> For the complete documentation index, see [llms.txt](https://docs.aurelia.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.aurelia.io/developer-guides/error-messages/runtime-html/aur0818.md).

# AUR0818

## Error Message

`AUR0818: "& signal" invalid usage. At least one signal name must be passed to the signal behavior, e.g. "expr & signal:'my-signal'"`

## Description

This error occurs when the `& signal` binding behavior is used in a binding expression without providing any signal names as arguments. The purpose of the `signal` behavior is to listen for specific named signals, so it requires at least one signal name to function correctly.

## Cause

The sole cause of this error is using the `signal` binding behavior without specifying which signal(s) it should listen for.

Examples of incorrect usage:

* `${myValue & signal}`
* `value.bind="myProp & signal"`

## Solution

Provide one or more signal names as string arguments to the `& signal` binding behavior. Signal names are separated by colons (`:`).

Examples of correct usage:

* `${myValue & signal:'my-signal-name'}`
* `value.bind="myProp & signal:'signal-a':'signal-b'"`

## Example

```html
<!-- Assume myData should update when 'data-updated' or 'user-changed' signals occur -->

<!-- Incorrect: No signal names provided -->
<p>${myData & signal}</p>

<!-- Correct: Provide one signal name -->
<p>${myData & signal:'data-updated'}</p>

<!-- Correct: Provide multiple signal names -->
<p>${myData & signal:'data-updated':'user-changed'}</p>
```

```typescript
// View Model Example
import { customElement, resolve } from 'aurelia';
import { ISignaler } from '@aurelia/runtime-html';

@customElement({ /* ... */ })
export class MyComponent {
  private signaler = resolve(ISignaler);
  myData = 'Initial Data';
  userInfo = 'User A';

  refreshData() {
    this.myData = `Refreshed at ${new Date().toLocaleTimeString()}`;
    this.signaler.dispatchSignal('data-updated');
  }

  changeUser() {
    this.userInfo = `User ${Math.random() > 0.5 ? 'B' : 'C'}`;
    // Assuming myData calculation depends on userInfo indirectly,
    // we might signal both. Or maybe a different binding listens for 'user-changed'.
    this.signaler.dispatchSignal('user-changed');
    this.signaler.dispatchSignal('data-updated'); // Or re-signal data if it depends on user
  }
}
```

## Debugging Tips

* Locate the binding expression using `& signal` in your template.
* Ensure that you have added at least one colon-separated string literal after `& signal` representing the name(s) of the signal(s) to listen for.
* Verify that the signal names are spelled correctly and match the names used when dispatching signals via `ISignaler.dispatchSignal()`.


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