Slots interrupting each other with just one Thread? - qt

Question

I have a small example using Slots called by mousemovement and mousewheel.
Now i have the problem that when i zoom and move at the same time, first the onZoom-slot is called and before it is finished it is calling the onMouseMoved-slot. That causes the first slot to lock the mutex (in my original program used by another thread) and the second one to wait for it.
How can I prevent the slots to interrupt each other (and why are they doing it in first place since they are in same thread?).
I read something about using Qt::QueuedConnection but that causes an access violation exception.
main.cpp
#include "ppi.h"
#include <QtGui/QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
PPI w;
w.show();
return a.exec();
}
ppi.h
#ifndef PPI_H
#define PPI_H
#include <QtGui/QMainWindow>
#include <QGraphicsView>
#include <QDebug>
#include <QWheelEvent>
#include <QgraphicsEllipseItem>
#include <QMouseEvent>
#include <QMutex>
#include <QThread>
#include <QGraphicsSceneMouseEvent>
//#include "ui_ppi.h"
class PPIView : public QGraphicsView
{
Q_OBJECT
public:
PPIView(QWidget * parent = 0)
: QGraphicsView(parent)
{};
~PPIView(){};
private slots:
void wheelEvent(QWheelEvent *event)
{emit zoom(event);};
signals:
void zoom(QWheelEvent *event);
};
class PPIScene : public QGraphicsScene
{
Q_OBJECT
public:
PPIScene(QObject *parent)
: QGraphicsScene(parent)
{};
~PPIScene(){};
private:
void mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{emit mouseMoved(event);};
signals:
void mouseMoved(QGraphicsSceneMouseEvent *event);
};
class PPI : public QMainWindow
{
Q_OBJECT
public:
PPI(QWidget *parent = 0, Qt::WFlags flags = 0)
: QMainWindow(parent, flags)
{
//ui.setupUi(this);
//ppiScene is inherited from QGraphicsScene, overriding mouseMoveEvent so it emits mouseMoved();
ppiScene = new PPIScene(this);
gVPPI = new PPIView(this);
gVPPI->setMinimumSize(1024,1024);
gVPPI->show();
test = new QGraphicsEllipseItem(-10, -10, 20, 20);
ppiScene->addItem(test);
gVPPI->adjustSize();
connect(ppiScene, SIGNAL(mouseMoved(QGraphicsSceneMouseEvent*)), this, SLOT(onMouseMoved(QGraphicsSceneMouseEvent*)));
connect(gVPPI, SIGNAL(zoom(QWheelEvent*)), this, SLOT(onZoom(QWheelEvent*)));
//ui.gVPPI is inherited from QGraphicsView, overriding wheelEvent, so it emits zoom()
gVPPI->setScene(ppiScene);
gVPPI->setMouseTracking(true);
};
~PPI(){};
QMutex mutex;
private:
//Ui::ppiClass ui;
PPIScene* ppiScene;
PPIView *gVPPI;
QGraphicsEllipseItem *test;
protected slots:
void onZoom(QWheelEvent *event)
{
qDebug() << "Zoom lock" << QThread::currentThreadId();
mutex.lock();
qDebug() << "Zoom locked";
if(event->delta() > 0)
gVPPI->scale(1.01, 1.01);
else
gVPPI->scale(1/1.01, 1/1.01);
qDebug() << "Zoom unlock";
mutex.unlock();
qDebug() << "Zoom unlocked";
};
void onMouseMoved(QGraphicsSceneMouseEvent *event)
{
qDebug() << "Move lock" << QThread::currentThreadId();
mutex.lock();
qDebug() << "move locked";
test->setPos(test->pos()+event->scenePos()-event->lastScenePos());
qDebug() << "Move unlock";
mutex.unlock();
qDebug() << "Move unlocked";
};
};
#endif // PPI_H
Output qDebug():
Move lock 0x1514
move locked
Move unlock
Move unlocked
Move lock 0x1514
move locked
Move unlock
Move unlocked
Zoom lock 0x1514
Zoom locked
Move lock 0x1514

Answer 1

You can use QObject::blockSignals to block signals for specific object,
but I think you have wrong idea about how your code works.
There is one GUI thread, and if signals + slots in the same GUI thread,
as in your case, then they called in sequence order. And if PPI::onZoom will called in another thread context then you have problem here, because of usage of things like ui.gVPPI->scale in non GUI thread is not allowed and may cause crash on assert, or just crash, or randomly works, UB as it is.
The situation when you see that slot 1 was called, then before finish of slot 1 slot 2 is called is likely because of you call some function inside slot 1 that call another function etc and some function deep inside call signal 2, which emit call of slot 2, just set breakpoints in your debugger and findout what is going on.

Answer 2

It doesn't make sense to use any form of locking in a single GUI thread that doesn't cooperate with other threads. Everything in the GUI thread is already serialized for you. You're fighting an imaginary problem. As long as your code is executing, nothing else will execute behind your back in the same thread.
onZoom-slot is called and before it is finished it is calling the onMouseMoved-slot
This is false. Nothing of the sort happens in the code you show. Perhaps ui.gVPPI->scale() is reentering the event loop. You really need to show a self-contained example: your problem is in the code you don't show.
How can I prevent the slots to interrupt each other
It's already prevented for you. You need to do nothing.
in my original program used by another thread
Generally speaking, objects living in the GUI thread cannot have methods directly invoked from other threads.
Most of the time, throwing a second thread at a problem makes you end up with two problems. Properly designed non-blocking code will work correctly, if perhaps a bit slow, even when run in a single thread. That'd be your point of departure - you then move objects to other threads simply to spread the work around. Ideally, though, async processing should be done by running work asynchronously through QtConcurrent::run.

Answer 3

I managed to set up my debugger and the problem seems QGraphicsView::scale() is calling QgraphicsScene::mouseMoveEvent() deep inside directly. So I need to introduce a variable which tells the mouseMoveEvent, whether it was called from QGraphicsView::scale() or from physical mouse movement.
protected slots:
void onZoom(QWheelEvent *event)
{
qDebug() << "Zoom lock" << QThread::currentThreadId();
mutex.lock();
qDebug() << "Zoom locked";
scale = true;
if(event->delta() > 0)
gVPPI->scale(1.01, 1.01);
else
gVPPI->scale(1/1.01, 1/1.01);
scale = false;
qDebug() << "Zoom unlock";
mutex.unlock();
qDebug() << "Zoom unlocked";
};
void onMouseMoved(QGraphicsSceneMouseEvent *event)
{
if(scale == false)
{
qDebug() << "Move lock" << QThread::currentThreadId();
mutex.lock();
qDebug() << "move locked";
}
test->setPos(test->pos()+event->scenePos()-event->lastScenePos());
if(scale == false)
{
qDebug() << "Move unlock";
mutex.unlock();
qDebug() << "Move unlocked";
}
};