How to use the QRemoteObjet class to share data between processes?

[Joeyy]

I'm trying to follow the Qt Remote Objects example found in this doc, but its very confuse.

init() on process 2 isn't getting called.

Also, when I emit the signal from process 1 by hitting F2 emit obj->mySignal(1000); process2 should trigged void P1Signal(int value) but it don't.

When I hit F2 on process 2 emit remoteObj->repply(1000); it doesn't trigger virtual void repplyFromP2(int value) on process 1.

Whats wrong?

//process 1

// p1.h
#pragma once

class MyObject : public QObject
{
    Q_OBJECT
public:
    MyObject(QObject *parent = nullptr) : QObject(parent) {}
    virtual void repplyFromP2(int value)
    {
        qDebug() << "P2 repplied with:" << value;
    }
signals:
    void mySignal(int value);
};

class P1 : public QMainWindow
{
    Q_OBJECT
public:
    MyObject* obj = nullptr;
    P1(QWidget *parent = nullptr);
};

// p1.cpp
#include "p1.h"
#include <QtRemoteObjects>
#include <QRemoteObjectRegistry>

P1::P1(QWidget* parent): QMainWindow(parent)
{
    obj = new MyObject();
    obj->setObjectName("MyObject");
    //QRemoteObjectHost sourceNode(QUrl(QStringLiteral("local:registry")));
    //sourceNode.enableRemoting(obj, QStringLiteral("MyObject"));

    QRemoteObjectRegistryHost regNode(QUrl(QStringLiteral("local:registry"))); // create node that hosts registry
    QRemoteObjectHost srcNode(QUrl(QStringLiteral("local:MyObject")), QUrl(QStringLiteral("local:registry"))); // create node that will host source and connect to registry
    srcNode.enableRemoting(obj); // enable remoting of source object

    QShortcut *shortcut = new QShortcut(QKeySequence(Qt::Key_F2), this);
    connect(shortcut, &QShortcut::activated, [this]
    {
        emit obj->mySignal(1000);
    });
}

//process2

// p2.h
#include <QtRemoteObjects>
#include <QRemoteObjectRegistry>

class RemoteObject : public QObject
{
    Q_OBJECT
public:
    RemoteObject(QSharedPointer<QRemoteObjectDynamicReplica> ptr) :
        QObject(nullptr), reptr(ptr)
    {
        //connect signal for replica valid changed with signal slot initialization
        connect(reptr.data(), &QRemoteObjectDynamicReplica::initialized, this,
            &RemoteObject::init);
    }

signals:
    // This signal is connected with the function 
    // repplyFromP2 of P1.
    void repply(int value);

public Q_SLOTS:
    // Slot to receive data from P1.
    void P1Signal(int value)
    {
        qDebug() << "received from P1:" << value;
        emit repply(999); // Emit signal to echo data
    }

    void init() //Slot to connect signals/slot on replica initialization
    {
       connect(reptr.data(), SIGNAL(mySignal()), this, SLOT(P1Signal()));
       connect(this, SIGNAL(repply(int)),reptr.data(), SLOT(repplyFromP2(int)));
    }

private:
    QSharedPointer<QRemoteObjectDynamicReplica> reptr;// holds reference to replica
 };

class P2 : public QMainWindow
{
    Q_OBJECT
public:
    RemoteObject* remoteObj = nullptr;
    P2(QWidget *parent = nullptr);
};

// p2.cpp
#include "p2.h"

P2::P2(QWidget *parent) : QMainWindow(parent)
{
    QSharedPointer<QRemoteObjectDynamicReplica> ptr; // shared pointer to hold replica
    QRemoteObjectNode repNode;
    repNode.connectToNode(QUrl(QStringLiteral("local:registry")));
    ptr.reset(repNode.acquireDynamic("MyObject")); // acquire replica of source from host node

    remoteObj = new RemoteObject(ptr); // create client switch object and pass replica reference to it

    QShortcut *shortcut = new QShortcut(QKeySequence(Qt::Key_F2), this);
    connect(shortcut, &QShortcut::activated, [this]
    {
        emit remoteObj->repply(1000);
    });
}

[Joeyy]

Could someone give a small example of how to share data between two processes using QRemoteObject?

[Joeyy]

I can't get this working, I rewrite it, but the client process does not compile, error:

Error	C2661	'SharedObject::SharedObject': no overloaded function takes 2 arguments	C:\Qt\6.3.0\qtremoteobjects\src\remoteobjects\qremoteobjectnode.h	78	

//server

// remoteobject.h
#include <QRemoteObjectRegistry>
#include <QRemoteObjectRegistryHost>
#include <QRemoteObjectHost>

class SharedObject : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString text READ text WRITE setText NOTIFY textChanged)

public:
    QString text() const { return m_text; }
    void setText(const QString& text)
    {
        if (m_text != text) {
            m_text = text;
            emit textChanged(m_text);
        }
    }

signals:
    void textChanged(const QString& text);

private:
    QString m_text;
};

// main.cpp
#include "remoteobject.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    SharedObject sharedObject;
    sharedObject.setObjectName("sharedObject");
    QRemoteObjectHost host;
    host.setHostUrl(QUrl("local:sharedObject"));
    host.enableRemoting(&sharedObject);

    return a.exec();
}

//client

// remoteobject.h
#include <QRemoteObjectRegistry>
#include <QRemoteObjectRegistryHost>
#include <QRemoteObjectHost>

class SharedObject : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString text READ text WRITE setText NOTIFY textChanged)

public:
    QString text() const { return m_text; }
    void setText(const QString& text)
    {
        if (m_text != text) {
            m_text = text;
            emit textChanged(m_text);
        }
    }

signals:
    void textChanged(const QString& text);

private:
    QString m_text;
};

// main.cpp
#include "remoteobject.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QRemoteObjectNode node;
    node.connectToNode(QUrl("local:sharedObject"));
    auto sharedObject = node.acquire<SharedObject>();

    QObject::connect(sharedObject, &SharedObject::textChanged, [&](const QString& text) {
        qDebug() << "Text changed to:" << text;
    });

    QTimer::singleShot(10000, [&]() {
        qDebug() << "Current text:" << sharedObject->text();
    });

    return a.exec();
}

[Cesar]

You need to generate your class .rep, see https://doc.qt.io/qt-6/qtremoteobjects-repc.html

[Joeyy]

@Cesar is not possible to get it working without a .rep file of the class?
When I add a .rep file to my project Visual Studio doesn't compile anymore
and it starts causing a lot of weird errors, like not finding files, the log:

1>Target QtWork:
1>  repc sharedobject.rep
1>   repc: D:\p2\sharedobject.rep:1: error: Unknown token encountered
1>  C:\Users\Local\QtMsBuild\qt_globals.targets(293,5): error MSB4181: The "QtRunWork" task returned false but did not log an error.

Do I need to configure something in VS to be able to compile this rep file?

[Cesar]

@Joeyy said in How to use the QRemoteObjet class to share data between processes?:

Do I need to configure something in VS to be able to compile this rep file?

I dont use Visual Studio, so i'm not sure, but searching around, i found this entry:
https://bugreports.qt.io/browse/QTBUG-71283

[Joeyy]

@Cesar

I think the errors were caused because I added an invalid rep file to the project.
I could not find how to make VS to auto-generate the rep file, all I could find was this question .

I tried to use the repc.exe tool, to generate the .rep file, but it also doesn't work:

C:\Qt\6.3.0\qtbase\bin>repc -i D:/P2/remoteobject.h -o remoteobject.rep
repc: Unknown input type"D:/P2/remoteobject.h".
Usage: repc [options] [json-file/rep-file] [rep-file/header-file]
repc tool v1.0.0 (Qt 6.3.0).

[Cesar]

@Joeyy
Without a rep file you can try something like this

class RemoteObject : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString text READ text WRITE setText NOTIFY textChanged)
 public:
    QString text() const { return m_text; };
public slots:
    void setText(const QString& text)
    {
        if (m_text != text) {
            m_text = text;
            emit textChanged(m_text);
        }
    };

signals:
    void textChanged(const QString& text);
private:
    QString m_text;
};


int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QRemoteObjectHost host;
    RemoteObject remoteObject;
    remoteObject.setObjectName("remoteObject");
    host.setHostUrl(QUrl("local:remoteObject"));
    host.enableRemoting(&remoteObject);
    return a.exec();
}

and in the client

class RemoteObject
{
    Q_OBJECT
public:
    RemoteObject(QString objectName)
    {
        repNode.connectToNode(QUrl(QStringLiteral("local:remoteObject")));
        if (repNode.lastError())
            qDebug() << "node error:" << repNode.lastError();

        ptr.reset(repNode.acquireDynamic(objectName));

        connect(ptr.data(), &QRemoteObjectDynamicReplica::initialized, this, [=]
        {
            if (initialized)
                return;
            initialized = true;
            connect(ptr.data(), SIGNAL(textChanged(QString)), this, SLOT(textChanged(QString)));
        });
    }

    void RemoteObject::text()
    {
        QString text = ptr.data()->property("text").toString();
    }

    void RemoteObject::setText(const QString& text)
    {
        QMetaObject::invokeMethod(ptr.data(), "setText", Q_ARG(QString, text));
    }

    void RemoteObject::textChanged(QString text)
    {
        qDebug() << "textChanged:" << text;
    }
}

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    RemoteObject remoteObject("remoteObject");
    return a.exec();
}

[Joeyy]

@Cesar Thanks!!! finally some help.
It's working with the string thing, but how to pass a struct from the client?
I tried to adapt your code:

struct MyStruct {
    int x = 0;
    QString test;
};

Q_DECLARE_METATYPE(MyStruct)

QDataStream &operator<<(QDataStream &out, const MyStruct &myStruct) {
    out << myStruct.x;
    return out;
}

QDataStream &operator>>(QDataStream &in, MyStruct &myStruct) {
    in >> myStruct.x;
    return in;
}

class RemoteObject : public QObject
{
    Q_OBJECT
    Q_PROPERTY(WRITE setStruct)
public:
public slots:
    void setStruct(MyStruct myStruct) {
	qDebug() << "myStruct.x:" << myStruct.x;
        qDebug() << "myStruct.test:" << myStruct.test;
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    qRegisterMetaType<MyStruct>("MyStruct");
    QRemoteObjectHost host;
    RemoteObject remoteObject;
    remoteObject.setObjectName("remoteObject");
    host.setHostUrl(QUrl("local:remoteObject"));
    host.enableRemoting(&remoteObject);
    return a.exec();
}

and on the client:

struct MyStruct {
    int x = 0;
    QString test;
};

Q_DECLARE_METATYPE(MyStruct)

QDataStream &operator<<(QDataStream &out, const MyStruct &myStruct) {
    out << myStruct.x;
    return out;
}

QDataStream &operator>>(QDataStream &in, MyStruct &myStruct) {
    in >> myStruct.x;
    return in;
}

class RemoteObject
{
    Q_OBJECT
public:
    QSharedPointer<QRemoteObjectDynamicReplica> ptr;
    QRemoteObjectNode repNode;
    RemoteObject(QString objectName) {
        repNode.connectToNode(QUrl(QStringLiteral("local:remoteObject")));
        ptr.reset(repNode.acquireDynamic(objectName));

        connect(ptr.data(), &QRemoteObjectDynamicReplica::initialized, this, [=]
        {
            qDebug() << "initialized...";
            MyStruct myStruct;
            myStruct.x = 1;
            myStruct.test = "test";
            setStruct(myStruct);
        });
    }

    void RemoteObject::setStruct(MyStruct myStruct) {
        QMetaObject::invokeMethod(ptr.data(), "setStruct", myStruct);
    }
}

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    qRegisterMetaType<MyStruct>("MyStruct");
    RemoteObject remoteObject("remoteObject");
    return a.exec();
}

But on process 1 when the function setStruct is called i get this error:
QVariant::load: unable to load type 65539.
and the data in the struct is empty.

Whats wrong?

[Cesar]

You are only deserializing the integer value 'x', but not the QString 'test'.

[Joeyy]

@Cesar i fixed it, but i continue getting the same error.