Can I get mouse events in a QGraphicsItem?

Question

I have subclassed QGraphicsRectItem, and it's not receiving any mouse events. I've seen other questions similar to this say I need to enable mouse tracking, but setMouseTracking is in QWidget, and QGraphicsItem does not appear to be a QWidget.

I've implemented paint, and that's working. In my subclassed QGraphicsView I am getting mouse events.

The docs seem to think I should just override the mousePressEvent function (for example) and I should start getting the events. Whether I forward the mousePressEvent to the superclass of my QGraphicsView or not doesn't seem to make any difference.

Answer 1

In your subclassed QGraphicsView, you need to call the default implementations of overridden mouse event methods if you want them to propagate down to the items. For example:

CustomView::mousePressEvent(QMouseEvent *event)
{
    // handle the event as you like

    QGraphicsView::mousePressEvent(event); // then call default implementation
}

If you want to accept hover events, you need to call QGraphicsItem::setAcceptHoverEvents(true);. Otherwise you do not need to enable any particular mouse tracking.

EDIT: Here is a full working example:

#include <QtGui>

class CustomView : public QGraphicsView
{
protected:
    void mousePressEvent(QMouseEvent *event)
    {
        qDebug() << "Custom view clicked.";
        QGraphicsView::mousePressEvent(event);
    }
};

class CustomItem : public QGraphicsRectItem
{
protected:
    void mousePressEvent(QGraphicsSceneMouseEvent *event)
    {
        qDebug() << "Custom item clicked.";
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    CustomItem item;
    item.setRect(20, 20, 60, 60);

    QGraphicsScene scene(0, 0, 100, 100);
    scene.addItem(&item);

    CustomView view;
    view.setScene(&scene);
    view.show();
    return a.exec();
}

Answer 2

I had a similar problem with a view not accepting mouse clicks. The problem was that I needed to enable the view ( ui->view->setEnabled(true) ).

Answer 3

I went through the same problems you have encountered and I wanted to add some insights on top of Anthony's really good answer. Here is an example I wrote showing some features that can be implemented using the mouse events and the keyboard events.

Note that the events do not propagate to QGraphicsItems in a QGraphicsItemGroup or in a QList<QGraphicsItem> (it took me a while to figure that out).

#include <QtGui>
#include <QGraphicsRectItem>
#include <QGraphicsView>
#include <QApplication>
#include <QGraphicsSceneMouseEvent>

class CustomItem : public QGraphicsEllipseItem
{
protected:
    void mousePressEvent(QGraphicsSceneMouseEvent *event)
    {
        if(event->button() == Qt::LeftButton) {
            if(event->modifiers() == Qt::ShiftModifier) {
                qDebug() << "Custom item left clicked with shift key.";
                // add the item to the selection
                setSelected(true);
            } else if(event->modifiers() == Qt::AltModifier){
                qDebug() << "Custom item left clicked with alt key.";
                // resize the item
                double radius = boundingRect().width() / 2.0;
                _center = QPointF(boundingRect().topLeft().x() + pos().x() + radius, boundingRect().topLeft().y() + pos().y() + radius);
                QPointF pos = event->scenePos();
                qDebug() << boundingRect() << radius << this->pos() << pos << event->pos();
                double dist = sqrt(pow(_center.x()-pos.x(), 2) + pow(_center.y()-pos.y(), 2));
                if(dist / radius > 0.8) {
                    qDebug() << dist << radius << dist / radius;
                    _isResizing = true;
                } else {
                    _isResizing = false;
                }
            } else {
                qDebug() << "Custom item left clicked.";
                QGraphicsItem::mousePressEvent(event);
                event->accept();
            }
        } else if(event->button() == Qt::RightButton) {
            qDebug() << "Custom item right clicked.";
            event->ignore();
        }
    }
    void mouseMoveEvent(QGraphicsSceneMouseEvent *event)
    {
        if(event->modifiers() == Qt::AltModifier && _isResizing){
            QPointF pos = event->scenePos();
            double dist = sqrt(pow(_center.x()-pos.x(), 2) + pow(_center.y()-pos.y(), 2));
            setRect(_center.x()-this->pos().x()-dist, _center.y()-this->pos().y()-dist, dist*2, dist*2);
        } else if(event->modifiers() != Qt::AltModifier) {
            qDebug() << "Custom item moved.";
            QGraphicsItem::mouseMoveEvent(event);
            qDebug()<<"moved"<<pos();
        }
    }
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
    {
        if(event->modifiers() == Qt::AltModifier && _isResizing) {
            _isResizing = false;
        } else if(event->modifiers() != Qt::ShiftModifier) {
            QGraphicsItem::mouseReleaseEvent(event);
        }
    }

    int type() const
    {
        // Enable the use of qgraphicsitem_cast with this item.
        return UserType+1;
    }
private:
    QPointF _center;
    bool _isResizing;
};

class CustomScene : public QGraphicsScene
{
protected:
    void mousePressEvent(QGraphicsSceneMouseEvent *event)
    {
        qDebug() << "Custom scene clicked.";
        QGraphicsScene::mousePressEvent(event);
        if(!event->isAccepted()) {
            if(event->button() == Qt::LeftButton) {
                // add a custom item to the scene
                QPointF pt = event->scenePos();
                CustomItem * item = new CustomItem();
                item->setRect(pt.x()-25, pt.y()-25, 50, 50);
                item->setFlags(QGraphicsItem::ItemIsSelectable|
                              QGraphicsItem::ItemIsMovable);
                addItem(item);
            } else if(event->button() == Qt::RightButton) {
                // check whether there is an item under the cursor
                QGraphicsItem * itemToRemove = NULL;
                foreach(auto item, items(event->scenePos())) {
                    if(item->type() == QGraphicsItem::UserType+1) {
                        itemToRemove = item;
                        break;
                    }
                }
                if(itemToRemove) {
                    // remove the item from the graphicsScene
                    removeItem(itemToRemove);
                }
            }
        }
    }
    void mouseMoveEvent(QGraphicsSceneMouseEvent *event)
    {
        qDebug() << "Custom scene moved.";
        QGraphicsScene::mouseMoveEvent(event);
    }
    void keyPressEvent(QKeyEvent * event) {
        if(event->key() == Qt::Key_Backspace) {
            // remove all selected items
            qDebug() << "selected items" << selectedItems().size();
            while(!selectedItems().isEmpty()) {
                removeItem(selectedItems().front());
            }
        } else {
            QGraphicsScene::keyPressEvent(event);
        }
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    CustomItem item;
    item.setRect(20, 20, 60, 60);
    item.setFlags(QGraphicsItem::ItemIsSelectable|
                  QGraphicsItem::ItemIsMovable);

    CustomScene scene;
    scene.setSceneRect(0, 0, 500, 500);
    scene.addItem(&item);

    QGraphicsView view;
    view.setScene(&scene);
    view.show();
    return a.exec();
}

Hope it helps too!