First attempt to use Qt networking on QML apps

qcoderpro

The code for that is:

QString Server::initServer() {

    tcpServer = new QTcpServer(this);
    if(!tcpServer->listen())
        return "Server Unable to start the server: " +
                tcpServer->errorString();

    QString ipAddress;
    QList<QHostAddress> ipAddressesList = QNetworkInterface::allAddresses();

    // use the first non-local IPv4 address
    for(int i=0; i<ipAddressesList.size(); ++i)
        if(ipAddressesList.at(i) != QHostAddress::LocalHost &&
                ipAddressesList.at(i).toIPv4Address()) {
            ipAddress = ipAddressesList.at(i).toString();
            break;
        }

    // if we did not find one, use IPv4 localhost
    if(ipAddress.isEmpty())
        ipAddress = QHostAddress(QHostAddress::LocalHost).toString();

    return " The server is running on\n\n IP: " +
            ipAddress + "\n port: " + QString::number(tcpServer->serverPort())
            + "\n\n Run the Client example now.";
}

GrecKo

You are calling initServer() twice, once in c++ and once in QML, thus creating 2 QTcpServer. You are connecting yout onNewConnection only to the first one but displaying the ip and port of the second one in your UI.

Your initServer function does too much stuff. It shouldn't return the text like you are doing. Expose the ip and port of your server as properties and display them properly in your QML. Don't call initServer from QML.

qcoderpro

@GrecKo
Yes, you're right. I removed the call from the constructor (in C++) but still nothing different in result. As for setting IP and port as properties on the QML side, that's not hard but I don't think there's a problem with exposing them inside a string to be shown on the server's UI. But if you think setting them as properties will solve the issue with the connection, OK I do that.

GrecKo

@qcoderpro I don't think that will solve the problem, I just think that's the proper and sane way to do it.

Are you connecting the newConnection signal in the QTcpServer created in the initServer() called from QML?

qcoderpro

@GrecKo

Are you connecting the newConnection signal in the QTcpServer created in the initServer() called from QML?

What did you mean, please? I didn't get this completely.
The connection:
connect(tcpServer, &QTcpServer::newConnection, this, &Server::onNewConnection); is defined in the serve's constructor. And I guess that signal (newConnection) is emitted when there's a connection coming from the client app (for example in the sendAddress slot there). That is, I guess when that slot in the client is called, the tcpSocket connects to host successfully, and on the other side, the server is that way notified with a new connection signal to it in turn calls the onNewConnection slot. Is it wrong to you?

GrecKo

The connection: connect(tcpServer, &QTcpServer::newConnection, this, &Server::onNewConnection); is defined in the serve's constructor.

but

I removed the call [to initServer] from the constructor (in C++) but still nothing different in result

You are connecting to a null tcpServer, so it can't work.

qcoderpro

@GrecKo
This is server's qml file:

ColumnLayout {
        anchors.fill: parent

        Label {
            text: myObj.initServer()
        }

        Label {
            id: msgLabel
            text: myObj.getMessage()
        }
    }

    ServerClass{
        id: myObj
    }

If I comment out the label's text text: myObj.initServer(), nothing will be shown on its UI. But by uncommenting it, the IP address and port are shown on the UI. then I manually write them on the client's UI and click on Send address there.

It's the only time the initServer is called and I'm using its output on the client's text fields, so why a null tcpServer, please?
The send button calls the sendAddress slot:

void Client::sendAddress(QString ip, QString port)
{
    tcpSocket->abort();
    tcpSocket->connectToHost(ip, port.toInt());

    connect(tcpSocket, &QAbstractSocket::connected, []() {
        qDebug() << "Connected to the host";
    });
}

Here, too, qDebug in the lambda prints the message. So there's been a proper connection seemingly, otherwise I'm missing something that makes the projects not work properly! :|

GrecKo

As I said, you are not connecting to your QTcpServer newConnection signal.
The connection you are trying to do in the constructor is too soon because you don't have any QTcpServer yet.

qcoderpro

@GrecKo

As I said, you are not connecting to your QTcpServer newConnection signal.

Yes, that's right and I know it but don't know how to solve it! :|

The connection you are trying to do in the constructor is too soon because you don't have any QTcpServer yet.

Yea, that makes sense. So I cut that connection there and pasted it in the initServer(). Still theonNewConnectionis not called! :|

qcoderpro

No further help! :(

jsulm

@qcoderpro said in First attempt to use Qt networking on QML apps:

So I cut that connection there and pasted it in the initServer(). Still theonNewConnectionis not called!

Please show your current code

qcoderpro

@jsulm

This is code for server.cpp:

#include "server.h"
#include <QtNetwork>
#include <QtCore>

Server::Server(QObject *parent) : QObject{parent}
    , tcpServer(new QTcpServer(this)) { }

QString Server::initServer() {
    // Called from the front-end

    tcpServer = new QTcpServer(this);
    if(!tcpServer->listen())
        return "Server Unable to start the server: " +
                tcpServer->errorString();

    QString ipAddress;
    QList<QHostAddress> ipAddressesList = QNetworkInterface::allAddresses();

    // use the first non-local IPv4 address
    for(int i=0; i<ipAddressesList.size(); ++i)
        if(ipAddressesList.at(i) != QHostAddress::LocalHost &&
                ipAddressesList.at(i).toIPv4Address()) {
            ipAddress = ipAddressesList.at(i).toString();
            break;
        }

    // if we did not find one, use IPv4 localhost
    if(ipAddress.isEmpty())
        ipAddress = QHostAddress(QHostAddress::LocalHost).toString();

    return " The server is running on\n\n IP: " +
            ipAddress + "\n port: " + QString::number(tcpServer->serverPort())
            + "\n\n Run the Client example now.";
    connect(tcpServer, &QTcpServer::newConnection, this, &Server::onNewConnection);
}

void Server::onNewConnection()
{
    qDebug() << "OnNewConnection was called!\n";

    QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
    connect(clientConnection, &QAbstractSocket::disconnected,
            clientConnection, &QObject::deleteLater);
    in.setDevice(clientConnection);
    in.setVersion(QDataStream::Qt_4_0);
    connect(clientConnection, &QAbstractSocket::readyRead, this, &Server::setMessage);
}

void Server::setMessage()
{
    in.startTransaction();

    QString msg;
    in >> msg;

    if (!in.commitTransaction())
        message = "commitTransaction error" ;
    else
        message = msg;
}

QString Server::getMessage() const
{
    return message;
}

Totally there're 6 files as follows:
server.h, server.cpp, server qml file, client.h, client.cpp and client qml file

If you like I can pack them all in a zip file and send it to a global resource for freely downloading and consideration.

jsulm

@qcoderpro said in First attempt to use Qt networking on QML apps:

ipAddress = ipAddressesList.at(i).toString();

What is ipAddress used for? You are not using it when you call listen().

But the actuall issue is that you have a return just before connect(), so connect() is never called...

qcoderpro

@jsulm

What is ipAddress used for? You are not using it when you call listen().

ipAddress is used to store the list of local/non-local IP addresses. But I didn't understand the part "You are not using it when you call listen()".

But the actuall issue is that you have a return just before connect(), so connect() is never called..

Right. I moved the connect() to the part right after the listen(). Now the onNewConnection() slot is successfully called and it prints the message "OnNewConnection was called" in Application Output window as expected.

Now there's still a problem remaining!
This is client.cpp:

#include "client.h"
#include <QtNetwork>

Client::Client(QObject *parent)
    : QObject{parent}
    , tcpSocket(new QTcpSocket(this))
{
    out.setDevice(tcpSocket);
    out.setVersion(QDataStream::Qt_4_0);
}

void Client::sendAddress(QString ip, QString port)
{
    tcpSocket->abort();
    tcpSocket->connectToHost(ip, port.toInt());

    connect(tcpSocket, &QAbstractSocket::connected, []() {
        qDebug() << "Connected to the host";
    });
}

void Client::sendMessage(const QString& message)
//called from front-end
{
    QByteArray block;
    QDataStream out(&block, QIODevice::WriteOnly);
    out.setVersion(QDataStream::Qt_5_10);
    out << message;
    tcpSocket->write(block);
}

The sendAddress slot prints the message of the lambda in output ("Connected to the host") so by that I assume the connection between the client and server the way above is properly established. The remaining problem is that the sendMessage slot still doesn't send its message (its string parameter) to the sever! :(

jsulm

@qcoderpro said in First attempt to use Qt networking on QML apps:

But I didn't understand the part "You are not using it when you call listen()".

You know that you can pass an IP to listen? See https://doc.qt.io/qt-5/qtcpserver.html#listen

Where and when do you call sendMessage ?
You also should add code for error handling: https://doc.qt.io/qt-5/qabstractsocket.html#errorOccurred
https://doc.qt.io/qt-5/qiodevice.html#errorString

qcoderpro

@jsulm

You know that you can pass an IP to listen? See https://doc.qt.io/qt-5/qtcpserver.html#listen

No, I didn't know that and if I use listen() that way I need to someway convert the ipAddress which a QString to a QHostAddress. But now the connection is properly established and the connect(...) calls the slot onNewConnection.

Where and when do you call sendMessage ?

In the front-end:

 ColumnLayout {
        anchors.fill: parent

        TextField {
            id: ipAddrs
        }
        TextField {
                id: portNum
            }
        Button {
            text: "Send Address"
            onClicked: myObj.sendAddress(ipAddrs.text.toString(), portNum.text.toString())
        }

        RowLayout {
            Layout.alignment: Qt.AlignBottom

            TextField {
                id: txtField
                Layout.fillWidth: true
            }
            Button {
                text: qsTr("Send")
                onClicked: myObj.sendMessage(txtField.text)
            }
        }
    }

    MyClass {
        id: myObj
    }

You also should add code for error handling: https://doc.qt.io/qt-5/qabstractsocket.html#errorOccurred
https://doc.qt.io/qt-5/qiodevice.html#errorString

For that, I added this connect() after the lambda in client.cpp:
connect(tcpSocket, &QAbstractSocket::errorOccurred, this, &Client::displayError); and this slot there:

void Client::displayError(QAbstractSocket::SocketError socketError) {
   // for this stage, printing the error messages using qDenug will be fine

    switch (socketError) {
    case QAbstractSocket::RemoteHostClosedError:
        break;
    case QAbstractSocket::HostNotFoundError:
        qDebug() <<"The host was not found. Please check the "
                   "host name and port settings.";
        break;
    case QAbstractSocket::ConnectionRefusedError:
        qDebug() << "The connection was refused by the peer. "
                    "Make sure the server is running, "
                    "and check that the host name and port "
                    "settings are correct.";
        break;
    default:
        qDebug() << "The following error occurred: " + tcpSocket->errorString();
    }
}

Still no message is sent nor there's any error.

jsulm

@qcoderpro said in First attempt to use Qt networking on QML apps:

For that, I added this connect() after the lambda in client.cpp:

The idea was actually to check the errors on server side

qcoderpro

@jsulm
Yeah, right.
I declared a QTcpSocket object in the server app and added the connect() and displayError slot as mentioned above in it. As before, the connection is established but no message is sent nor is an error displayed!
I shared both projects in a zip file in link below. If you have time, take a look at them please. I'm sure the remaining problem will be clear to you. I tried to keep the projects as simple as possible just to do the job: https://www.4shared.com/s/f_Gct6Kjwiq

GrecKo

This file sharing is unusable without account it seems.

If you want to share multiple files, using https://gist.github.com would be better for the people that want to help you.

qcoderpro

@GrecKo
That's a code sharing platform (instead of files) but if code is meant to be easier than the projects both in a zip file then as usual it's feasible to share them here.
server.h:

#include <QObject>
#include <QAbstractSocket>

class QTcpServer;
class QTcpSocket;

class Server : public QObject
{
    Q_OBJECT
public:
    explicit Server(QObject *parent = nullptr);

public slots:
    QString initServer();
    void setMessage();
    QString getMessage() const;
    void onNewConnection();
    void displayError(QAbstractSocket::SocketError);

private:
    QTcpServer* tcpServer { nullptr };
    QTcpSocket* tcpSocket { nullptr };
    QDataStream in;
    QString message;
};

server.cpp:

#include "server.h"
#include <QtNetwork>
#include <QtCore>

Server::Server(QObject *parent) : QObject{parent}
    , tcpServer(new QTcpServer(this))
    , tcpSocket(new QTcpSocket(this)) { }

QString Server::initServer() {
    // Called from the front-end

    tcpServer = new QTcpServer(this);
    if(!tcpServer->listen())
        return "Server Unable to start the server: " +
                tcpServer->errorString();

    connect(tcpServer, &QTcpServer::newConnection, this, &Server::onNewConnection);
    connect(tcpSocket, &QAbstractSocket::errorOccurred, this, &Server::displayError);

    QString ipAddress;
    QList<QHostAddress> ipAddressesList = QNetworkInterface::allAddresses();

    // use the first non-local IPv4 address
    for(int i=0; i<ipAddressesList.size(); ++i)
        if(ipAddressesList.at(i) != QHostAddress::LocalHost &&
                ipAddressesList.at(i).toIPv4Address()) {
            ipAddress = ipAddressesList.at(i).toString();
            break;
        }

    // if we did not find one, use IPv4 localhost
    if(ipAddress.isEmpty())
        ipAddress = QHostAddress(QHostAddress::LocalHost).toString();

    return " The server is running on\n\n IP: " +
            ipAddress + "\n port: " + QString::number(tcpServer->serverPort())
            + "\n\n Run the Client example now.";
}

void Server::onNewConnection()
{
    qDebug() << "OnNewConnection was called!\n";

    QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
    connect(clientConnection, &QAbstractSocket::disconnected,
            clientConnection, &QObject::deleteLater);
    in.setDevice(clientConnection);
    in.setVersion(QDataStream::Qt_4_0);
    connect(clientConnection, &QAbstractSocket::readyRead, this, &Server::setMessage);
}

void Server::displayError(QAbstractSocket::SocketError socketError)
{
    // for this stage, printing the error messages using qDenug will be fine

     switch (socketError) {
     case QAbstractSocket::RemoteHostClosedError:
         break;
     case QAbstractSocket::HostNotFoundError:
         qDebug() <<"The host was not found. Please check the "
                    "host name and port settings.";
         break;
     case QAbstractSocket::ConnectionRefusedError:
         qDebug() << "The connection was refused by the peer. "
                     "Make sure the server is running, "
                     "and check that the host name and port "
                     "settings are correct.";
         break;
     default:
         qDebug() << "The following error occurred: " + tcpSocket->errorString();
     }
}

void Server::setMessage()
{
    in.startTransaction();

    QString msg;
    in >> msg;

    if (!in.commitTransaction())
        message = "commitTransaction error" ;
    else
        message = msg;
}

QString Server::getMessage() const
{
    return message;
}

server's project qml file:

import QtQuick
import QtQuick.Controls
import QtQuick.Layouts
import SomeClass 1.0

Window {
    width: 300
    height: 200
    visible: true
    title: qsTr("Server")
    color: "lightblue"

    ColumnLayout {
        anchors.fill: parent

        Label {
            text: myObj.initServer()
        }

        Label {
            id: msgLabel
            text: myObj.getMessage()
        }
    }

    MyClass {
        id: myObj
    }
}

client.h:

#include <QDataStream>
#include <QObject>

class QTcpSocket;

class Client : public QObject
{
    Q_OBJECT

public:
    explicit Client(QObject *parent = nullptr);

public slots:
    void sendAddress(QString, QString);
    void sendMessage(const QString&);

private:
    QTcpSocket* tcpSocket { nullptr };
    QDataStream out;
};

client.cpp:

#include "client.h"
#include <QtNetwork>

Client::Client(QObject *parent)
    : QObject{parent}
    , tcpSocket(new QTcpSocket(this))
{
    out.setDevice(tcpSocket);
    out.setVersion(QDataStream::Qt_4_0);
}

void Client::sendAddress(QString ip, QString port)
{
    tcpSocket->abort();
    tcpSocket->connectToHost(ip, port.toInt());

    connect(tcpSocket, &QAbstractSocket::connected, []() {
        qDebug() << "Connected to the host";
    });
}

void Client::sendMessage(const QString& message) {
      //called from front-end
    QByteArray block;
    QDataStream out(&block, QIODevice::WriteOnly);
    out.setVersion(QDataStream::Qt_5_10);
    out << message;
    tcpSocket->write(block);
}

client's project qml file:

import QtQuick
import QtQuick.Controls
import QtQuick.Layouts
import SomeClass 1.0

Window {
    width: 300
    height: 200
    visible: true
    title: qsTr("Client")
    color: "lightblue"

    ColumnLayout {
        anchors.fill: parent

        TextField {
            id: ipAddrs
        }
        TextField {
                id: portNum
            }
        Button {
            text: "Send Address"
            onClicked: myObj.sendAddress(ipAddrs.text.toString(), portNum.text.toString())
        }

        RowLayout {
            Layout.alignment: Qt.AlignBottom

            TextField {
                id: txtField
                Layout.fillWidth: true
            }
            Button {
                text: qsTr("Send")
                onClicked: myObj.sendMessage(txtField.text)
            }
        }
    }

    MyClass {
        id: myObj
    }
}

Hopefully we can now find the issue. :(