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;
};
使用说明：
通过子类提供的getPainter()方法获取QPainter指针以便进行绘制，比如:
paintWidget.getPainter()->drawLine(...);
paintWidget.update() ; //绘制之后记得通过paintWidget.update() 通知页面刷新
附：代码中的resizeEvent是为了保证缓存的QPixmap始终比widget大，根据情况你可以直接初始化为所希望的大小
原文：http://www.cnblogs.com/wiki3d/p/4227307.html