f8/sdk/dart/package-fidl_fidl/decodeMessageWithCallback.md

# decodeMessageWithCallback<A> function










A decodeMessageWithCallback
<A>([IncomingMessage](../package-fidl_fidl/IncomingMessage-class.md) message, int inlineSize, A f([Decoder](../package-fidl_fidl/Decoder-class.md) decoder))



<p>Decodes a FIDL message with multiple parameters.  The callback parameter
provides a decoder that is initialized on the provided Message, which
callers can use to decode specific types.  The return result of the callback
(e.g. the decoded parameters, wrapped in a containing class/struct) is
returned as the result of the function.  This functionality (decoding
multiple parameters) is implemented using a callback because returning a
list would be insufficient: the list would be of type List<fidltype>,
whereas we want to retain concrete types of each decoded parameter.  The
only way to accomplish this in Dart is to pass in a function that collects
these multiple values into a bespoke, properly typed class.</fidltype></p>



## Implementation

```dart
A decodeMessageWithCallback<A>(
    IncomingMessage message, int inlineSize, A Function(Decoder decoder) f) {
  final int size = kMessageHeaderSize + inlineSize;
  final Decoder decoder = Decoder(message)..claimBytes(size, 0);
  A out = f(decoder);
  final int padding = align(size) - size;
  decoder.checkPadding(size, padding);
  _validateDecoding(decoder);
  return out;
}
```