Appsync Events implementation for flutter SDK

[matta-pie]

Would it be possible to add the implementation for publishing/subscribing to AWS AppSync Events endpoints?

There's already an implementation for all javascript based frameworks
javascript doc

[jamilsaadeh97]

I would also add that this would be very useful for us if it would be supported out of the box

[tyllark]

Hello @matta-pie, thanks for taking the time to open this issue. We will keep this open as a feature parity request, please give your description a thumbs to help us gauge community interest.

In the meantime you might be able to use our aws_signature_v4 package to call AWS APIs Amplify doesn't covered yet.

[matta-pie]

@tyllark could you explain more about the alternative solution you proposed? in particular i need to subscribe to an appsync event (namespace/channel) using JWT auth. (i guess auth type doesn't change much). from what im seeing, your proposed solution could work to send messages, not subscribing to events.

[tyllark]

Hello @matta-pie, I used an API key to test the following code, but you should be able to update the authorization mapping to match you preferred subprotocol format as documented here. While I used an API key for simplicity I recommend using Authorization in your production frontend code.

'authorization': {
  'x-api-key': _apiKey,
  'host': _httpDomain,
},

Your DNS Endpoints (and API key) can be found at AWS Console -> AWS AppSync -> APIs -> [Your Event API] -> Settings. I got my AWSCredentials via the aws cli command aws sts get-session-token. Finally you will need add web_socket_channel: ^3.0.2 and uuid: ^4.5.1 to your pubspec.yaml.

final String _httpDomain = '[YOUR_HTTP_DOMAIN]';
final String _realtimeDomain = '[YOUR_REALTIME_DOMAIN]';
final String _apiKey = '[YOUR_API_KEY]';

Future<void> eventApiExample() async {
  String channel = 'default/myChannel';
  final events = [
    '{"event 1" : "data 1"}',
    '{"event 2" : "data 2"}',
    '{"event 3" : "data 3"}',
    '{"event 4" : "data 4"}',
  ];

  final websocket = await _createEventApiWebsocket(channel);
  websocket.stream.listen(_onEvent);

  await _callEventApiRest(channel, events);
}

void _onEvent(dynamic event) {
  safePrint('Event Recieved: $event');
}

Future<void> _callEventApiRest(String channel, List<String> events) async {
  final signer = AWSSigV4Signer(
    credentialsProvider: AWSCredentialsProvider(
      AWSCredentials(
        AwsConstants.accessKey,
        AwsConstants.secretAccessKey,
        AwsConstants.sessionToken,
        AwsConstants.expiration,
      ),
    ),
  );

  final scope = AWSCredentialScope(
    region: AwsConstants.region,
    service: AWSService.appSync,
  );

  final jsonBody = json.encode({'channel': channel, 'events': events});
  final request = AWSHttpRequest(
    method: AWSHttpMethod.post,
    headers: {
      AWSHeaders.contentType: 'application/json',
      'x-api-key': _apiKey,
    },
    uri: Uri.https(_httpDomain, '/event'),
    body: jsonBody.codeUnits,
  );

  final signedRequest = await signer.sign(request, credentialScope: scope);

  final operation = signedRequest.send();
  final response = await operation.response;
  final body = await response.decodeBody();

  safePrint(response.statusCode);
  safePrint(response.headers);
  safePrint(body);
}

Future<WebSocketChannel> _createEventApiWebsocket(String channel) async {
  final signer = AWSSigV4Signer(
    credentialsProvider: AWSCredentialsProvider(
      AWSCredentials(
        AwsConstants.accessKey,
        AwsConstants.secretAccessKey,
        AwsConstants.sessionToken,
        AwsConstants.expiration,
      ),
    ),
  );

  final scope = AWSCredentialScope(
    region: AwsConstants.region,
    service: AWSService.appSync,
  );

  final jsonBody = json.encode({'channel': channel});
  final request = AWSHttpRequest(
    method: AWSHttpMethod.get,
    headers: {
      AWSHeaders.contentType: 'application/json',
      'x-api-key': _apiKey,
      'host': _httpDomain,
    },
    uri: Uri(scheme: 'wss', host: _realtimeDomain, path: '/event/realtime'),
    body: jsonBody.codeUnits,
  );

  final uri = await signer.presign(
    request,
    credentialScope: scope,
    expiresIn: Duration(hours: 1),
  );

  final headerJson = jsonEncode({'x-api-key': _apiKey, 'host': _httpDomain});
  var header = base64.encode(utf8.encode(headerJson));
  header = header.replaceAll('+', '-');
  header = header.replaceAll('/', '-');
  header = header.replaceAll('=', '');

  final websocket = WebSocketChannel.connect(
    uri,
    protocols: ['aws-appsync-event-ws', 'header-$header'],
  );
  await websocket.ready;

  final uuid = Uuid();
  final subscribe = {
    'type': 'subscribe',
    'id': uuid.v4(),
    'channel': channel,
    'authorization': {
      'x-api-key': _apiKey,
      'host': _httpDomain,
    },
  };

  websocket.sink.add(jsonEncode(subscribe));

  final event = {
    'type': 'publish',
    'id': uuid.v4(),
    'channel': channel,
    'events': ['{"socket event 1" : "socket data 1"}',],
    'authorization': {
      'x-api-key': _apiKey,
      'host': _httpDomain,
    },
  };

  websocket.sink.add(jsonEncode(event));

  return websocket;
}