一、简介

GraphicsView支持事件传播体系结构，可以使图元在场景scene中得到提高了已被的精确交互能力。图形视图框架中的事件都是首先由视图进行接收，然后传递给场景，再由场景给相应的图形项。

对于键盘鼠标事件，scene会传递给获得焦点的图形项。如果场景没有获得焦点，那键盘事件会丢弃；如果调用场景setFocus()或者场景中的一个图形项获得了焦点，那么场景会自动获得焦点；如果场景丢失了焦点（如调用clearFocus()）而其中一个图形项获得焦点那场景会保存这个图形项的焦点信息。

图形项默认无法接收悬停事件，可以使用QGraphicsItem的setAcceptHoverEvents()函数使图形项可以接收悬停事件。

二、运行图

（1）五个图形项的运行图如下图所示。

三、详解

1、QGraphicsScene

#ifndef MYSCENE_H
#define MYSCENE_H

#include <QGraphicsScene>
#include <QGraphicsSceneMouseEvent>
#include <QPaintEvent>
#include <QKeyEvent>

class MyScene : public QGraphicsScene
{
    Q_OBJECT
public:
    explicit MyScene(QObject *parent = 0);

protected:
    void keyPressEvent(QKeyEvent *event);
    void mousePressEvent(QGraphicsSceneMouseEvent *event);

signals:

public slots:

};

#endif // MYSCENE_H

#include "myscene.h"

MyScene::MyScene(QObject *parent) :
    QGraphicsScene(parent)
{
    clearFocus();
}

void MyScene::keyPressEvent(QKeyEvent *event)
{
    qDebug("*********MyScene::keyPressEvent***************");
    return QGraphicsScene::keyPressEvent(event);
}

void MyScene::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug("*********MyScene::mousePressEvent***************");
    QGraphicsScene::mousePressEvent(event);
}

2、QGraphicsView

#ifndef MYVIEW_H
#define MYVIEW_H

#include <QGraphicsView>

class MyView : public QGraphicsView
{
    Q_OBJECT
public:
    explicit MyView(QWidget *parent = 0);

protected:
    void keyPressEvent(QKeyEvent *event);
    void mousePressEvent(QMouseEvent *event);
    void paintEvent(QPaintEvent * event);
    void mouseMoveEvent(QMouseEvent *event);
signals:

public slots:

};

#endif // MYVIEW_H

#include "myview.h"
#include <QKeyEvent>

MyView::MyView(QWidget *parent) :
    QGraphicsView(parent)
{
}

void MyView::keyPressEvent(QKeyEvent *event)
{
   qDebug("*********MyView::keyPressEvent***************");
    switch (event->key())
    {
    case Qt::Key_Left :
        scale(1.2, 1.2);
        break;
    case Qt::Key_Right :
        scale(1 / 1.2, 1 / 1.2);
        break;
    case Qt::Key_Up :
        rotate(30);
        break;
    }
    QGraphicsView::keyPressEvent(event);
}

void MyView::mousePressEvent(QMouseEvent *event)
{
    qDebug("************MyView::mousePressEvent*****************");
    QGraphicsView::mousePressEvent(event);
}

void MyView::paintEvent(QPaintEvent *event)
{
    qDebug("************MyView::paintEvent*****************");
    QGraphicsView::paintEvent(event);
}

void MyView::mouseMoveEvent(QMouseEvent *event)
{
    //qDebug("************MyView::mouseMoveEvent*****************");
    QGraphicsView::mouseMoveEvent(event);
}

3、QGraphicsItem

#ifndef MYITEM_H
#define MYITEM_H

#include <QGraphicsItem>
#include <QGraphicsSceneEvent>
class MyItem : public QGraphicsItem
{
public:
    MyItem();
    QRectF boundingRect() const;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
                  QWidget *widget);
    void setColor(const QColor &color) { brushColor = color; }

protected:
    void keyPressEvent(QKeyEvent *event);
    void mousePressEvent(QGraphicsSceneMouseEvent *event);
    void hoverEnterEvent(QGraphicsSceneHoverEvent *event);
    void hoverLeaveEvent (QGraphicsSceneHoverEvent * event);
    void contextMenuEvent(QGraphicsSceneContextMenuEvent *event);
    void mouseMoveEvent(QGraphicsSceneMouseEvent *event);
private:
    QColor brushColor;

};

#endif // MYITEM_H

#include "myitem.h"
#include <QPainter>
#include <QCursor>
#include <QKeyEvent>
#include <QGraphicsSceneHoverEvent>
#include <QGraphicsSceneContextMenuEvent>
#include <QMenu>

MyItem::MyItem()
{
    brushColor = Qt::red;
    setFlag(QGraphicsItem::ItemIsFocusable);
    setFlag(QGraphicsItem::ItemIsMovable);
    //setAcceptHoverEvents(true);
}

QRectF MyItem::boundingRect() const
{
    qreal adjust = 0.5;
    return QRectF(-10 - adjust, -10 - adjust,
                  20 + adjust, 20 + adjust);
}

void MyItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
                   QWidget *widget)
{   qDebug("************MyItem::paint*****************");
    if(hasFocus()) {
        painter->setPen(QPen(QColor(255,255,255,200)));
    } else {
        painter->setPen(QPen(QColor(100,100,100,100)));
    }
    painter->setBrush(brushColor);
    painter->drawRect(-10, -10, 20, 20);
}

// 鼠标按下事件处理函数，设置被点击的图形项获得焦点，并改变光标外观
void MyItem::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug("************MyItem::mousePressEvent*****************");
    setFocus();
    setCursor(Qt::ClosedHandCursor);
}

// 键盘按下事件处理函数，判断是否是向下方向键，如果是，则向下移动图形项
void MyItem::keyPressEvent(QKeyEvent *event)
{
    qDebug("************MyItem::keyPressEvent*****************");
    if(event->key() == Qt::Key_Down)
        moveBy(0, 10);
}

// 悬停事件处理函数，设置光标外观和提示
void MyItem::hoverEnterEvent(QGraphicsSceneHoverEvent *event)
{
    qDebug("************MyItem::hoverEnterEvent*****************");
    setCursor(Qt::OpenHandCursor);
    setToolTip("I am item");
}

void MyItem::hoverLeaveEvent(QGraphicsSceneHoverEvent *event)
{
    qDebug("************MyItem::hoverLeaveEvent*****************");
    setCursor(Qt::ArrowCursor);
}


// 右键菜单事件处理函数，为图形项添加一个右键菜单
void MyItem::contextMenuEvent(QGraphicsSceneContextMenuEvent *event)
{
    QMenu menu;
    QAction *moveAction = menu.addAction("move back");
    QAction *actAction = menu.addAction("test");
    QAction *selectedAction = menu.exec(event->screenPos());
    if(selectedAction == moveAction) {
        setPos(0, 0);
    }
}

void MyItem::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug("************MyItem::mouseMoveEvent*****************");
    QGraphicsItem::mouseMoveEvent(event);
}

4、main及运行

#include <QApplication>
#include "myitem.h"
#include "myview.h"
#include "myscene.h"
#include <QTime>

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

    qsrand(QTime(0, 0, 0).secsTo(QTime::currentTime()));

    MyScene scene;
    scene.setSceneRect(-200, -150, 400, 300);
    for(int i = 0; i < 5; ++i) {
        MyItem *item = new MyItem;
        item->setColor(QColor(qrand() % 256, qrand() % 256, qrand() % 256));
        item->setPos(i * 50 - 90, -50);
        scene.addItem(item);
    }


    MyView view;
    view.setScene(&scene);
    view.setBackgroundBrush(QPixmap(":/background.png"));
    view.show();

    return app.exec();
}