如何在Qt的 paintEvent之外进行绘制

QPainter 默认只能在paintEvent中进行绘制

这在有些情况下会很不方便，有时候我们希望可以在任意地方直接进行绘制

为了实现这个目的，可以采用以下方法：

继承QWidget，通过子类提供直接绘制的方法，并将所有绘制保存到中间的QPixmap

最后在重载的paintEvent中将QPixmap复制显示:

#pragma once
 
#include <QtWidgets/QWidget>
#include <QPainter>
#include <QResizeEvent>
 
class PaintWidget: public QWidget
{
    Q_OBJECT
public:
    PaintWidget(QWidget * parent = 0, Qt::WindowFlags f = 0)
        :QWidget(parent, f)
    {
        mPixmap = new QPixmap(width(), height());
        mPainter = new QPainter(mPixmap);
    }
 
    ~PaintWidget()
    {
        delete mPainter;
        delete mPixmap;
    }
 
    void resizeEvent(QResizeEvent *event)
    {
        if(event->size().width() > mPixmap->width() || event->size().height() > mPixmap->height())
        {
            QPixmap* pixmap = new QPixmap((std::max)(event->size().width(), mPixmap->width()), (std::max)(event->size().height(), mPixmap->height()));
            delete mPainter;
            mPainter = new QPainter(pixmap);
            mPainter->drawPixmap(0, 0, *mPixmap);
            delete mPixmap;
            mPixmap = pixmap;
        }
    }
 
    void paintEvent(QPaintEvent *event)
    {
        QPainter painter(this);
        painter.drawPixmap(0, 0, *mPixmap);
    }
 
    QPainter* getPainter()
    {
        return mPainter;
    }
private:
    QPixmap* mPixmap;
    QPainter* mPainter;
};