Example of how to generate GRPC APIs from protobufs and use them in Dart.

grpc_googleapis_example.dart

// Copyright 2019 Google LLC.
// SPDX-License-Identifier: Apache-2.0
//
// Experiments with dart, grpc and googleapis
// ==========================================
//
// 1. Install `protoc`
// 2. `pub global activate protoc_plugin`
// 3. `git clone https://github.com/googleapis/googleapis`
// 4. `protoc --dart_out=grpc:lib/src/generated -I googleapis/ -I /usr/local/include/ googleapis/google/cloud/vision/v1/image_annotator.proto`
//
import 'package:dart_grpc_googleapis/src/generated/google/cloud/vision/v1/image_annotator.pb.dart';
import 'package:dart_grpc_googleapis/src/generated/google/cloud/vision/v1/image_annotator.pbgrpc.dart';
import 'package:path/path.dart';
import 'package:grpc/grpc.dart';

import 'dart:io';

// Assuming image.jpg and credentials.json are next the current file.
var _imageFile = join(dirname(Platform.script.path), 'image.jpg');
var _serviceAccountFile =
    join(dirname(Platform.script.path), 'credentials.json');

void main() async {
  // Find the service end-point in .proto files or reference documentation.
  final serviceName = "vision.googleapis.com";
  final port = 443;

  // Setup a channel
  var channel = ClientChannel(serviceName,
      port: port,
      options: ChannelOptions(
        credentials: ChannelCredentials.secure(),
      ));

  // Authenticate using credentials
  var auth = JwtServiceAccountAuthenticator(
      await File(_serviceAccountFile).readAsString());
  var client = ImageAnnotatorClient(channel, options: auth.toCallOptions);

  // Use the generated API clients
  var res = await client.batchAnnotateImages(BatchAnnotateImagesRequest()
    ..requests.add(AnnotateImageRequest()
      ..image = (Image()..content = await File(_imageFile).readAsBytes())
      ..features.add(Feature()..type = Feature_Type.LABEL_DETECTION)));

  for (var label in res.responses[0].labelAnnotations) {
    print(label.description);
  }

  // Close the channel
  await channel.shutdown();
}