# Watchers

React to state changes by triggering actions when watched paths update.

## The `watch` Field

Elements can have an optional `watch` field that maps state paths to action bindings. When the value at a watched path changes, the bound actions fire automatically.

`watch` is a **top-level field** on the element (sibling of `type`, `props`, `children`) — not inside `props`.

```json
{
  "type": "Select",
  "props": {
    "label": "Country",
    "value": { "$bindState": "/form/country" },
    "options": ["US", "Canada", "UK"]
  },
  "watch": {
    "/form/country": {
      "action": "loadCities",
      "params": { "country": { "$state": "/form/country" } }
    }
  },
  "children": []
}
```

When the user selects a different country, the `loadCities` action fires with the new country value. The action handler can fetch city data and update state, causing a dependent city Select to re-render with new options.

## Cascading Selects

A common pattern is cascading dropdowns where selecting a value in one field loads options for another:

```json
{
  "root": "form",
  "elements": {
    "form": {
      "type": "Stack",
      "props": { "direction": "vertical", "gap": "md" },
      "children": ["country-select", "city-select"]
    },
    "country-select": {
      "type": "Select",
      "props": {
        "label": "Country",
        "value": { "$bindState": "/form/country" },
        "options": ["US", "Canada", "UK"]
      },
      "watch": {
        "/form/country": [
          { "action": "loadCities", "params": { "country": { "$state": "/form/country" } } },
          { "action": "setState", "params": { "statePath": "/form/city", "value": "" } }
        ]
      },
      "children": []
    },
    "city-select": {
      "type": "Select",
      "props": {
        "label": "City",
        "value": { "$bindState": "/form/city" },
        "options": { "$state": "/availableCities" },
        "placeholder": "Select a city"
      },
      "children": []
    }
  },
  "state": {
    "form": { "country": "", "city": "" },
    "availableCities": []
  }
}
```

The watcher on `country-select` fires two actions when the country changes:
1. `loadCities` — fetches and writes city options to `/availableCities`
2. `setState` — resets the city selection

The city Select reads its options from `{ "$state": "/availableCities" }`, so it automatically updates when the data is loaded.

### Action Handler

```typescript
const handlers = {
  loadCities: async (params) => {
    const cities = await fetchCities(params.country);
    // setState is called by the runtime to write the result
    return cities;
  },
};
```

Or with `defineRegistry`:

```typescript
const { registry, handlers } = defineRegistry(catalog, {
  components: { /* ... */ },
  actions: {
    loadCities: async (params, setState) => {
      const response = await fetch(`/api/cities?country=${params.country}`);
      const cities = await response.json();
      setState('/availableCities', cities);
    },
  },
});
```

## Multiple Watchers

An element can watch multiple state paths. Each path maps to one or more action bindings:

```json
{
  "watch": {
    "/form/startDate": { "action": "validateDateRange" },
    "/form/endDate": { "action": "validateDateRange" },
    "/form/quantity": [
      { "action": "recalculateTotal" },
      { "action": "checkInventory", "params": { "qty": { "$state": "/form/quantity" } } }
    ]
  }
}
```

## Behavior

- Watchers only fire on **value changes**, not on the initial render
- Comparison is by reference (`===`), not deep equality
- Action params support the same expressions as event bindings (`$state`, `$item`, `$index`)
- Multiple action bindings on the same path execute sequentially

## When to Use `watch` vs `on`

<table>
<thead>
<tr>
<th>Mechanism</th>
<th>Trigger</th>
<th>Use Case</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>on</code></td>
<td>User interaction (press, change, blur)</td>
<td>Button clicks, input changes, form submissions</td>
</tr>
<tr>
<td><code>watch</code></td>
<td>State value change (any source)</td>
<td>Cascading data, derived state, cross-field sync</td>
</tr>
</tbody>
</table>

Use `on` when reacting to direct user actions. Use `watch` when a state change (from any source — user input, action handler, or external store update) should trigger side effects.

## Next

- [Data Binding](/docs/data-binding) — connect elements to state
- [Computed Values](/docs/computed-values) — derive prop values
- [Visibility](/docs/visibility) — conditionally show or hide elements