Qt How to move view with mouseMoveEvent - c++

Question

While using a QGraphicsScene and QGraphicsView I wanted to achieve that if the mouse is at one of the borders of the screen the view moves with it (like it is the case in most RTS games). However when dealing with the mouseMoveEvent I only get a stack overflow, most likely because the event is called infinitely many times once the mouse is at a certain location.
My camera class has a pointer to the view and inherits QGraphicsRectItem and is added to the scene in the main class.
Is there a way to prevent this event from happening at a certain point? Or is there even an elegant solution to this? One additional problem with my attempt is that the camera class has to grab the mouse when i want the mouseMoveEvent to work.
void Camera::mouseMoveEvent(QGraphicsSceneMouseEvent* e)
{
int view_x = view->mapFromScene(e->pos()).x();
int view_y = view->mapFromScene(e->pos()).y();
int horizontalSliderPos = view->horizontalScrollBar()->sliderPosition();
int verticalSliderPos = view->verticalScrollBar()->sliderPosition();
if (view_x < 100) {
view->horizontalScrollBar()->setSliderPosition(horizontalSliderPos - 5);
}
if (view_x > Constants::VIEWWIDTH - 100) {
view->horizontalScrollBar()->setSliderPosition(view->horizontalScrollBar()->sliderPosition() + 5);
}
if (view_y < 100) {
view->verticalScrollBar()->setSliderPosition(view->verticalScrollBar()->sliderPosition() - 5);
}
if (view_y > Constants::VIEWHEIGHT - 100) {
view->verticalScrollBar()->setSliderPosition(view->verticalScrollBar()->sliderPosition() + 5);
}
}

Answer 1

After some trying and experiments I obtained a solution, that might be a good starting point. As you already mentioned you have to break the recursive calls of mouseMoveEvent. I broke the recursive call with a simple boolean variable, but maybe also QSignalBlocker can be helpful here, even though it blocks all signals.
Another feature I tried to implement is that there is still a moving window in case of a non-moving mouse at the margin. I did this by using a QTimer, that fires every 25ms.
main.cpp
#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QMainWindow>
#include "MyScene.h"
int main(int argc, char* argv[]) {
QApplication app(argc, argv);
auto view = new QGraphicsView;
auto model = new MyScene;
view->setMouseTracking(true);
view->setScene(model);
model->setView(view);
model->addRect(QRectF(20, 20, 20, 20));
model->addRect(QRectF(300, 20, 20, 20));
model->addRect(QRectF(0, 0, 500, 500), QPen(Qt::blue)); // Complete Scene
view->show();
view->setSceneRect(QRectF(120, 20, 20, 20));
return app.exec();
}
MyScene.h
#pragma once
#include <QDebug>
#include <QGraphicsSceneEvent>
#include <QGraphicsView>
#include <QTimer>
class MyScene : public QGraphicsScene {
Q_OBJECT
public:
MyScene(QWidget* parent=nullptr) : QGraphicsScene(parent) {
}
void setView(QGraphicsView* view) {
mView = view;
}
protected:
void mouseMoveEvent(QGraphicsSceneMouseEvent* event) override {
if (mTranslating) return;
delete mRepeater; // Destroys connect
mRepeater = new QTimer;
if (!mView) return;
mTranslating = true;
int tx{ 0 };
int ty{ 0 };
const int margin = 20;
int sx = mView->mapFromGlobal(event->screenPos()).x();
int sy = mView->mapFromGlobal(event->screenPos()).y();
if (sx < margin) {
tx = -1;
}
else if (sx > mView->width() - margin) {
tx = 1;
}
if (sy < margin) {
ty = -1;
}
else if (sy > mView->height() - margin) {
ty = 1;
}
if (tx != 0 || ty != 0) {
auto rect = mView->sceneRect();
rect.translate(QPointF{ (qreal)tx,(qreal)ty });
mView->setSceneRect(rect);
connect(mRepeater, &QTimer::timeout, [=]() { // Moves even if mouse is not moved
auto rect = mView->sceneRect();
rect.translate(QPointF{ (qreal)tx,(qreal)ty });
mView->setSceneRect(rect);
});
mRepeater->start(25);
}
mTranslating = false;
}
QGraphicsView* mView{ nullptr };
bool mTranslating{ false };
QTimer* mRepeater{ nullptr };
};

Answer 2

I think this problem could be avoided if you would handle the mouse move event in your QGraphicsView class instead of doing it in the QGraphicsScene class.