> ## Documentation Index
> Fetch the complete documentation index at: https://www.canva.dev/docs/apps/llms.txt
> Use this file to discover all available pages before exploring further.

# appProcess.registerOnMessage

API reference for the appProcess.registerOnMessage method.

Registers a callback that listens for broadcasted messages.

## Usage: Listen for inter-process messages

```typescript
import { appProcess } from '@canva/platform';

const messageDisposer = appProcess.registerOnMessage(async (sender, message) => {
  const { appProcessId, surface } = sender;
  // Handle message from other process
});

// Later: cleanup the listener
await messageDisposer();
```

## Parameters

<Prop.List>
  <Prop name="callback" required type="OnMessageCallback" sourceLineNumbers={[120]}>
    The callback that listens for broadcasted messages.

    **Parameters**

    <Prop.List>
      <Prop name="sender" required type="object" sourceLineNumbers={[518]}>
        Information about the process that sent the message.

        * sender.appProcessId - The ID of the process that sent the message.
        * sender.surface - The surface of the process that sent the message.

        <PillAccordion defaultExpanded={false} title={<>Properties of <strong>sender</strong></>}>
          <Prop.List>
            <Prop name="appProcessId" required type="AppProcessId" sourceLineNumbers={[519]}>
              The unique identifier of an app process.
            </Prop>

            <Prop name="surface" required type="AppSurface" sourceLineNumbers={[520]}>
              The type of surface on which an app process can run.

              The possible surfaces include:

              * `"headless"` - A surface for when there is no visible user interface.
              * `"object_panel"` - A surface that renders a user interface in the side panel of the Canva editor.
              * `"selected_image_overlay"` - A surface that can be opened on top of a selected image.

              <Prop.Extras>
                **Available values**:

                * `"object_panel"`
                * `"selected_image_overlay"`
                * `"headless"`
              </Prop.Extras>
            </Prop>
          </Prop.List>
        </PillAccordion>
      </Prop>

      <Prop name="message" required type="any" sourceLineNumbers={[518]}>
        The broadcasted message.
      </Prop>
    </Prop.List>

    **Returns**

    `Promise<void>`
  </Prop>
</Prop.List>

## Returns

A disposer function that cleans up the registered callback.

() => `Promise<void>`