可以通过QT的重绘事件和鼠标事件来绘制多边形，最简单的办法就是在继承QWidget的窗体中重写paintEvent、mousePressEvent等事件处理函数。QT提供了图形绘制接口QPainter，通过该接口可以绘制多种图形，包括多边形。
新建一个基于QWidget的QT界面类GraphicsPainter，将其放置到想要显示的窗体中。该类的具体代码：
GraphicsPainter.h:
#ifndef GRAPHICSPAINTER_H
#define GRAPHICSPAINTER_H
#include <QWidget>
class GraphicsPainter : public QWidget
{
Q_OBJECT
public:
explicit GraphicsPainter(QWidget *parent = nullptr);
void SetDraw(bool bDraw);
signals:
void singalDrawOver();
public slots:
protected:
void paintEvent(QPaintEvent *); //绘制
void mousePressEvent(QMouseEvent *e); //按下
void mouseMoveEvent(QMouseEvent *e); //移动
void mouseReleaseEvent(QMouseEvent *e); //松开
void mouseDoubleClickEvent(QMouseEvent *event); //双击
bool bDraw; //是否处于绘制状态
bool bLeftClick; //是否已经开始左键点击，同时标识是否开始进行绘制
bool bMove; //是否处于绘制时的鼠标移动状态
QVector<QPointF> pointList;
QPointF movePoint;
};
#endif // GRAPHICSPAINTER_HGraphicsPainter.cpp:
#include "graphicspainter.h"
#include <QPainter>
#include <QMouseEvent>
#include <QDebug>
GraphicsPainter::GraphicsPainter(QWidget *parent) : QWidget(parent)
{
//填充背景色
setAutoFillBackground(true);
setBackgroundRole(QPalette::Base);
bDraw = false;
bLeftClick = false;
bMove = false;
setMouseTracking(true);
}
void GraphicsPainter::SetDraw(bool bDraw)
{
this->bDraw = bDraw;
pointList.clear();
}
//重新实现paintEvent
void GraphicsPainter::paintEvent(QPaintEvent *)
{
QPainter painter(this);
if(bDraw)
{
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(bMove&&pointList.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 GraphicsPainter::mousePressEvent(QMouseEvent *e)
{
if(bDraw)
{
if(!bLeftClick)
{
pointList.clear();
bLeftClick = true;
}
}
//qDebug()<<"Press";
}
//移动
void GraphicsPainter::mouseMoveEvent(QMouseEvent *e)
{
if(bDraw&&bLeftClick)
{
movePoint = e->pos();
bMove = true;
this->update();
}
//qDebug()<<"Move";
}
//松开
void GraphicsPainter::mouseReleaseEvent(QMouseEvent *e)
{
if(bDraw&&bLeftClick)
{
pointList.push_back(QPointF(e->x(), e->y()));
bMove = false;
this->update();
}
//qDebug()<<"Release";
}
//双击
void GraphicsPainter::mouseDoubleClickEvent(QMouseEvent *event)
{
if(bDraw)
{
bLeftClick = false;
pointList.push_back(pointList[0]);
this->update();
singalDrawOver();
}
//qDebug()<<"DoubleClick";
}在重新实现的重绘事件中，通过QPainter绘制了一系列线组成线串，最后会首尾相连形成多边形。这里的bMove标识是否处于绘制时的鼠标移动状态，只有鼠标左键点击后才会确定为真正的节点：
//重新实现paintEvent
void GraphicsPainter::paintEvent(QPaintEvent *)
{
QPainter painter(this);
if(bDraw)
{
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(bMove&&pointList.size()>0)
{
QLineF line(QPointF(pointList[pointList.size()-1].x(), pointList[pointList.size()-1].y()), movePoint);
lines.push_back(line);
}
painter.drawLines(lines);
}
}鼠标按下事件中，主要是通过bLeftClick值来确定是否已经处于左键点击状态，同时还能标识是否开始进行绘制。一旦开始，就会把上次绘制的节点清除。
//按下
void GraphicsPainter::mousePressEvent(QMouseEvent *e)
{
if(bDraw)
{
if(!bLeftClick)
{
pointList.clear();
bLeftClick = true;
}
}
//qDebug()<<"Press";
}
一旦鼠标松开，就可以确定一个节点，此时需要调用update()进行重绘：
//松开
void GraphicsPainter::mouseReleaseEvent(QMouseEvent *e)
{
if(bDraw&&bLeftClick)
{
pointList.push_back(QPointF(e->x(), e->y()));
bMove = false;
this->update();
}
//qDebug()<<"Release";
}
当开始进行绘制后，移动鼠标就会处于绘制时的鼠标移动状态，这时就会确定bMove为true，重绘事件就会将该鼠标点绘制出来，从而达到待选节点的效果：
//移动
void GraphicsPainter::mouseMoveEvent(QMouseEvent *e)
{
if(bDraw&&bLeftClick)
{
movePoint = e->pos();
bMove = true;
this->update();
}
//qDebug()<<"Move";
}
鼠标双击后，将第一个点加入到当前多边形的节点中后，达到首尾相连的效果，此时就会结束绘制：
//双击
void GraphicsPainter::mouseDoubleClickEvent(QMouseEvent *event)
{
if(bDraw)
{
bLeftClick = false;
pointList.push_back(pointList[0]);
this->update();
singalDrawOver();
}
//qDebug()<<"DoubleClick";
}
这里一定要注意，当进行双击操作时，首先会触发一次mousePressEvent，然后触发一次mouseReleaseEvent，接着才会触发一次mouseDoubleClickEvent，最后还会触发一次mouseReleaseEvent。所以这就是这里设置bLeftClick这个参数原因：当触发mouseDoubleClickEvent后，bLeftClick设置为false，第二次触发mouseReleaseEvent时内部就不会在做任何操作了。
最终运行的结果如下所示：
