Qt實現畫筆功能

#ifndef MyPaint_h__
#define MyPaint_h__

#include <QtWidgets/QWidget>

class MyPaint :public QWidget
{
    Q_OBJECT
public:
    MyPaint(QWidget *parent = nullptr);

    void mousePressEvent(QMouseEvent *event);
    void mouseMoveEvent(QMouseEvent *event);
    void mouseReleaseEvent(QMouseEvent *event);
    void paintEvent(QPaintEvent *event);

private:
    std::vector<std::vector<QPoint>> _lines;

};

#endif // MyPaint_h__

#include "MyPaint.h"
#include <QPainter>
#include <QMouseEvent>


MyPaint::MyPaint(QWidget *parent):QWidget(parent)
{

}

void MyPaint::mousePressEvent(QMouseEvent *event) //鼠標按下的時候新增加一條線，並記錄起點
{
    std::vector<QPoint> line;  
    line.push_back(event->pos());
    _lines.push_back(line);
}

void MyPaint::mouseMoveEvent(QMouseEvent *event)//鼠標移動的時候新增加點
{
    auto &line = _lines[_lines.size() - 1];
    line.push_back(event->pos());

    update(); //更新，重新繪制窗口，自動調用paintEvent
}

void MyPaint::mouseReleaseEvent(QMouseEvent *event)
{
    auto &line = _lines[_lines.size() - 1];
    line.push_back(event->pos());
}

void MyPaint::paintEvent(QPaintEvent *event) //繪制所有的線
{
    //QPainter painter(this);
    //painter.setPen(QPen(Qt::red, 3, Qt::DashLine));
    //painter.setBrush(Qt::blue);
    //painter.drawRect(QRect(10, 10, 100, 30));
    //painter.drawEllipse(130, 10, 50, 50);
    //painter.drawLine(QLine(200, 10, 300, 20));
    //painter.drawText(QPoint(10, 70), "hello" );
    QPainter painter(this);
    painter.setPen(QPen(Qt::green, 3));
    for (const auto &line : _lines)
    {
        for (int i=0;i<line.size()-1;i++)
        {
            painter.drawLine(line.at(i), line.at(i + 1));
        }
    }
}