registerServiceExtension method

  1. @protected
void registerServiceExtension({
  1. required String name,
  2. required ServiceExtensionCallback callback,
})

Registers a service extension method with the given name (full name "ext.flutter.name").

The given callback is called when the extension method is called. The callback must return a Future that either eventually completes to a return value in the form of a name/value map where the values can all be converted to JSON using json.encode() (see JsonEncoder), or fails. In case of failure, the failure is reported to the remote caller and is dumped to the logs.

The returned map will be mutated.

A registered service extension can only be activated if the vm-service is included in the build, which only happens in debug and profile mode. Although a service extension cannot be used in release mode its code may still be included in the Dart snapshot and blow up binary size if it is not wrapped in a guard that allows the tree shaker to remove it (see sample code below).

The following code registers a service extension that is only included in debug builds.
link 
void myRegistrationFunction() {
  assert(() {
    // Register your service extension here.
    return true;
  }());
}

A service extension registered with the following code snippet is available in debug and profile mode.
link 
void myOtherRegistrationFunction() {
  // kReleaseMode is defined in the 'flutter/foundation.dart' package.
  if (!kReleaseMode) {
    // Register your service extension here.
  }
}

Both guards ensure that Dart's tree shaker can remove the code for the service extension in release builds.

Implementation

@protected
void registerServiceExtension({
  required String name,
  required ServiceExtensionCallback callback,
}) {
  final String methodName = 'ext.flutter.$name';
  developer.registerExtension(methodName, (String method, Map<String, String> parameters) async {
    assert(method == methodName);
    assert(() {
      if (debugInstrumentationEnabled) {
        debugPrint('service extension method received: $method($parameters)');
      }
      return true;
    }());

    // VM service extensions are handled as "out of band" messages by the VM,
    // which means they are handled at various times, generally ASAP.
    // Notably, this includes being handled in the middle of microtask loops.
    // While this makes sense for some service extensions (e.g. "dump current
    // stack trace", which explicitly doesn't want to wait for a loop to
    // complete), Flutter extensions need not be handled with such high
    // priority. Further, handling them with such high priority exposes us to
    // the possibility that they're handled in the middle of a frame, which
    // breaks many assertions. As such, we ensure they we run the callbacks
    // on the outer event loop here.
    await debugInstrumentAction<void>('Wait for outer event loop', () {
      return Future<void>.delayed(Duration.zero);
    });

    late Map<String, dynamic> result;
    try {
      result = await callback(parameters);
    } catch (exception, stack) {
      FlutterError.reportError(FlutterErrorDetails(
        exception: exception,
        stack: stack,
        context: ErrorDescription('during a service extension callback for "$method"'),
      ));
      return developer.ServiceExtensionResponse.error(
        developer.ServiceExtensionResponse.extensionError,
        json.encode(<String, String>{
          'exception': exception.toString(),
          'stack': stack.toString(),
          'method': method,
        }),
      );
    }
    result['type'] = '_extensionType';
    result['method'] = method;
    return developer.ServiceExtensionResponse.result(json.encode(result));
  });
}