How to mix mouseEnterEvent and mouseMove? - c++

Question

in my application I try to connect nodes with lines. I use a QGraphicsView with a QGraphicsScene and my own QGraphicsItems. Now if I click on an item I want to draw a line to another node. To give a visual feedback, the goal should change color if the mouse hovers over the goal. The basics works so far, but my problem is that if I drag a line with the mouse (via mouseMoveEvent), I do not get any hoverEvents any more. I replicated the behaviour with this code:
Header File:
#pragma once
#include <QtWidgets/Qwidget>
#include <QGraphicsItem>
#include <QGraphicsScene>
class HaggiLearnsQt : public QWidget
{
Q_OBJECT
public:
HaggiLearnsQt(QWidget *parent = Q_NULLPTR);
};
class MyScene : public QGraphicsScene
{
public:
MyScene(QObject* parent = 0);
void mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent);
};
class MyItem : public QGraphicsItem
{
public:
MyItem(QGraphicsItem* parent = Q_NULLPTR);
QRectF boundingRect() const;
void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
void hoverEnterEvent(QGraphicsSceneHoverEvent *event);
void hoverLeaveEvent(QGraphicsSceneHoverEvent *event);
bool mouseOverItem;
};
Implementation:
#include "HaggiLearnsQt.h"
#include <QMessageBox>
#include <QFrame>
#include <QHBoxLayout>
#include <QGraphicsView>
MyScene::MyScene(QObject* parent)
{}
void MyScene::mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
QGraphicsScene::mouseMoveEvent(mouseEvent);
}
MyItem::MyItem(QGraphicsItem* parent) : mouseOverItem(false)
{
setAcceptHoverEvents(true);
}
QRectF MyItem::boundingRect() const
{
return QRectF(-50, -50, 50, 50);
}
void MyItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
QBrush b = QBrush(Qt::black);
if(mouseOverItem)
b = QBrush(Qt::yellow);
painter->setBrush(b);
painter->drawRect(boundingRect());
}
void MyItem::hoverEnterEvent(QGraphicsSceneHoverEvent *event)
{
mouseOverItem = true;
QGraphicsItem::hoverEnterEvent(event);
}
void MyItem::hoverLeaveEvent(QGraphicsSceneHoverEvent *event)
{
mouseOverItem = false;
QGraphicsItem::hoverLeaveEvent(event);
}
HaggiLearnsQt::HaggiLearnsQt(QWidget *parent)
: QWidget(parent)
{
QHBoxLayout* layout = new QHBoxLayout(this);
MyScene* graphicsScene = new MyScene();
QGraphicsView* graphicsView = new QGraphicsView();
graphicsView->setRenderHint(QPainter::RenderHint::Antialiasing, true);
graphicsView->setScene(graphicsScene);
layout->addWidget(graphicsView);
graphicsView->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
graphicsView->setMinimumHeight(200);
graphicsView->setMinimumWidth(200);
graphicsView->setStyleSheet("background-color : gray");
MyItem* myitem = new MyItem();
myitem->setPos(50, 50);
graphicsScene->addItem(myitem);
}
And the default main.cpp:
#include "HaggiLearnsQt.h"
#include <QtWidgets/QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
HaggiLearnsQt w;
w.show();
return a.exec();
}
If you run the code, a box appears in the middle of the window. If you hover over the box, it changes color. Now try to klick outside the box and drag wiht pressed button into the box. The box does not receive a hover and does not change color.
So my question is: Can I somehow change the item while I move the mouse with a pressed button?

Answer 1

You can get the hovered item passing mouseEvent->scenePos() to the QGraphicsScene::itemAt method inside the scene mouse move event handler.
Have a pointer to a MyItem instance, in MyScene:
class MyScene : public QGraphicsScene
{
MyItem * hovered;
//...
initialize it to zero in MyScene constructor:
MyScene::MyScene(QObject* parent)
{
hovered = 0;
}
then use it to track the current highlighted item (if there's one):
void MyScene::mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
if(mouseEvent->buttons())
{
QGraphicsItem * item = itemAt(mouseEvent->scenePos(), QTransform());
MyItem * my = dynamic_cast<MyItem*>(item);
if(my != 0)
{
qDebug() << mouseEvent->scenePos();
if(!my->mouseOverItem)
{
my->mouseOverItem = true;
my->update();
hovered = my;
}
}
else
{
if(hovered != 0)
{
hovered->mouseOverItem = false;
hovered->update();
hovered = 0;
}
}
}
QGraphicsScene::mouseMoveEvent(mouseEvent);
}
The line if(mouseEvent->buttons()) at the beginning prevents the check to be performed if no mouse button is held.
Don't forget to initialize mouseOverItem to false in MyItem constructor:
MyItem::MyItem(QGraphicsItem* parent) : mouseOverItem(false)
{
setAcceptHoverEvents(true);
mouseOverItem = false;
}