# AUR0803

## Error Message

`AUR0803: "& updateTrigger" invalid usage. This binding behavior can only be applied to two-way/ from-view bindings.`

## Description

This error occurs when the `& updateTrigger` binding behavior is applied to a binding expression that is not a two-way (`.bind`) or from-view (`.from-view`) binding.

## Cause

The `& updateTrigger` binding behavior modifies *when* the view model is updated based on changes in the view (specifically, by listening to DOM events on the target element). This concept only applies to bindings that transfer data *from* the view *to* the view model.

These binding types are:

* **Two-way (`.bind`):** Data flows in both directions (view-to-model and model-to-view).
* **From-view (`.from-view`):** Data flows only from the view to the model.

Applying `& updateTrigger` to other binding types, such as:

* One-way (`.to-view`)
* One-time (`.one-time`)
* String interpolation (`${}`)
* Event bindings (`.trigger`, `.capture`)
* Reference bindings (`ref`) is invalid because these bindings do not inherently update the view model based on view changes, rendering the `updateTrigger` behavior meaningless in these contexts.

## Solution

1. **Apply Only to `.bind` or `.from-view`:** Ensure that `& updateTrigger` is only used with the `.bind` or `.from-view` binding commands.
2. **Remove from Other Bindings:** If `& updateTrigger` has been applied to an incompatible binding type, remove it.
3. **Choose Correct Binding Mode:** If you intended to update the view model based on view events, ensure you are using `.bind` or `.from-view` as the binding command.

## Example

```html
<!-- Incorrect: Applying updateTrigger to .to-view -->
<span textcontent.to-view="statusMessage & updateTrigger:'mousemove'"></span>

<!-- Incorrect: Applying updateTrigger to .one-time -->
<img src.one-time="imageUrl & updateTrigger:'load'">

<!-- Incorrect: Applying updateTrigger to .trigger -->
<button click.trigger="save() & updateTrigger:'mousedown'">Save</button>

<!-- Correct: Applying updateTrigger to .bind -->
<input type="text" value.bind="username & updateTrigger:'blur'">

<!-- Correct: Applying updateTrigger to .from-view -->
<select value.from-view="selectedOption & updateTrigger:'change'">
  <!-- options -->
</select>
```

```typescript
// View Model
import { customElement } from 'aurelia';

@customElement({ /* ... */ })
export class MyComponent {
  statusMessage = 'Ready';
  imageUrl = 'logo.png';
  username = 'user';
  selectedOption = '';

  save() { /* ... */ }
}
```

## Debugging Tips

* Check the binding command used immediately before the property name in the binding expression (e.g., `value.bind`, `textcontent.to-view`).
* Ensure the command is either `.bind` or `.from-view` when using `& updateTrigger`.
* Review why you added `& updateTrigger` to the binding. If it wasn't intended for a two-way or from-view scenario, it's likely unnecessary.


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