重新实现鼠标事件时对 QGraphicsView 的奇怪影响

Posted 2023-02-17

【中文标题】重新实现鼠标事件时对 QGraphicsView 的奇怪影响

【英文标题】：Strange effect on QGraphicsView upon Reimplementation of Mouse Event

【发布时间】：2016-12-13 19:39:49

【问题描述】：

我使用这个源代码开始了一个小项目：

#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsRectItem>
#include <QGraphicsLineItem>

class CustomRectItem : public QGraphicsRectItem

public:
    CustomRectItem (const QRectF& rect) : QGraphicsRectItem(rect) 
    setFlag(QGraphicsItem::ItemIsMovable);
    setFlag(QGraphicsItem::ItemSendsScenePositionChanges);
    setAcceptHoverEvents(true);


void addLine(QGraphicsLineItem *line1, QGraphicsLineItem *line2) 
    if (this->data(0).toString() == "_p1") 
        this->leftLine = line1;
        this->topLine = line2;
    
    if (this->data(0).toString() == "_p2") 
        this->topLine = line1;
        this->rightLine = line2;
    
    if (this->data(0).toString() == "_p3") 
        this->rightLine = line1;
        this->bottomLine = line2;
    
    if (this->data(0).toString() == "_p4") 
        this->bottomLine = line1;
        this->leftLine = line2;
    


QPointF center(void) 
    return QPointF((rect().x() + rect().width() / 2),
                   (rect().y() + rect().height() / 2));


QVariant itemChange(GraphicsItemChange change, const QVariant &value)

    if (change == ItemPositionChange && scene()) 
        // value is the new position.
        QPointF newPos = value.toPointF();

        moveLineToCenter(newPos, this->data(0).toString());
    
    return QGraphicsItem::itemChange(change, value);


void moveLineToCenter(QPointF newPos, QString pointString) 
    // Converts the polygon upper left position
    // to the upper left "handle"-rect center position
    qreal xOffset = rect().x() + rect().width()/2;
    qreal yOffset = rect().y() + rect().height()/2;

    QPointF newCenterPos = QPointF(newPos.x() + xOffset, newPos.y() + yOffset);

    QPointF p1;
    QPointF p2;

 // upper-left point
    if (pointString == "_p1") 
        p1 = leftLine->line().p1();
        p2 = newCenterPos;
        leftLine->setLine(QLineF(p1, p2));
        p1 = newCenterPos;
        p2 = topLine->line().p2();
        topLine->setLine(QLineF(p1, p2));
  // upper-right point
     else if (pointString == "_p2") 
        p1 = topLine->line().p1();
        p2 = newCenterPos;
        topLine->setLine(QLineF(p1, p2));
        p1 = newCenterPos;
        p2 = rightLine->line().p2();
        rightLine->setLine(QLineF(p1, p2));
  // lower-right point
     else if (pointString == "_p3") 
        p1 = rightLine->line().p1();
        p2 = newCenterPos;
        rightLine->setLine(QLineF(p1, p2));
        p1 = newCenterPos;
        p2 = bottomLine->line().p2();
        bottomLine->setLine(QLineF(p1, p2));
  // lower-left point
     else if (pointString == "_p4") 
        p1 = bottomLine->line().p1();
        p2 = newCenterPos;
        bottomLine->setLine(QLineF(p1, p2));
        p1 = newCenterPos;
        p2 = leftLine->line().p2();
        leftLine->setLine(QLineF(p1, p2));
     else 
        return;
    


/////////////////////////////////////////////////////
/* -- comment-in this block to observe problem --  //
/////////////////////////////////////////////////////

protected:
virtual void hoverEnterEvent(QGraphicsSceneHoverEvent *event) 
    Q_UNUSED(event);
    this->setCursor(Qt::SizeAllCursor);

virtual void hoverLeaveEvent(QGraphicsSceneHoverEvent *event) 
    Q_UNUSED(event);
    this->unsetCursor();

virtual void mousePressEvent(QGraphicsSceneMouseEvent *event) 
    Q_UNUSED(event);
    this->setCursor(Qt::BlankCursor);

virtual void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) 
    Q_UNUSED(event);
    this->unsetCursor();
    this->data(0).toString();
    if (this->isUnderMouse()) 
           this->setCursor(Qt::SizeAllCursor);
    


/////////////////////////////////////////////////////
//       -- end of problem-causing block --        //
///////////////////////////////////////////////////// */

private:
QGraphicsLineItem *topLine;
QGraphicsLineItem *rightLine;
QGraphicsLineItem *bottomLine;
QGraphicsLineItem *leftLine;
;

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

    QGraphicsScene scene;

    CustomRectItem *custRect1 = new CustomRectItem(QRectF(30, 30, 10, 10));
    scene.addItem(custRect1);
    custRect1->setData(0, "_p1");

    CustomRectItem *custRect2 = new CustomRectItem(QRectF(70, 30, 10, 10));
    scene.addItem(custRect2);
    custRect2->setData(0, "_p2");

    CustomRectItem *custRect3 = new CustomRectItem(QRectF(70, 70, 10, 10));
    scene.addItem(custRect3);
    custRect3->setData(0, "_p3");

    CustomRectItem *custRect4 = new CustomRectItem(QRectF(30, 70, 10, 10));
    scene.addItem(custRect4);
    custRect4->setData(0, "_p4");

    QGraphicsLineItem *topLine = scene.addLine(QLineF(custRect1->center(), custRect2->center()));
    QGraphicsLineItem *rightLine = scene.addLine(QLineF(custRect2->center(), custRect3->center()));
    QGraphicsLineItem *bottomLine = scene.addLine(QLineF(custRect3->center(), custRect4->center()));
    QGraphicsLineItem *leftLine = scene.addLine(QLineF(custRect4->center(), custRect1->center()));

    custRect1->addLine(leftLine, topLine);
    custRect2->addLine(topLine, rightLine);

    custRect3->addLine(rightLine, bottomLine);
    custRect4->addLine(bottomLine, leftLine);

    QGraphicsView view(&scene);
    view.show();

    return a.exec();

编译上面的代码，你应该会得到一个小窗口，其中只包含 QGraphivsView、一个多边形和四个附加到多边形顶点的句柄。 现在使用鼠标移动手柄。再次移动它。 一切都应该像您在任何图形编辑器中所期望的那样运行。 接下来，在代码块中添加注释，该代码块重新实现了一些鼠标事件，基本上只是在悬停和拖动手柄时改变光标。 重新编译并重新开始移动一个句柄。再次移动相同的手柄，然后……看看会发生什么！怎么回事？它跳回到它的初始位置。 有谁知道如何解释这种行为以及如何解决它？

谢谢！

【参考方案1】：

确保在实现后调用基类事件以确保正常功能保持不变。

virtual void hoverEnterEvent(QGraphicsSceneHoverEvent *event) 
    this->setCursor(Qt::SizeAllCursor);
    //
    QGraphicsRectItem::hoverEnterEvent( event );

virtual void hoverLeaveEvent(QGraphicsSceneHoverEvent *event) 
    this->unsetCursor();
    //
    QGraphicsRectItem::hoverLeaveEvent( event );


virtual void mousePressEvent(QGraphicsSceneMouseEvent *event) 
    this->setCursor(Qt::BlankCursor);
    //
    QGraphicsRectItem::mousePressEvent( event );

virtual void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) 
    this->unsetCursor();
    this->data(0).toString();
    if (this->isUnderMouse()) 
           this->setCursor(Qt::SizeAllCursor);
    
    //
    QGraphicsRectItem::mouseReleaseEvent( event );

【参考方案2】：

您需要为 mousePressEvent 和 mouseReleaseEvent 方法调用基类实现：

QGraphicsRectItem::mousePressEvent (event);

和

QGraphicsRectItem::mouseReleaseEvent (event);

根据文档，基本实现处理鼠标抓取、移动和选择，因此在这种情况下您需要调用它们。手柄完全移动了，我有点惊讶，它们显然是移动的，但是项目的位置在内部没有得到正确处理。