PyQt QTcpServer: How to return data to multiple clients?

Question

I am looking to create a QTcpServer using PyQt that can simultaneously return data to 2 or more clients. I assume that this will require threading.

Using the threadedfor

Answer 1

As was probably exasperatingly obvious to most of you, I didn't fully understand how to deal with threads! Not to worry, I have discovered a way to design a server that can send data to multiple clients with nary a secondary thread to be found.

Quite simple, really, but I'm not the quickest of cats at the best of times.

SERVER:

#!/usr/bin/env python3

import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *
from PyQt4.QtNetwork import *

PORT = 9999
SIZEOF_UINT32 = 4

class ServerDlg(QPushButton):

    def __init__(self, parent=None):
        super(ServerDlg, self).__init__(
                "&Close Server", parent)
        self.setWindowFlags(Qt.WindowStaysOnTopHint)

        self.tcpServer = QTcpServer(self)               
        self.tcpServer.listen(QHostAddress("0.0.0.0"), PORT)
        self.connect(self.tcpServer, SIGNAL("newConnection()"), 
                    self.addConnection)
        self.connections = []

        self.connect(self, SIGNAL("clicked()"), self.close)
        font = self.font()
        font.setPointSize(24)
        self.setFont(font)
        self.setWindowTitle("Server")

    def addConnection(self):
        clientConnection = self.tcpServer.nextPendingConnection()
        clientConnection.nextBlockSize = 0
        self.connections.append(clientConnection)

        self.connect(clientConnection, SIGNAL("readyRead()"), 
                self.receiveMessage)
        self.connect(clientConnection, SIGNAL("disconnected()"), 
                self.removeConnection)
        self.connect(clientConnection, SIGNAL("error()"), 
                self.socketError)

    def receiveMessage(self):
        for s in self.connections:
            if s.bytesAvailable() > 0:
                stream = QDataStream(s)
                stream.setVersion(QDataStream.Qt_4_2)

                if s.nextBlockSize == 0:
                    if s.bytesAvailable() < SIZEOF_UINT32:
                        return
                    s.nextBlockSize = stream.readUInt32()
                if s.bytesAvailable() < s.nextBlockSize:
                    return

                textFromClient = stream.readQString()
                s.nextBlockSize = 0
                self.sendMessage(textFromClient, 
                                 s.socketDescriptor())
                s.nextBlockSize = 0

    def sendMessage(self, text, socketId):
        for s in self.connections:
            if s.socketDescriptor() == socketId:
                message = "You> {}".format(text)
            else:
                message = "{}> {}".format(socketId, text)
            reply = QByteArray()
            stream = QDataStream(reply, QIODevice.WriteOnly)
            stream.setVersion(QDataStream.Qt_4_2)
            stream.writeUInt32(0)
            stream.writeQString(message)
            stream.device().seek(0)
            stream.writeUInt32(reply.size() - SIZEOF_UINT32)
            s.write(reply)

    def removeConnection(self):
        pass

    def socketError(self):
        pass


app = QApplication(sys.argv)
form = ServerDlg()
form.show()
form.move(0, 0)
app.exec_()

CLIENT

import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *
from PyQt4.QtNetwork import *

PORTS = (9998, 9999)
PORT = 9999
SIZEOF_UINT32 = 4

class Form(QDialog):

    def __init__(self, parent=None):
        super(Form, self).__init__(parent)

        # Ititialize socket
        self.socket = QTcpSocket()

        # Initialize data IO variables
        self.nextBlockSize = 0
        self.request = None

        # Create widgets/layout
        self.browser = QTextBrowser()
        self.lineedit = QLineEdit("Enter text here, dummy")
        self.lineedit.selectAll()
        self.connectButton = QPushButton("Connect")
        self.connectButton.setEnabled(True)
        layout = QVBoxLayout()
        layout.addWidget(self.browser)
        layout.addWidget(self.lineedit)
        layout.addWidget(self.connectButton)
        self.setLayout(layout)
        self.lineedit.setFocus()

        # Signals and slots for line edit and connect button
        self.lineedit.returnPressed.connect(self.issueRequest)
        self.connectButton.clicked.connect(self.connectToServer)

        self.setWindowTitle("Client")
        # Signals and slots for networking
        self.socket.readyRead.connect(self.readFromServer)
        self.socket.disconnected.connect(self.serverHasStopped)
        self.connect(self.socket,
                     SIGNAL("error(QAbstractSocket::SocketError)"),
                     self.serverHasError)

    # Update GUI
    def updateUi(self, text):
        self.browser.append(text)

    # Create connection to server
    def connectToServer(self):
        self.connectButton.setEnabled(False)
        self.socket.connectToHost("localhost", PORT)

    def issueRequest(self):
        self.request = QByteArray()
        stream = QDataStream(self.request, QIODevice.WriteOnly)
        stream.setVersion(QDataStream.Qt_4_2)
        stream.writeUInt32(0)
        stream.writeQString(self.lineedit.text())
        stream.device().seek(0)
        stream.writeUInt32(self.request.size() - SIZEOF_UINT32)
        self.socket.write(self.request)
        self.nextBlockSize = 0
        self.request = None
        self.lineedit.setText("")

    def readFromServer(self):
        stream = QDataStream(self.socket)
        stream.setVersion(QDataStream.Qt_4_2)

        while True:
            if self.nextBlockSize == 0:
                if self.socket.bytesAvailable() < SIZEOF_UINT32:
                    break
                self.nextBlockSize = stream.readUInt32()
            if self.socket.bytesAvailable() < self.nextBlockSize:
                break
            textFromServer = stream.readQString()
            self.updateUi(textFromServer)
            self.nextBlockSize = 0

    def serverHasStopped(self):
        self.socket.close()
        self.connectButton.setEnabled(True)

    def serverHasError(self):
        self.updateUi("Error: {}".format(
                self.socket.errorString()))
        self.socket.close()
        self.connectButton.setEnabled(True)


app = QApplication(sys.argv)
form = Form()
form.show()
app.exec_()

To summarize, each client connection opens a socket, and the socket is appended to a list of all client sockets. Then, when one of the clients sends text, the server loops over the client sockets, finds the one that has bytesAvailable, reads it in, and then sends the message to the other clients.

I would be curious to hear what other people may think of this approach. Pitfalls, issues, etc.

Thanks!