QGraphicsItem to react on CTRL left click

[JanLaloux]

I have a QGraphicsScene with different items such a rectangles and circles. It must be possible to click on them in different ways, eg a Control left click, and that should trigger a function call with one or more parameters. What is the best way to achieve this?

[mrjj]

hi
if you override mousePressEvent(QMouseEvent *event)
you can check for ctrl there

virtual void mousePressEvent(QMouseEvent *event) {
if (event->modifiers() == Qt::ControlModifier && event->button() == Qt::LeftButton)
call whatever
}

you can do that for
QGraphicsItem or QGraphicsScene depending on your needs.
see
http://www.qtforum.org/article/20343/qgraphicsscenemouseevent-event-has-no-value.html
for code sample.
Note the use of scenePos() and not Pos()

You can use QGraphicsScene::itemAt to find out which items was under mouse.

[JanLaloux]

I tried the approach but it seems not working. I made a simple example with a class Circle that inherts from QGraphicsEllipseItem. Clicking on a circle does not trigger the Circle::mousePressEvent(). What am I missing?

#include <QApplication>
#include <QMessageBox>
#include <QtWidgets/QMainWindow>
#include <QtWidgets/QGraphicsScene>
#include <QtWidgets/QGraphicsView>
#include <QtWidgets/QGridLayout>
#include <QtWidgets/QGraphicsEllipseItem>

class Circle : public QGraphicsEllipseItem {
public:
	Circle::Circle(const qreal ax, const qreal ay, const qreal wx, const qreal wy, QGraphicsScene *scene) :
		QGraphicsEllipseItem(ax, ay, wx, wy)
	{
		setPen( QPen(QBrush(Qt::black), 2, Qt::SolidLine, Qt::SquareCap, Qt::MiterJoin) );
		setBrush( QBrush(Qt::gray) );
		setRect(ax, ay, wx, wy);
		setAcceptedMouseButtons(Qt::LeftButton | Qt::RightButton);
		scene->addItem(this);
	}

	~Circle(){}

	void Circle::mousePressEvent(QMouseEvent *event) {
		QMessageBox msgBox;
		QString QResult{ QString::fromLatin1("Circle clicked") };
		msgBox.setText( QResult );
		msgBox.exec();
	}
};



class MainWindow : public QMainWindow
{
public:
	QWidget *baseWidget;
	QGridLayout *baseWidgetGrid;
	QGraphicsScene *graphicsScene;
	QGraphicsView *graphicsView;

public:
	MainWindow::MainWindow(QWidget *parent=0) :
		QMainWindow(parent)
	{
		graphicsScene = new QGraphicsScene(0, 0, 100, 100);
		new Circle(  5,  5, 20, 20, graphicsScene );
		new Circle( 35, 35, 20, 20, graphicsScene );
		new Circle( 65, 65, 20, 20, graphicsScene );

		baseWidget = new QWidget(this);
		baseWidgetGrid = new QGridLayout(baseWidget);
			graphicsView = new QGraphicsView(baseWidget);
			graphicsView->setScene(graphicsScene);
		baseWidgetGrid->addWidget(graphicsView, 0, 0, 1, 1);

		setCentralWidget(baseWidget);
	}

	~MainWindow(){}
};


int main(int argc, char *argv[])
{
	QApplication Application(argc, argv);
	MainWindow MainWin;
	MainWin.show();
	return Application.exec();
}

[mrjj]

Hmmm, you have listed it under public:
Try move it to
protected:
Also normally its virtual

so try with

protected: virtual void Circle::mousePressEvent(QMouseEvent *event) { .. }

note:
you can remove the Circle:: when the function is implemented in the .H file.

[JohanSolo]

@mrjj said:

Also normally its virtual

so try with

protected: virtual void Circle::mousePressEvent(QMouseEvent *event) { .. }

From a purely technical point of view, the virtual is not necessary when reimplementing a virtual method, the `virtualness' is inherited from the base class. I therefore don't think this should help.

[JanLaloux]

No help, not with protected: nor protected: virtual ...

[mrjj]

@JanLaloux
Ok, Im on mobile so cant check the code.
Have a look at the 40000 sample (in chip.h)
It has a clickable item.

[JanLaloux]

I noticed in Chip that the event passed is of type QGraphicsSceneMouseEvent * instead of QMouseEvent *. Changed it and now it works, even as public. Thanks for the support!

[mrjj]

@JanLaloux
Good spotted! :)

[Chris Kawa]

On a side note: when overriding virtual methods use the override specifier. This way you'll get an instant error message at compile time if you make a typo or mismatch arguments like in this case.

[JanLaloux]

OK, now I understand the meaning of the Q_DECL_OVERRIDE macro I used to see in the examples...

[JanLaloux]

I have modified my example. The mouse click works but I experience something strange. After something was clicked whatever click anywhere on the scene triggers the same mouse click method of the first clicked object! Any ideas on this?

#include <string>;
#include <QtWidgets/QApplication>
#include <QtWidgets/QMessageBox>
#include <QtWidgets/QMainWindow>
#include <QtWidgets/QGraphicsScene>
#include <QtWidgets/QGraphicsView>
#include <QtWidgets/QGridLayout>
#include <QtWidgets/QGraphicsEllipseItem>

using namespace std;


class Circle : public QGraphicsEllipseItem {
public:
	Circle::Circle(int Id, const qreal ax, const qreal ay, const qreal wx, const qreal wy, QGraphicsScene *scene) :
		QGraphicsEllipseItem(ax, ay, wx, wy),
		Id(Id)
	{
		setPen( QPen(QBrush(Qt::black), 1, Qt::SolidLine, Qt::SquareCap, Qt::MiterJoin) );
		setBrush( QBrush(Qt::gray) );
		setRect(ax, ay, wx, wy);
		setAcceptedMouseButtons(Qt::LeftButton | Qt::RightButton);
		scene->addItem(this);
	}

	~Circle(){}

	void Circle::mousePressEvent(QGraphicsSceneMouseEvent *event) {
		QMessageBox msgBox;
		string msg{ "Circle " + to_string(Id) + " clicked" };
		QString QResult{ QString::fromLatin1(msg.c_str()) };
		msgBox.setText( QResult );
		msgBox.exec();
	}
	int Id;
};




class MainWindow : public QMainWindow
{
public:
	QWidget *baseWidget;
	QGridLayout *baseWidgetGrid;
	QGraphicsScene *graphicsScene;
	QGraphicsView *graphicsView;

public:
	MainWindow::MainWindow(QWidget *parent=0) :
		QMainWindow(parent)
	{
		graphicsScene = new QGraphicsScene(0, 0, 120, 120);
		graphicsScene->addRect( 10, 10,  100, 100 );
		new Circle( 1, 20, 20, 20, 20, graphicsScene );
		new Circle( 2, 75, 75, 20, 20, graphicsScene );

		baseWidget = new QWidget(this);
		baseWidgetGrid = new QGridLayout(baseWidget);
			graphicsView = new QGraphicsView(baseWidget);
			graphicsView->setRenderHint(QPainter::Antialiasing);
			graphicsView->setScene(graphicsScene);
		baseWidgetGrid->addWidget(graphicsView, 0, 0, 1, 1);

		setCentralWidget(baseWidget);
	}

	~MainWindow(){}
};


int main(int argc, char *argv[])
{
	QApplication Application(argc, argv);
	MainWindow MainWin;
	MainWin.show();
	return Application.exec();
}

[mrjj]

@JanLaloux said:

so when clicking outside, it would still say
"Circle " bla bla from the msgBox ?

I wonder if scene or view need
setMouseTracking(true);

[JanLaloux]

@mrjj
Yes, wherever you click on the scene you get the messagebox with the ID of the circle that was first clicked. The code I included is "ready to go" if you want to try it.

[mrjj]

@JanLaloux
Hi , very strange. it said nothing when I clicked. not on circle or outside.

update:
Ahh I see that older post used QGraphicsSceneMouseEvent and not QMouseEvent
so just copy & paste error I guess.

I then added

protected:
    virtual void mousePressEvent(QGraphicsSceneMouseEvent *event) {
    qDebug() << "test";
    }

to Circle and now it print test when click on circle but not outside.

So not really sure whats going on.

Does that work for you too?

#include <QApplication>
#include <QMessageBox>
#include <QtWidgets/QMainWindow>
#include <QtWidgets/QGraphicsScene>
#include <QtWidgets/QGraphicsView>
#include <QtWidgets/QGridLayout>
#include <QtWidgets/QGraphicsEllipseItem>
#include <qDebug>

class Circle : public QGraphicsEllipseItem {
public:
    Circle(const qreal ax, const qreal ay, const qreal wx, const qreal wy, QGraphicsScene *scene) :
        QGraphicsEllipseItem(ax, ay, wx, wy)
    {
        setPen( QPen(QBrush(Qt::black), 2, Qt::SolidLine, Qt::SquareCap, Qt::MiterJoin) );
        setBrush( QBrush(Qt::gray) );
        setRect(ax, ay, wx, wy);
        setAcceptedMouseButtons(Qt::LeftButton | Qt::RightButton);
        scene->addItem(this);
    }

    ~Circle(){}
protected:
    virtual void mousePressEvent(QGraphicsSceneMouseEvent *event) {
    qDebug() << "test";
    }

   virtual void mousePressEvent(QMouseEvent *event) {
        QMessageBox msgBox;
        QString QResult("DEER");
        msgBox.setText( QResult );
        msgBox.exec();
    }
};



class MainWindow : public QMainWindow
{
public:
    QWidget *baseWidget;
    QGridLayout *baseWidgetGrid;
    QGraphicsScene *graphicsScene;
    QGraphicsView *graphicsView;

public:
    MainWindow(QWidget *parent=0) :
        QMainWindow(parent)
    {
        graphicsScene = new QGraphicsScene(0, 0, 100, 100);
        new Circle(  5,  5, 20, 20, graphicsScene );
        new Circle( 35, 35, 20, 20, graphicsScene );
        new Circle( 65, 65, 20, 20, graphicsScene );

        baseWidget = new QWidget(this);
        baseWidgetGrid = new QGridLayout(baseWidget);
            graphicsView = new QGraphicsView(baseWidget);
            graphicsView->setScene(graphicsScene);
        baseWidgetGrid->addWidget(graphicsView, 0, 0, 1, 1);

        setCentralWidget(baseWidget);
    }

    ~MainWindow(){}
};


int main(int argc, char *argv[])
{
    QApplication Application(argc, argv);
    MainWindow MainWin;
    MainWin.show();
    return Application.exec();
}

[JanLaloux]

It's the msgBox that is the culprit!
Replace the qDebug output with the messagebox I used in my code and it goes wrong. The behaviour is as it should without it, I suppose this is a bug...
Remark: it is not needed to make the member protected
Anyway, thanks a lot for the help!

[mrjj]

aha!
Pretty strange.

ok, its a habit for mousepressed for widgets :)

np. Good with code you can just run :)

[willems1994]

For people having the same problem that the QMessageBox keeps appearing anywhere you click, you can fix it by ungrabbing the mouse after closing the box:

scene->mouseGrabberItem()->ungrabMouse();

@JanLaloux said in QGraphicsItem to react on CTRL left click:

@mrjj
Yes, wherever you click on the scene you get the messagebox with the ID of the circle that was first clicked. The code I included is "ready to go" if you want to try it.