Struggling with mouse events

[SteveFalco]

I am trying to catch both keyboard and mouse events. I am able to get both KeyPress and KeyRelease events, but I'm only able to get MouseButtonPress events. Something is eating the MouseButtonRelease events before I can see them.

Below is a very stripped down app that shows the problem. I'm using QtCreator and I simply added a QGraphicsView, to which I attach the eventFilter.

Interestingly, in the code below, if I comment out the ui->graphicsView->setScene(m_scene); line then I don't even get the MouseButtonPress events, but I still get the KeyPress and KeyRelease events.

One last comment - if I change the else clause of the eventFilter to always return true instead of QObject::eventFilter(obj, event); then I do get both MouseButtonPress and MouseButtonRelease events, but the graphicsView doesn't work properly, because it is not getting its events.

#include "map.h"
#include "./ui_map.h"

Map::Map(QWidget *parent)
	: QMainWindow(parent), ui(new Ui::Map)
{
	ui->setupUi(this);

	m_scene = new QGraphicsScene();
	ui->graphicsView->setScene(m_scene);

	ui->graphicsView->installEventFilter(this);
}

Map::~Map()
{
	delete ui;
}

bool
Map::eventFilter(QObject *obj, QEvent *event)
{
	qInfo() << "event " << event;

	if(event->type() == QEvent::KeyPress) {
		QKeyEvent *keyEvent = static_cast<QKeyEvent *>(event);
		qDebug("Ate key press %d", keyEvent->key());
		return true;
	} else if(event->type() == QEvent::KeyRelease) {
		QKeyEvent *keyEvent = static_cast<QKeyEvent *>(event);
		qDebug("Ate key release %d", keyEvent->key());
		return true;
	} else if(event->type() == QEvent::MouseButtonPress) {
		QMouseEvent *mouseEvent = static_cast<QMouseEvent *>(event);
		qDebug("Ate mouse press %d", mouseEvent->button());
		return true;
	} else if(event->type() == QEvent::MouseButtonRelease) {
		QMouseEvent *mouseEvent = static_cast<QMouseEvent *>(event);
		qDebug("Ate mouse release %d", mouseEvent->button());
		return true;
	} else {
		// standard event processing
		return QObject::eventFilter(obj, event);
	}
}

For completeness, here is the header file:

#ifndef MAP_H
#define MAP_H

#include <QMainWindow>
#include <QKeyEvent>
#include <QMouseEvent>
#include <QGraphicsScene>
#include <QGraphicsView>

QT_BEGIN_NAMESPACE
namespace Ui {
	class Map;
}
QT_END_NAMESPACE

class Map : public QMainWindow
{
	Q_OBJECT

	public:
		explicit Map(QWidget *parent = nullptr);
		~Map();

	private:
		Ui::Map *ui;

		QGraphicsScene* m_scene;

		bool eventFilter(QObject *obj, QEvent *event);
};

#endif // MAP_H

And here is the ui file:

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>Map</class>
 <widget class="QMainWindow" name="Map">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>425</width>
    <height>824</height>
   </rect>
  </property>
  <property name="sizePolicy">
   <sizepolicy hsizetype="Preferred" vsizetype="Preferred">
    <horstretch>0</horstretch>
    <verstretch>0</verstretch>
   </sizepolicy>
  </property>
  <property name="sizeIncrement">
   <size>
    <width>0</width>
    <height>0</height>
   </size>
  </property>
  <property name="baseSize">
   <size>
    <width>0</width>
    <height>0</height>
   </size>
  </property>
  <property name="windowTitle">
   <string>Map</string>
  </property>
  <widget class="QWidget" name="centralwidget">
   <layout class="QVBoxLayout" name="verticalLayout">
    <item>
     <widget class="QGraphicsView" name="graphicsView">
      <property name="sizePolicy">
       <sizepolicy hsizetype="Expanding" vsizetype="Expanding">
        <horstretch>0</horstretch>
        <verstretch>0</verstretch>
       </sizepolicy>
      </property>
      <property name="alignment">
       <set>Qt::AlignLeading|Qt::AlignLeft|Qt::AlignTop</set>
      </property>
     </widget>
    </item>
   </layout>
  </widget>
  <widget class="QMenuBar" name="menubar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>425</width>
     <height>23</height>
    </rect>
   </property>
  </widget>
  <widget class="QStatusBar" name="statusbar"/>
 </widget>
 <resources/>
 <connections/>
</ui>

[Pl45m4]

@SteveFalco

QGraphicsView doesn't like to propagate mouseEvents (I know there is something but cant remember the correct explanation right now).
The better solution is to re-implement the mousePressEvent and mouseReleaseEvent in a QGraphicsView subclass directly.

Edit:

Btw, if you still want to stick to your solution, install the filter on the QGraphicsScene (not view), and catch like this:

        if(event->type() == QGraphicsSceneMouseEvent::GraphicsSceneMouseRelease) {
            QGraphicsSceneMouseEvent *mouseEvent = static_cast<QGraphicsSceneMouseEvent *>(event);
            qDebug("Ate Graphics mouse release %d", mouseEvent->button());
            return false;  // or true, if you want to swallow the event here.
                           // but this might cause unwanted behavior within the view/scene
        }
        if(event->type() == QGraphicsSceneMouseEvent::GraphicsSceneMousePress) {
            QGraphicsSceneMouseEvent *mouseEvent = static_cast<QGraphicsSceneMouseEvent *>(event);
            qDebug("Ate Graphics mouse press %d", mouseEvent->button());
            return false; // same here
        }

(Note the QGraphicsSceneMouseEvent-type)

The normal key event will also still work :)

[SteveFalco]

@Pl45m4 said in Struggling with mouse events:

QGraphicsView subclass

Installing the filter on the scene works perfectly. Thanks!