Using Socket in Flutter apps?

Question

Asked by user9239214 on June 28, 2018 (source).

I create a Flutter App. I need to connect my app to local network socket services. As shown below, I can use telnet Connect, Send data and Receive data from the server. I use Flutter web_socket plugin and example. I can connect to the server and send the data but I cannot catch (or get data, it doesn't show anything.) the data. In Flutter google groups one person advised me to use stream instead of StreamBuilder.

To send data I use;         Q101:_:49785:_:ABCDE
And receive data I get;     1:_:2:_:119351:_:NİYAZİ TOROS 

And when I use this example (https://flutter.io/cookbook/networking/web-sockets/) I am getting error on my socket service as:

Q: 28.06.2018 08:53:57->GET / HTTP/1.1
A: 28.06.2018 08:53:57 ->:1:_:1:_:FAIL1

Example:

Last login: Tue Jun 26 15:01:44 on ttys000
Niyazis-MBP:~ niyazitoros$ telnet
telnet> telnet 192.168.1.22 1024
Trying 192.168.1.22...
Connected to 192.168.1.22.
Escape character is '^]'.
Q101:_:49785:_:*************
1:_:2:_:119351:_:NİYAZİ TOROS

Based on @Richard Heap suggestion:

import 'dart:async';
import 'dart:convert';
import 'dart:io';

void connect(InternetAddress clientAddress, int port) {
  Future.wait([RawDatagramSocket.bind(InternetAddress.anyIPv4, 0)]).then(
      (values) {
    RawDatagramSocket _socket = values[0];
    _socket.listen((RawSocketEvent e) {
      print(e);
      switch (e) {
        case RawSocketEvent.read:
          Datagram dg = _socket.receive();
          if (dg != null) {
            dg.data.forEach((x) => print(x));
          }
          _socket.writeEventsEnabled = true;
          break;
        case RawSocketEvent.write:
          _socket.send(
              new Utf8Codec().encode('Hello from client'), clientAddress, port);
          break;
        case RawSocketEvent.closed:
          print('Client disconnected.');
      }
    });
  });
}

main(List<String> arguments) {
  print("Connecting to server..");
  var address = new InternetAddress('192.168.1.22');
  int port = 1024;
  connect(address, port);
}

And I get this:

/Users/niyazitoros/flutter/bin/cache/dart-sdk/bin/dart --enable-asserts --enable-vm-service:59683 /Users/niyazitoros/IdeaProjects/github/untitled/bin/main.dart
Observatory listening on http://127.0.0.1:59683/

Connecting to the server.
RawSocketEvent.write

Answer

Question answered by MahMoos (source).

As attdona mentioned,

Your server does not speak the websocket protocol but it exposes a plain tcp socket.

So you need a TCP socket and there is a great tutorial on Sockets and ServerSockets which you can find here.

Here's a snippet:

import 'dart:io';
import 'dart:async';

Socket socket;

void main() {
   Socket.connect("localhost", 4567).then((Socket sock) {
   socket = sock;
   socket.listen(dataHandler, 
      onError: errorHandler, 
      onDone: doneHandler, 
      cancelOnError: false);
   }).catchError((AsyncError e) {
      print("Unable to connect: $e");
   });
   //Connect standard in to the socket 
   stdin.listen((data) => socket.write(new String.fromCharCodes(data).trim() + '\n'));
}

void dataHandler(data){
   print(new String.fromCharCodes(data).trim());
}

void errorHandler(error, StackTrace trace){
   print(error);
}

void doneHandler(){
   socket.destroy();
}
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