A small program on QPainter

    In Qt,we must put our painting operation in the related paintEvent() funciton.

#include<QtGui>

class MyPaint:public QWidget
{
  Q_OBJECT
public:
  MyPaint(QWidget *parent=0,int i=1);
  QSize sizeHint()const;
#if 1
  void drawAnEllipse(QPainter *p);
  void drawAPie(QPainter *p);
  void drawACure(QPainter *p);
#endif
protected:
  void paintEvent(QPaintEvent * );
private:
  int num;
};

MyPaint::MyPaint(QWidget *parent,int i)
  :QWidget(parent),num(i)
{

}
void MyPaint::drawAPie(QPainter *pa1)
{
  pa1->setPen(QPen(Qt::black,15,Qt::SolidLine,Qt::RoundCap,Qt::MiterJoin));
  pa1->setBrush(QBrush(Qt::blue,Qt::DiagCrossPattern));
  pa1->drawPie(0,0,200,140,60*16,270*16);
}
void MyPaint::drawACure(QPainter *pa2)
{
  pa2->setRenderHint(QPainter::Antialiasing,true);
  QPainterPath path;
  path.moveTo(0,220);
  path.cubicTo(100,0,220,0,380,220);
  pa2->setPen(QPen(Qt::black,8));
  pa2->drawPath(path);
}
void MyPaint::drawAnEllipse(QPainter *pa3)
{
  pa3->setRenderHint(QPainter::Antialiasing,true);
  pa3->setPen(QPen(Qt::black,12,Qt::DashDotLine,Qt::RoundCap));
//#define GRADIENTPATTERN


#ifndef GRADIENTPATTERN
  pa3->setBrush(QBrush(Qt::green));
#else

// 该句也有问题，目前不知道如何解决，不过 Qt::LinearGradientPattern换成

// Qt::SolidePattern的话，程序正常。
  pa3->setBrush(QBrush(Qt::green,Qt::LinearGradientPattern));
#endif


// 还有另外两种渐变模式，分别为：QRadialGradient,QConicalGradient

#ifndef GRADIENTPATTERN
  QLinearGradient gradient(0,0,200,140);
  gradient.setColorAt(0.0,Qt::red);
  gradient.setColorAt(0.2,Qt::green);
  gradient.setColorAt(1.0,Qt::black);
  pa3->setBrush(gradient);
#endif
  pa3->drawEllipse(0,0,200,140);

}

QSize MyPaint::sizeHint()const
{
  return QSize(400,400);
}

void MyPaint::paintEvent(QPaintEvent *)
{
  QPainter pa1(this);
  switch (num)
    {
    case 1:
     drawAnEllipse(&pa1);
     break;
    case 2:
     drawAPie(&pa1);
     break;
    case 3:
     drawACure(&pa1);
     break;
    default:
     break;
    }
}

#include"main.moc" //?目前还不清楚这里面是什么，但必须包含进去，否则链接时候会出错。

int main(int argc,char *argv[])
{
  QApplication app(argc,argv);
  QMainWindow *win = new QMainWindow;
  MyPaint *mp1 = new MyPaint(win,1);
  MyPaint *mp2 = new MyPaint(win,2);
  MyPaint *mp3 = new MyPaint(win,3);
  
  QSplitter sp(win);
  sp.addWidget(mp1);
  sp.addWidget(mp2);
  sp.addWidget(mp3);
  win->setCentralWidget(&sp);
#if 0
  mp1->show();
  mp2->show();
  mp3->show();
#endif
  win->show();
  return app.exec();
}