先创建一个继承自QWidget的控件：

class MyPainterWidget:public QWidget

重写各种鼠标方法：

protected:void paintEvent(QPaintEvent *) override;void mousePressEvent(QMouseEvent *e) override;       //按下void mouseMoveEvent(QMouseEvent *e) override;        //移动void mouseReleaseEvent(QMouseEvent *e) override;     //松开void mouseDoubleClickEvent(QMouseEvent *event) override;

在构造函数中声明跟踪鼠标：

#include <QMouseEvent>
setMouseTracking(true);

定义两个标志位作为开始绘制和鼠标移动的标志：

bool m_bStartDraw = false;    //是否已经开始左键点击，同时标识是否开始进行绘制bool bMove = false;           //是否处于绘制时的鼠标移动状态

创建一个点集记录每次按下的坐标点：

QVector<QPointF> pointList;

在程序一开始的时候先清除点集：

MyPainterWidget::MyPainterWidget(QWidget *parent) : QWidget(parent)
{setMouseTracking(true);pointList.clear();
}

按下鼠标时将开始绘制标志位设为true：

// 按下
void MyPainterWidget::mousePressEvent(QMouseEvent *e)
{if (e->button() == Qt::LeftButton){if(!m_bStartDraw){pointList.clear();m_bStartDraw = true;}}   
}

创建一个点用于获取鼠标移动时的当前位置：

QPointF movePoint;

重写移动事件：

// 移动
void MyPainterWidget::mouseMoveEvent(QMouseEvent *e)
{if(m_bStartDraw){movePoint = e->pos();this->update();//重新进入painEvent事件进行绘制// 先刷新再设为true, 防止第一点和（0,0）连在一块bMove = true;}
}

重写释放事件：

// 松开
void MyPainterWidget::mouseReleaseEvent(QMouseEvent *e)
{if (e->button() == Qt::LeftButton){if (m_bStartDraw){// 鼠标松开后的点需要添加到路径中pointList.push_back(QPointF(e->x(), e->y()));bMove = false;this->update();//刷新绘制}}
}

写一个表示结束绘制的功能函数：

void MyPainterWidget::endDraw()
{m_bStartDraw = false;//需要把第一个点连起来pointList.push_back(pointList[0]);this->update();
}

实现清除所有已绘的功能函数：

void MyPainterWidget::clearPath()
{pointList.clear();this->update();
}

主窗口：

#ifndef MAINWINDOW_H
#define MAINWINDOW_H#include <QMainWindow>
#include <QMenu>QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACEclass MainWindow : public QMainWindow
{Q_OBJECTpublic:MainWindow(QWidget *parent = nullptr);~MainWindow();protected:// 右键菜单需要重写的类void contextMenuEvent(QContextMenuEvent* event) override;private:Ui::MainWindow *ui;QMenu* m_pMenu;
};
#endif // MAINWINDOW_H

/*使用方式左键点击，移动鼠标开始绘制，双击左键结束绘制，或者右键点击结束绘制*/#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QMenu>
#include <QAction>MainWindow::MainWindow(QWidget *parent): QMainWindow(parent), ui(new Ui::MainWindow)
{ui->setupUi(this);m_pMenu = new QMenu(this);QAction* pAc1 = new QAction(QString::fromLocal8Bit("结束绘制"), this);pAc1->setShortcut(QKeySequence("Ctrl+E"));QAction* pAc2 = new QAction(QString::fromLocal8Bit("清除"), this);pAc2->setShortcut(QKeySequence("Ctrl+D"));// 这是个假动作，为了让菜单消失，且不影响绘制路径QAction* pAc3 = new QAction(QString::fromLocal8Bit("退出菜单"), this);m_pMenu->addAction(pAc1);m_pMenu->addAction(pAc2);m_pMenu->addAction(pAc3);m_pMenu->setStyleSheet("QMenu{font:18px;}");connect(pAc1, &QAction::triggered, [=] {ui->graphicsPainter->endDraw();});connect(pAc2, &QAction::triggered, [=] {ui->graphicsPainter->clearPath();});
}MainWindow::~MainWindow()
{delete ui;
}// 右键菜单
void MainWindow::contextMenuEvent(QContextMenuEvent* event)
{m_pMenu->move(cursor().pos());m_pMenu->show();
}

完整代码：

#ifndef GRAPHICSPAINTER_H
#define GRAPHICSPAINTER_H#include <QWidget>class MyPainterWidget : public QWidget
{Q_OBJECTpublic:explicit MyPainterWidget(QWidget *parent = nullptr);void endDraw();void clearPath();protected:void paintEvent(QPaintEvent *) override;void mousePressEvent(QMouseEvent *e) override;       //按下void mouseMoveEvent(QMouseEvent *e) override;        //移动void mouseReleaseEvent(QMouseEvent *e) override;     //松开void mouseDoubleClickEvent(QMouseEvent *event) override;     //双击bool m_bStartDraw = false;    //是否已经开始左键点击，同时标识是否开始进行绘制bool bMove = false;           //是否处于绘制时的鼠标移动状态QVector<QPointF> pointList;QPointF movePoint;
};#endif // GRAPHICSPAINTER_H

#include "MyPainterWidget.h"
#include <QPainter>
#include <QMouseEvent>MyPainterWidget::MyPainterWidget(QWidget *parent) : QWidget(parent)
{setMouseTracking(true);pointList.clear();
}void MyPainterWidget::paintEvent(QPaintEvent *)
{QPainter painter(this);painter.setPen(QColor(255,0,0));QVector<QLineF> lines;for(int i = 0; i < pointList.size()-1; i++){QLineF line(QPointF(pointList[i].x(), pointList[i].y()), QPointF(pointList[i+1].x(), pointList[i+1].y()));lines.push_back(line);}if (m_bStartDraw){int size = pointList.size();if (bMove && size > 0){QLineF line(QPointF(pointList[pointList.size() - 1].x(), pointList[pointList.size() - 1].y()),movePoint);lines.push_back(line);}}painter.drawLines(lines);
}// 按下
void MyPainterWidget::mousePressEvent(QMouseEvent *e)
{if (e->button() == Qt::LeftButton){if(!m_bStartDraw){pointList.clear();m_bStartDraw = true;}}   
}// 移动
void MyPainterWidget::mouseMoveEvent(QMouseEvent *e)
{if(m_bStartDraw){movePoint = e->pos();this->update();//重新进入painEvent事件进行绘制// 先刷新再设为true, 防止第一点和（0,0）连在一块bMove = true;}
}// 松开
void MyPainterWidget::mouseReleaseEvent(QMouseEvent *e)
{if (e->button() == Qt::LeftButton){if (m_bStartDraw){// 鼠标松开后的点需要添加到路径中pointList.push_back(QPointF(e->x(), e->y()));bMove = false;this->update();//刷新绘制}}
}// 双击结束绘制
void MyPainterWidget::mouseDoubleClickEvent(QMouseEvent *event)
{endDraw();
}void MyPainterWidget::endDraw()
{m_bStartDraw = false;//需要把第一个点连起来pointList.push_back(pointList[0]);this->update();
}void MyPainterWidget::clearPath()
{pointList.clear();this->update();
}