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之前项目上需要用Qt来绘制可拖拽改变形状的矩形。看了Qt Graphics相关的内容，虽然对Qt怎么添加图元的有了些了解，但是具体如何实现拖拽效果，一时也没有什么好的想法。还好网上有人分享的例子，很受启发。后来又回顾了一下这部分的代码，发现了一种新的实现方式。希望这个例子也能帮助到需要的人吧。
这几篇文章对这个例子的顺利实现很有帮助，非常感谢。
通过QGraphicsItem绘制可拖拽，改变大小的矩形_qt 绘制多个可拖动矩形-CSDN博客
QGraphicsSceneBspTree::climbTree崩溃（自记）-CSDN博客
关于QGraphicsItem的scenePos一直为(0,0)的解决方案_qt scenepos-CSDN博客
运行效果：
完整代码如下：
GraphicsViewTest2.pro
QT += core guigreaterThan(QT_MAJOR_VERSION, 4): QT += widgetsCONFIG += c++11# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000 # disables all the APIs deprecated before Qt 6.0.0SOURCES += \main.cpp \mainwindow.cpp \mylane.cpp \testrect.cpp \testview.cppHEADERS += \mainwindow.h \mylane.h \testrect.h \testview.hFORMS += \mainwindow.ui# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target
mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H#include <QMainWindow>
#include <QGraphicsScene>
#include "mylane.h"
#include "testrect.h"QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACEclass MainWindow : public QMainWindow
{Q_OBJECTpublic:MainWindow(QWidget *parent = nullptr);~MainWindow();QList<MyLane*> laneList;
private slots:void on_pushButton_clicked();void on_pushButton_2_clicked();private:Ui::MainWindow *ui;QGraphicsScene *scene;
};
#endif // MAINWINDOW_Hmylane.h
#ifndef MYLANE_H
#define MYLANE_H#include <QGraphicsObject>
#include "testrect.h"class MyLane : public QGraphicsObject
{Q_OBJECT
public:explicit MyLane(QGraphicsObject *parent = nullptr);~MyLane();QRectF boundingRect() const override;void paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget) override;void addToScene();void removeFromScene();
public:TestRect *item1;TestRect *item2;TestRect *item3;TestRect *item4;public slots:void updateLane();
};#endif // MYLANE_Htestrect.h
#ifndef TESTRECT_H
#define TESTRECT_H#include <QGraphicsRectItem>class TestRect : public QObject, public QGraphicsRectItem
{Q_OBJECT
public:TestRect(qreal x, qreal y, qreal w, qreal h, QGraphicsRectItem *parent = nullptr);~TestRect();void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override;void mousePressEvent(QGraphicsSceneMouseEvent *event) override;TestRect &operator=(const TestRect& pt);QPointF point;
public slots:void moveMove(QPointF point);
signals:void sendPos(QPointF point);
};#endif // TESTRECT_H
testview.h
#ifndef TESTVIEW_H
#define TESTVIEW_H#include <QGraphicsView>class TestView : public QGraphicsView
{Q_OBJECT
public:TestView(QWidget *parent = nullptr);void mousePressEvent(QMouseEvent *event) override;void mouseMoveEvent(QMouseEvent *event) override;
signals:void mouseMovePoint(QPoint point);void mouseClicked(QPoint point);
};#endif // TESTVIEW_Hmain.cpp
#include "mainwindow.h"#include <QApplication>int main(int argc, char *argv[])
{QApplication a(argc, argv);MainWindow w;w.show();return a.exec();
}
mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QGraphicsItem>
#include <QGraphicsRectItem>
#include <QDebug>MainWindow::MainWindow(QWidget *parent): QMainWindow(parent), ui(new Ui::MainWindow)
{ui->setupUi(this);QRectF rect(-200, -100, 400, 200);scene = new QGraphicsScene(rect);//不加下面这行发现删除矩形的时候程序会概率性的挂在QGraphicsSceneBspTree函数里，什么原因我也不清楚，有清楚的欢迎留言指正scene->setItemIndexMethod(QGraphicsScene::NoIndex);QGraphicsRectItem *item = new QGraphicsRectItem(rect);//item->setFlags(QGraphicsItem::ItemIsFocusable | QGraphicsItem::ItemIsSelectable);QPen pen;pen.setWidth(2);item->setPen(pen);scene->addItem(item);ui->graphicsView->setScene(scene);
}MainWindow::~MainWindow()
{delete ui;
}void MainWindow::on_pushButton_clicked()
{if(!laneList.isEmpty()){MyLane* lane = laneList.first();lane->removeFromScene();scene->removeItem(lane);laneList.removeOne(lane);delete lane;lane = nullptr;}
}void MainWindow::on_pushButton_2_clicked()
{MyLane* lane = new MyLane;scene->addItem(lane);lane->addToScene();laneList.append(lane);
}mylane.cpp
#include "mylane.h"
#include "testrect.h"
#include <QGraphicsScene>
#include <QGraphicsRectItem>
#include <QPainter>
#include <QDebug>MyLane::MyLane(QGraphicsObject *parent) : QGraphicsObject(parent)
{item1 = new TestRect(0, 0, 10, 10);item1->setFlags(QGraphicsItem::ItemIsFocusable |QGraphicsItem::ItemIsSelectable |QGraphicsItem::ItemIsMovable);item1->setPos(0, 0);item2 = new TestRect(0, 0, 10, 10);item2->setFlags(QGraphicsItem::ItemIsFocusable |QGraphicsItem::ItemIsSelectable |QGraphicsItem::ItemIsMovable);item2->setPos(100, 0);item3 = new TestRect(0, 0, 10, 10);item3->setFlags(QGraphicsItem::ItemIsFocusable |QGraphicsItem::ItemIsSelectable |QGraphicsItem::ItemIsMovable);item3->setPos(100, -100);item4 = new TestRect(0, 0, 10, 10);item4->setFlags(QGraphicsItem::ItemIsFocusable |QGraphicsItem::ItemIsSelectable |QGraphicsItem::ItemIsMovable);item4->setPos(0, -100);connect(item1, &TestRect::sendPos, item2, &TestRect::moveMove);connect(item2, &TestRect::sendPos, item1, &TestRect::moveMove);connect(item3, &TestRect::sendPos, item4, &TestRect::moveMove);connect(item4, &TestRect::sendPos, item3, &TestRect::moveMove);connect(item1, &TestRect::sendPos, this, &MyLane::updateLane);connect(item2, &TestRect::sendPos, this, &MyLane::updateLane);connect(item3, &TestRect::sendPos, this, &MyLane::updateLane);connect(item4, &TestRect::sendPos, this, &MyLane::updateLane);}
MyLane::~MyLane(){qDebug() << "~MyLane";delete item1;delete item2;delete item3;delete item4;item1 = nullptr;item2 = nullptr;item3 = nullptr;item4 = nullptr;
}
QRectF MyLane::boundingRect() const{QVector<float> vecX;QVector<float> vecY;vecX.append(item1->x());vecX.append(item2->x());vecX.append(item3->x());vecX.append(item4->x());vecY.append(item1->y());vecY.append(item2->y());vecY.append(item3->y());vecY.append(item4->y());float minX = *std::min_element(std::begin(vecX), std::end(vecX));float minY = *std::min_element(std::begin(vecY), std::end(vecY));float maxX = *std::max_element(std::begin(vecX), std::end(vecX));float maxY = *std::max_element(std::begin(vecY), std::end(vecY));QPointF startPoint(minX, minY);QPointF endPoint(maxX, maxY);QPointF pointOffset(10, 10);return { startPoint, endPoint + pointOffset};
}void MyLane::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget){
// qDebug() << "item1:" << item1->pos();
// qDebug() << "item2:" << item2->pos();
// qDebug() << "item3:" << item3->pos();
// qDebug() << "item4:" << item4->pos();painter->save();painter->setPen(Qt::blue);painter->drawLine(item1->pos(), item2->pos());painter->drawLine(item2->pos(), item3->pos());painter->drawLine(item3->pos(), item4->pos());painter->drawLine(item4->pos(), item1->pos());
// painter->drawRect(boundingRect());
// qDebug() << "boundingRect:" << boundingRect().x() << boundingRect().y() << boundingRect().width() << boundingRect().height();painter->restore();
}void MyLane::addToScene(){if(scene()){scene()->addItem(item1);scene()->addItem(item2);scene()->addItem(item3);scene()->addItem(item4);}
}
void MyLane::removeFromScene(){if(scene()){scene()->removeItem(item1);scene()->removeItem(item2);scene()->removeItem(item3);scene()->removeItem(item4);}
}void MyLane::updateLane(){scene()->update();
}
testrect.cpp
#include "testrect.h"
#include <QGraphicsSceneMouseEvent>
#include <QDebug>TestRect::TestRect(qreal x, qreal y, qreal w, qreal h, QGraphicsRectItem *parent): QGraphicsRectItem(x, y, w, h, parent)
{}
void TestRect::mouseMoveEvent(QGraphicsSceneMouseEvent *event){emit sendPos(event->pos() - point);QGraphicsRectItem::mouseMoveEvent(event);
}
void TestRect::mousePressEvent(QGraphicsSceneMouseEvent *event){point = event->pos();QGraphicsRectItem::mousePressEvent(event);
}void TestRect::moveMove(QPointF point){moveBy(point.x(), point.y());
}
TestRect &TestRect::operator=(const TestRect& pt){setPos(pt.pos());return *this;
}
TestRect::~TestRect(){qDebug() << "~TestRect";
}
testview.cpp
#include "testview.h"#include <QMouseEvent>
#include <QDebug>TestView::TestView(QWidget *parent)
{}void TestView::mousePressEvent(QMouseEvent *event){if(event->button() == Qt::LeftButton){QPoint point = event->pos();qDebug() << "view:" << point;qDebug() << "scene:" << mapToScene(point);}QGraphicsView::mousePressEvent(event);
}void TestView::mouseMoveEvent(QMouseEvent *event){QGraphicsView::mouseMoveEvent(event);
}
mainwindow.ui
<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0"><class>MainWindow</class><widget class="QMainWindow" name="MainWindow"><property name="geometry"><rect><x>0</x><y>0</y><width>800</width><height>600</height></rect></property><property name="windowTitle"><string>MainWindow</string></property><widget class="QWidget" name="centralwidget"><layout class="QGridLayout" name="gridLayout"><item row="0" column="0"><widget class="QGraphicsView" name="graphicsView"/></item><item row="1" column="0"><layout class="QHBoxLayout" name="horizontalLayout"><item><spacer name="horizontalSpacer"><property name="orientation"><enum>Qt::Horizontal</enum></property><property name="sizeHint" stdset="0"><size><width>40</width><height>20</height></size></property></spacer></item><item><widget class="QPushButton" name="pushButton_2"><property name="text"><string>Add</string></property></widget></item><item><widget class="QPushButton" name="pushButton"><property name="text"><string>Remove</string></property></widget></item></layout></item></layout></widget><widget class="QMenuBar" name="menubar"><property name="geometry"><rect><x>0</x><y>0</y><width>800</width><height>26</height></rect></property></widget><widget class="QStatusBar" name="statusbar"/></widget><resources/><connections/>
</ui>