flutter-rp-example/lib/services/grpc_service.dart

import 'package:grpc/grpc.dart';
import 'package:grpc/grpc_or_grpcweb.dart';
import 'package:sampleapp/services/auth/auth.dart';

class AuthInterceptor extends ClientInterceptor {
  final AuthService authService;

  AuthInterceptor(this.authService);

  @override
  ResponseFuture<R> interceptUnary<Q, R>(
      ClientMethod<Q, R> method, Q request, CallOptions options, invoker) {
    final metadata = <String, String>{};
    metadata['authorization'] = 'Bearer ${authService.accessToken}';
    options = options.mergedWith(CallOptions(metadata: metadata));
    return invoker(method, request, options);
  }
}

class GrpcClient {
  static Client initializeClient(
    AuthService authService,
    String host,
    int port,
    Client Function(GrpcOrGrpcWebClientChannel, List<ClientInterceptor>)
        clientFactory,
  ) {
    final interceptor = AuthInterceptor(authService);
    final channel = GrpcOrGrpcWebClientChannel.toSingleEndpoint(
      host: host,
      port: port,
      transportSecure: true,
    );
    return clientFactory(channel, [interceptor]);
  }
}