Qt - Send key press event to GLWidget from a QMainWindow

Question

I have a Qt/OpenGL/OpenCL application with a QMainWindow containing a GLWidget subclass widget. I would like to add zoom functionalities (with key press) on the GLWidget from the menu of the QMainWindow.

Originally, I did zoom In/Out directly from the GLWidget as follows (GLWidget.cpp) :

void GLWidget::keyPressEvent(QKeyEvent* event) 
{
  switch(event->key()) {
    case Qt::Key_W:
      zoomScale(zoomFactor_in);
      zoomGlobal = 1.0f / zoomFactor_in;
      break;
    case Qt::Key_X:
      zoomScale(zoomFactor_out);
      zoomGlobal = 1.0f / zoomFactor_out;
      break;        
    default:
      event->ignore();
      break;
  }
}

So, in order to add this functioanlity from an action of QMainWindow menu, I create signal/slot (MainWindow.cpp) :

void Ui_MainWindow::createSignals()
{ 
...
  connect(actionZoom_In, SIGNAL(triggered()), this, SLOT(zoomIn()));     
  connect(actionZoom_Out, SIGNAL(triggered()), this, SLOT(zoomOut()));  

}

with the following slots (I try this way to send the keypress signal to the GLwidget "widget_2") :

void Ui_MainWindow::zoomIn()
{
  QKeyEvent event1 = QKeyEvent(QEvent::KeyPress, Qt::Key_W, Qt::NoModifier);
  QApplication::sendEvent(widget_2, &event1);  
}

void Ui_MainWindow::zoomOut()
{
  QKeyEvent event2 = QKeyEvent(QEvent::KeyPress, Qt::Key_X, Qt::NoModifier);
  QApplication::sendEvent(widget_2, &event2);
}

widget_2 is the GLWidget_2 for which I send the KeyPress 'X' or 'W'.

Unfortunately, this doesn't work. When I press 'X' or 'W', I have a segfault and I don't understand why ?

Here's the backtrace :

(gdb) bt
#0  0x00007ffff6c70e80 in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#1  0x00007ffff6c7127f in QMainWindow::event(QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#2  0x00007ffff685d70c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#3  0x00007ffff6862704 in QApplication::notify(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#4  0x00007ffff6328b5e in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#5  0x00007ffff688f8cc in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#6  0x00007ffff6863869 in QApplication::notify(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#7  0x00007ffff6328b5e in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#8  0x000000000047af4e in Ui_MainWindow::zoomIn() ()
#9  0x00007ffff633e54f in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#10 0x00007ffff6857502 in QAction::triggered(bool) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#11 0x00007ffff68576f0 in QAction::activate(QAction::ActionEvent) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#12 0x00007ffff6857857 in QAction::event(QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#13 0x00007ffff685d70c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#14 0x00007ffff6861b8a in QApplication::notify(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#15 0x00007ffff6328b5e in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#16 0x00007ffff688e474 in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#17 0x00007ffff688f983 in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#18 0x00007ffff6863869 in QApplication::notify(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#19 0x00007ffff6328b5e in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#20 0x000000000047af4e in Ui_MainWindow::zoomIn() ()
#21 0x00007ffff633e54f in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#22 0x00007ffff6857502 in QAction::triggered(bool) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#23 0x00007ffff68576f0 in QAction::activate(QAction::ActionEvent) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#24 0x00007ffff6857857 in QAction::event(QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#25 0x00007ffff685d70c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#26 0x00007ffff6861b8a in QApplication::notify(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#27 0x00007ffff6328b5e in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#28 0x00007ffff688e474 in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4

...

Same problem when I click on actionZoom_In/Out from the MainWindow menu except that segfault occurs at the second click.

Anyone could see what's wrong ?

Answer 1

Don't send the key event yourself; let the system do its job.

Create two slots: one for zoom in and one for out. Call them directly from the keyPressEvent via connections from the main window.

Answer 2

ALWAYS check if an event (any pointer tbh) is nullptr. Even event->ignore() causes segfault on a nullptr. (And yes, it does happen. Functions in Qt sometimes get called with a nullptr event)