知识点

双缓冲就是在内存区申请一块缓存；然后显卡直接从这块内存读取数据.。
这样就不用鼠标边画，经过IO来读取这个环节；
鼠标驱动 — 内存缓冲区— 显卡

drawwidget

drawwidget.h

#ifndef DRAWWIDGET_H
#define DRAWWIDGET_H#include <QMainWindow>
#include <QtGui>
#include <QMouseEvent>
#include <QPaintEvent>
#include <QResizeEvent>
#include <QColor>
#include <QPixmap>
#include <QPoint>
#include <QPainter>
#include <QPalette>
#include <QPen>class drawWidget : public QWidget
{Q_OBJECTpublic:explicit drawWidget(QWidget* parent = 0);public:void mousePressEvent(QMouseEvent*);void mouseMoveEvent(QMouseEvent*);void paintEvent(QPaintEvent*);void resizeEvent(QResizeEvent*);public slots:void setStyle(int);void setWidth(int);void setColor(QColor);void clear();
private:QPixmap* pix;QPoint startPos;QPoint endPos;int style;int weight;QColor color;};
#endif // DRAWWIDGET_H

drawwidget.cpp

#include "drawwidget.h"drawWidget::drawWidget(QWidget *parent): QWidget(parent)
{setAutoFillBackground(true);setPalette(QPalette(Qt::white));pix = new QPixmap(size());pix->fill(Qt::white);setMinimumSize(600,400);
}void drawWidget::mousePressEvent(QMouseEvent * e)
{startPos = e->pos();
}void drawWidget::mouseMoveEvent(QMouseEvent *e)
{QPainter *painter = new QPainter;QPen pen;pen.setStyle((Qt::PenStyle)style);pen.setWidth(weight);pen.setColor(color);painter->begin(pix);painter->setPen(pen);painter->drawLine(startPos,e->pos());painter->end();startPos = e->pos();update();}void drawWidget::paintEvent(QPaintEvent *)
{//把图片内存的数据  绘制到窗体控件中QPainter painter(this);painter.drawPixmap(QPoint(0,0),*pix);
}void drawWidget::resizeEvent(QResizeEvent *event)
{if(height()>pix->height() || width()>pix->width())//如果图片尺寸发生改变{QPixmap *newPix = new QPixmap(size());newPix->fill(Qt::white);QPainter p(newPix);p.drawPixmap(QPoint(0,0),*pix);pix = newPix;}QWidget::resizeEvent(event);
}void drawWidget::setStyle(int s)
{style = s;
}void drawWidget::setWidth(int w)
{weight = w;}void drawWidget::setColor(QColor c)
{color = c;
}void drawWidget::clear()
{QPixmap *clearPix = new QPixmap(size());clearPix->fill(Qt::white);pix = clearPix;update();
}

mainwindow

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QToolButton>
#include <QLabel>
#include <QComboBox>
#include <QSpinBox>
#include <QToolBar>
#include <QColorDialog>#include "drawwidget.h"class MainWindow : public QMainWindow
{Q_OBJECT
public:MainWindow(QWidget *parent = nullptr);~MainWindow();void createToolBar();signals:
public slots:void ShowStyle();void ShowColor();
private:drawWidget *drawwidget;QLabel* styleLabel;QComboBox* styleComboBox;QLabel* widthLabel;QSpinBox* widthSpinBox;QToolButton* colorBtn;QToolButton* clearBtn;
};#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"MainWindow::MainWindow(QWidget *parent): QMainWindow{parent}
{drawwidget = new drawWidget;setCentralWidget(drawwidget);createToolBar();setMinimumSize(600,400);ShowStyle();drawwidget->setWidth(widthSpinBox->value());drawwidget->setColor(Qt::black);
}MainWindow::~MainWindow()
{}void MainWindow::createToolBar()
{QToolBar* toolBar = addToolBar("Tool");styleLabel = new QLabel(tr("线型风格："));styleComboBox = new QComboBox;styleComboBox->addItem(tr("SolidLine"),static_cast<int>(Qt::SolidLine));styleComboBox->addItem(tr("DashLine"),static_cast<int>(Qt::DashLine));styleComboBox->addItem(tr("DotLine"),static_cast<int>(Qt::DotLine));styleComboBox->addItem(tr("DashDotLine"),static_cast<int>(Qt::DashDotLine));styleComboBox->addItem(tr("DashDotDotLine"),static_cast<int>(Qt::DashDotDotLine));connect(styleComboBox,static_cast<void (QComboBox::*)(int)>(&QComboBox::activated),this,&MainWindow::ShowStyle);widthLabel = new QLabel(tr("线宽："));widthSpinBox =new QSpinBox;connect(widthSpinBox,static_cast<void (QSpinBox::*)(int)>(&QSpinBox::valueChanged),drawwidget,&drawWidget::setWidth);colorBtn = new QToolButton;QPixmap pixmap(20,20);pixmap.fill(Qt::black);colorBtn->setIcon(QIcon(pixmap));connect(colorBtn,&QToolButton::clicked,this,&MainWindow::ShowColor);clearBtn = new QToolButton;clearBtn->setText(tr("清除"));connect(clearBtn,&QToolButton::clicked,drawwidget,&drawWidget::clear);toolBar->addWidget(styleLabel);toolBar->addWidget(styleComboBox);toolBar->addWidget(widthLabel);toolBar->addWidget(widthSpinBox);toolBar->addWidget(colorBtn);toolBar->addWidget(clearBtn);}void MainWindow::ShowStyle()
{drawwidget->setStyle(styleComboBox->itemData(styleComboBox->currentIndex(),Qt::UserRole).toInt());}void MainWindow::ShowColor()
{QColor color =QColorDialog::getColor(static_cast<int>(Qt::black));if(color.isValid()){drawwidget->setColor(color);QPixmap p(20,20);p.fill(color);colorBtn->setIcon(QIcon(p));}}

main.cpp

#include "mainwindow.h"#include <QApplication>
#include <QFont>int main(int argc, char *argv[])
{QApplication a(argc, argv);QFont font("ZYSong18030",12);a.setFont(font);MainWindow w;w.show();return a.exec();
}

运行图