f9/sdk/dart/package-fidl_fidl/ProxyController/getCallback.md

# getCallback method








Function getCallback
(int txid)





<p>Returns the callback associated with the given response message.</p>
<p>Used by subclasses of <a href="../../package-fidl_fidl/Proxy-class.md">Proxy&lt;T&gt;</a> to retrieve registered callbacks when
handling response messages.</p>



## Implementation

```dart
Function? getCallback(int txid) {
  final Function? result = _callbackMap.remove(txid);
  if (result == null) {
    proxyError('Message had unknown request id: $txid');
    return null;
  }
  return result;
}
```