonical_board)),
        )),
    }
}
```

Server-authoritative on conflict. No Redux sagas. No React Query cache fades.

---

## 8. Presence

```rust
#[slhx_sync::presence]
pub fn user_joined(user: UserPresence) -> impl IntoEffect {
    Effect::append_keyed(slots::PRESENCE_USER, user.id, PresenceBadge::from(user))
}
```

Browser receives raw `EffectBatch` over WebSocket/SSE:

```text
Op::AppendKeyed(slot=PRESENCE_USER, key=user_id, html=...)
Op::RemoveKeyed(slot=PRESENCE_USER, key=user_id)
```

The runtime does not know "presence". It executes ops.

---

## 9. What the browser receives

Initial SSR:

```html
<section data-sid="0" data-aid="0">
    ...
    <article data-sid="3" data-sk="42" data-hid="1">
        Fix login bug
    </article>
    ...
</section>
<script src="/slhx.js"></script>
<script type="application/slhx-state">
BASE64URL_POSTCARD_INITIAL_ATOMS
</script>
```

Runtime attachment:

```js
document.addEventListener("submit", dispatch)
document.addEventListener("click", dispatch)
document.addEventListener("pointerdown", dispatch)
document.addEventListener("pointermove", dispatch)
document.addEventListener("pointerup", dispatch)
```

No framework download. No VDOM. No hydration. No game loop.

---

## 10. Why this is not a React/Vue/htmx app

| Concern | React/Vue | htmx+SSR | slhx |
|---|---|---|---|
| SSR | RSC/Vue SSR | native | native (hemplate) |
| 60fps drag | 100ms re-render + React-DnD | custom JS | WASM handler, EffectBatch |
| Optimistic update | useOptimistic | impossible | `board.update` → `SyncEffect::send_patch` |
| Offline support | Service Worker + custom | impossible | patch queue in `slhx-sync` |
| Conflict resolution | manual / Yjs CRDT | impossible | server-authoritative patch |
| Presence | WebSocket + custom state | SSE possible | `Effect::broadcast` over channel |
| Keyed DOM | React key | not a concern | `KeydSlot<T, K>` compile-time |
| Forms | React Hook Form | HTML native, but no validation bridge | `Fork<T>` derived from `.heml` surface |
| Routing | React Router / Vue Router | HTML links, but no state routing | `Effect::navigate` with scroll/title |
| Total JS shipped | ~300KB+ | ~20KB htmx + custom | ~3KB slhx.js interpreter |

---

## 11. The claim

```text
A local-first multiplayer board where all high-frequency UI runs as Rust/WASM effects,
all durable state syncs through slhx-sync,
all HTML is hemplate-rendered,
and the browser runtime only executes typed postcard DOM ops.
```

Not:

```text
server Rust here
client TypeScript there
shared schema somewhere
validation duplicated
DOM identity by selectors
state sync by convention
```

But:

```text
Rust owns types.
hemplate owns structure.
slhx owns interaction.
browser executes ops.
```