[Qt-interest] cannot send posted events for objects in another thread

[Yifei Li]

Hi all,

I know there're already lots of threads on this issue, but none of them  
answer my question.   Here's a self-contained small example  that shows  
how  I got the above error message:

I have a global object of Foo  and all the work is done in Foo:doWork, 
Foo has a signal telling us its progress.  Then in WorkerThread::run(),  
Foo::doWork is called.  I connet Foo's signal to a slot in WorkerThread, 
and in the slot I updated the value of a progress dialog.

WorkerThread::run() is called somewhere else in the application.   
However, I get  the following errors when trying to run the program:

QCoreApplication::sendPostedEvents: Cannot send posted events for 
objects in another thread
( the above message is repeated here)
MyApp: xcb_lock.c:77: _XGetXCBBuffer: Assertion `((int) ((xcb_req) - 
(dpy->request)) >= 0)' failed.

I'm pretty sure it's that global object that causes the problem.   But 
my questions are:

1) why this happens?
2) what I need to do to make the program working without getting rid of 
the global object  ( in real apps,  that global object is defined in a 
3rd party libarary)

Any help is highly appreciated!

Yifei

=======Foo.h==========
class Foo : public QObject
{
    Q_OBJECT
signals:
    void currentStep(int)

public:
    void doWork();
}

extern Foo foo;                     // my global object

======Foo.cpp======
Foo foo;

void Foo::doWork()
{
    // do something
    emit currentStep(i);       // tell us its progress
}


=====WorkerThread.h======
class WorkerThread : public QThread
{
public:
    WorkerThread();
private slots:
    void currentStep(int);
protected:
    void run();
private:
    QProgressDialog * pd;
}

======WorkerThread.cpp=====
WorkerThread::WorkerThread()
{
    pd = new QProgressDialog(...);
    connect( &foo, SIGNAL(currentStep(int)), this, SLOT(currentStep(int)) );
}

void WorkerThread::currentStep(int i)
{
    pd->setValue(i);      // update progress dialog
}

void WorkerThread::run()
{
    pd->show();
    foo.doWork();      // do the work
}