Message handler question

[jacek]

Using qDebug causes and infinite loop back to the message handler.

That's how the message handler works --- it collects messages from qDebug(), qFatal() and similar and processes them in some way. On windows the default message handler should send those messages to the debugger.

[bruccutler]

Thanks.

Let me be more specific:
*** questions below ***

I call:
qInstallMsgHandler(myMsgHandler);
to install my own message handler, which follows:

void myMsgHandler(QtMsgType type , const char *msg)
{
QString text;
if (strstr(msg, "Object::connect:") != NULL)
{
return;
}
switch (type)
{
case QtDebugMsg:
text = QString ("Debug: %1\n").arg(msg);
break;
case QtWarningMsg:
text = QString ("Warning: %1\n").arg(msg);
break;
case QtCriticalMsg:
text = QString ("Critical: %1\n").arg(msg);
break;
case QtFatalMsg:
text = QString ("Fatal: %1\n").arg(msg);
break;
}

// Display a message to the user
if (this thread is gui thread) *** How do I get the thread to see if this is a GUI thread? ***
{
QMessageBox::information(NULL, QString("Error"), text); // this only works if this handler was called from a GUI thread
}
else
{
*** how to display error to user from a non-gui thread? ***
}

#ifndef NDEBUG // when in debug mode, do this
#ifdef WINDOWS
OutputDebugStringA(text.toAscii().data()); // this works if we are in debug
#else // non-windows
fprintf(stderr(text.toAscii()));
#endif
}
#endif
if (type == QtFatalMsg)
{
abort();
}
return;
}

[jpn]

// only windows has a "default" msg handler, 0 is returned on other platforms!
QtMsgHandler msgHandler = qInstallMsgHandler(myMsgHandler);
 
void myMsgHandler(QtMsgType type , const char *msg)
{
    ...
 
    if (QThread::currentThread() == qApp->thread())
    {
        // gui thread
    }
 
    if (msgHandler)
    {
        // call the default windows handler
        msgHandler(type, msg);
    }
    else
    {
        fprintf(stderr, qPrintable(text));
    }
 
    ...
}

// only windows has a "default" msg handler, 0 is returned on other platforms!
QtMsgHandler msgHandler = qInstallMsgHandler(myMsgHandler);

void myMsgHandler(QtMsgType type , const char *msg)
{
    ...

    if (QThread::currentThread() == qApp->thread())
    {
        // gui thread
    }

    if (msgHandler)
    {
        // call the default windows handler
        msgHandler(type, msg);
    }
    else
    {
        fprintf(stderr, qPrintable(text));
    }

    ...
}

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[jacek]

how to display error to user from a non-gui thread?

You can send an event with error message (for example to a QApplication subclass) and show that message box in the event handler.

[marcel]

You can send an event with error message (for example to a QApplication subclass) and show that message box in the event handler.

Or you can emit a signal from that thread if you don't want to implement a custom event.

[bruccutler]

Emitting a signal might be a good way to go.

Thanks.
- Bruce

[bruccutler]

One other quick question - how can I tell if the current thread is the GUI thread?

[jacek]

One other quick question - how can I tell if the current thread is the GUI thread?

See the post #5.

[bruccutler]

Oops -- Seeing I could not see. Now I've seen.