Problem with plugin signals/slots

I am developing under Windows and I have a Qt plugin project that produces a dll, and I have another app that can load that dll dynamically as a plugin via the usual Qt methods. Basically what I wanted to do was have the plugin report its status back to this app that's using it (i.e. the plugin emits a signal, the app receives it via a slot). So after I successfully detect and read in the plugin (i.e. the dll file), I do the usual connect call to set up the signal/slot (which returns true), but the connection doesn't seem to actually be working, as my slot code never gets executed in the app. I tried to strip down the pertinent code below.


UpgradeInterface.h (this is the interface a plugin extends):

Code:

---

class UpgradeInterface : public QObject
{
public:
 
    virtual ~UpgradeInterface() {}
    virtual bool performUpgrade() = 0;
 
signals:
      void statusUpdate(QString statusMsg);
};
 
Q_DECLARE_INTERFACE(UpgradeInterface, "com.UpgradeInterface/1.0")

---

//----------------------------------------------------------

UpgradePlugin.h:

Code:

---

class UpgradePlugin : public QObject, public UpgradeInterface
{
    Q_OBJECT
    Q_INTERFACES(UpgradeInterface)
 
public:
    bool performUpgrade();
 
signals:
      void statusUpdate(QString statusMsg);
};

---

//----------------------------------------------------

UpgradePlugin.cpp:

Code:

---

bool UpgradePlugin::performUpgrade()
{
    emit statusUpdate("Upgrade started!");
    return true;
}
 
Q_EXPORT_PLUGIN2(UpgradePlugin, UpgradePlugin)

---

//-------------------------------------------------

The following is part of the app code that detects/loads the plugin.

MyDialog.h:

Code:

---

class MyDialog : public QDialog
{
      Q_OBJECT
//...
//...
protected slots:
      void statusUpdateFromUpgradePlugin(QString statusMsg);
};

---

//------------------------------------------------------

MyDialog.cpp:

Code:

---

void MyDialog::loadPlugins(QDir dir)
{
    // Load dynamic plugins
 
    foreach (QString fileName, dir.entryList(QDir::Files)) 
      {
        QPluginLoader loader(dir.absoluteFilePath(fileName));
        QObject *plugin = loader.instance();
 
        if (plugin) 
            {
                  UpgradeInterface * iUpgrade = qobject_cast<UpgradeInterface *>(plugin);
                  if (iUpgrade)
                  {
                        bool success = connect(iUpgrade, SIGNAL(statusUpdate(QString)), this, SLOT(statusUpdateFromUpgradePlugin(QString))); // returns true
                        iUpgrade->performUpgrade();               
                  }
            }
      }
}
 
void MyDialog::statusUpdateFromUpgradePlugin(QString statusMsg)
{
      cout << statusMsg;  // Never gets here!
}

---

AFAIK your code won't compile because for me code like this:

Code:

---

QString str("asdasd");
std::cout << str;

---

result in this:

Quote:

---

error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'QString' (or there is no acceptable conversion)

---

so I don't know how do you achieved compilation, but you can tell me how to do it :]
next thing is that maybe you forgot CONFIG += console or sth. Try using:

Code:

---

#include <QDebug>
 
...
 
qDebug() << statusMsg;

---

except for that everything should work in your code in my opinion

I don't actually have the "cout" statement in my code... it was there just as a generic line of code. So my code does actually compile :). Put anything you want there... the point I was trying to illustrate is that it simply doesn't ever get there. I put break points, try to pop up message dialogs, etc. so I know it does not actually execute... thx.

Hi,

try to add Q_OBJECT to the UpgradeInterface declaration. It works written in that way in my code, I hope it will help you.

have a nice day

ok, I've done some research and I think, like my preposter, that you should add Q_OBJECT macro in interface declaration, and you should not redefine signal in implementation, just emit it.

Hmm when I add Q_OBJECT, now when I compile my UpgradePlugin project, I get:

Error 3 error LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __thiscall UpgradeInterface::metaObject(void)const " (?metaObject@UpgradeInterface@@UBEPBUQMetaObject@@ XZ) UpgradePlugin.obj UpgradePlugin
Error 4 error LNK2001: unresolved external symbol "public: virtual void * __thiscall UpgradeInterface::qt_metacast(char const *)" (?qt_metacast@UpgradeInterface@@UAEPAXPBD@Z) UpgradePlugin.obj UpgradePlugin
Error 5 error LNK2001: unresolved external symbol "public: virtual int __thiscall UpgradeInterface::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall@UpgradeInterface@@UAEHW4Call@QMetaOb ject@@HPAPAX@Z) UpgradePlugin.obj UpgradePlugin
Error 6 fatal error LNK1120: 3 unresolved externals ..\UpgradePlugin.dll UpgradePlugin

Hmm when I call emit from the class that implements UpgradeInterface (UpgradePlugin), no luck :(. Keep in mind UpgradePlugin is its own separate project (and produces a dll) and references the same UpgradeInterface.h file as the app (exe) project does... not that that helps, just reiterating...

Error 3 error LNK2019: unresolved external symbol "protected: void __thiscall UpgradeInterface::statusUpdate(class QString)" (?statusUpdate@UpgradeInterface@@IAEXVQString@@@Z) referenced in function "public: virtual bool __thiscall UpgradePlugin::performUpgrade(void)" (?performUpgrade@UpgradePlugin@@UAE_NXZ) UpgradePlugin.obj UpgradePlugin
Error 4 error LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __thiscall UpgradeInterface::metaObject(void)const " (?metaObject@UpgradeInterface@@UBEPBUQMetaObject@@ XZ) UpgradePlugin.obj UpgradePlugin
Error 5 error LNK2001: unresolved external symbol "public: virtual void * __thiscall UpgradeInterface::qt_metacast(char const *)" (?qt_metacast@UpgradeInterface@@UAEPAXPBD@Z) UpgradePlugin.obj UpgradePlugin
Error 6 error LNK2001: unresolved external symbol "public: virtual int __thiscall UpgradeInterface::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall@UpgradeInterface@@UAEHW4Call@QMetaOb ject@@HPAPAX@Z) UpgradePlugin.obj UpgradePlugin
Error 7 fatal error LNK1120: 4 unresolved externals ..\UpgradePlugin.dll UpgradePlugin

nmake clean + qmake again :]
this works for me:
interface.h:

Code:

---

#ifndef INTERFACE_H
#define INTERFACE_H
 
#include <QtPlugin>
 
class PluginInterface : public QObject
{
	Q_OBJECT
public:
	PluginInterface(QObject *parent) : QObject(parent) {}
	virtual void someMethod() = 0;
signals:
	void someSignal();
};
 
Q_DECLARE_INTERFACE(PluginInterface, "pl.faldzip.PluginInterface")
#endif // INTERFACE_H

---

myplugin.h:

Code:

---

#ifndef MYPLUGIN_H
#define MYPLUGIN_H
 
#include "plugin1_global.h"
#include "interface.h"
 
class MyPlugin : public PluginInterface
{
	Q_OBJECT
	Q_INTERFACES(PluginInterface)
public:
	MyPlugin(QObject *parent = 0);
	void someMethod();
};
 
#endif // MYPLUGIN_H

---

myplugin.cpp:

Code:

---

#include "myplugin.h"
 
MyPlugin::MyPlugin(QObject *parent)
		: PluginInterface(parent)
{
}
 
void MyPlugin::someMethod()
{
	emit someSignal();
}
 
Q_EXPORT_PLUGIN2(myplugin, MyPlugin);

---

and this was one project (with pro file containing TEMPLATE = lib and CONFIG += plugin),
and second project - app project:
main.cpp:

Code:

---

#include <QtGui>
#include <QtCore>
#include <QtPlugin>
#include "interface.h"
 
class SomeClass : public QObject
{
	Q_OBJECT
public:
	SomeClass(PluginInterface *pi, QObject *parent = 0) : QObject(parent), m_pi(pi) { connect(m_pi, SIGNAL(someSignal()), SLOT(someSlot2())); }
public slots:
	void someSlot() { m_pi->someMethod(); }
	void someSlot2() { qDebug("someSlot2"); }
private:
	PluginInterface *m_pi;
};
 
int main(int argc, char *argv[])
{
	QApplication a(argc, argv);
	QPluginLoader loader(a.applicationDirPath() + QDir::separator() + "plugin1.dll");
	QObject *obj = loader.instance();
	PluginInterface *pi = qobject_cast<PluginInterface *>(obj);
	if (!pi)
	{
		qDebug("plugin error");
		return 1;
	}
	QPushButton pb("Button");
	SomeClass sc(pi, &pb);
	pb.connect(&pb, SIGNAL(clicked()), &sc, SLOT(someSlot()));
	pb.show();
	return a.exec();
}
 
#include "main.moc"

---

so it shows a button, and when I click on it, it's callingSomeClass::someSlot() which is calling PluginInterface::someMethod(), whis is emitting someSignal() connected to someSlot2() so the "someSlot2" appears on the screen.

Thanks for the help... finally got it!

One problem was the Q_OBJECT thing... but after adding that I got some link errors, as I mentioned. Basically I reorganized my projects so that the UpgradeInterface was in its own separate project (lib), and I had the plugin project and app project both reference that project. Works :)