如何用QPainter绘画？

Question

我最近开始学习Qt.
我不清楚如何使用QPainter课程画画.假设我只想在窗口中放几个点:

class PointDrawer: public QWidget {
    Q_OBJECT
private:
    QPainter p;
public:
    PointDrawer(QWidget* obj=0): QWidget(obj), p(this) {}
    virtual void paintEvent(QPaintEvent*) {
        p.setPen(QPen(Qt::black, 3));
        int n = 8;
        while(...) {
            qreal fAngle = 2 * 3.14 * i / n;
            qreal x = 50 + cos(fAngle) * 40;
            qreal y = 50 + sin(fAngle) * 40;
            p.drawPoint(QPointF(x, y));
                        i++;
        }
    }
}

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    PointDrawer drawer;

    drawer.resize(200, 200);
    drawer.show();

    return app.exec();
}

在那之后,我什么都没得到!
你能告诉我我哪里错了吗？

Answer 1

void SimpleExampleWidget::paintEvent(QPaintEvent *)
{
     QPainter painter(this);
     painter.setPen(Qt::blue);
     painter.setFont(QFont("Arial", 30));
     painter.drawText(rect(), Qt::AlignCenter, "Qt");
}

http://doc.qt.digia.com/4.4/qpainter.html

Answer 2

我认为问题是你的QPainter初始化.

你可以QPainter在hydroes的回答中创建类似的东西,它看起来像这样:

class PointDrawer: public QWidget {
    Q_OBJECT
public:
    PointDrawer(QWidget* obj=0): QWidget(obj) {}
    virtual void paintEvent(QPaintEvent*) {
        QPainter p(this)
        p.setPen(QPen(Qt::black, 3));
        int n = 8;
        while(...) {
                qreal fAngle = 2 * 3.14 * i / n;
                qreal x = 50 + cos(fAngle) * 40;
                qreal y = 50 + sin(fAngle) * 40;
                p.drawPoint(QPointF(x, y));
                        i++;
        }
    }
}

它也可以使用这样的东西,但我不推荐它(我只是喜欢其他解决方案):

class PointDrawer: public QWidget {
    Q_OBJECT
private:
    QPainter p;
public:
    PointDrawer(QWidget* obj=0): QWidget(obj) {}
    virtual void paintEvent(QPaintEvent*) {
        p.begin(this);
        p.setPen(QPen(Qt::black, 3));
        int n = 8;
        while(...) {
                qreal fAngle = 2 * 3.14 * i / n;
                qreal x = 50 + cos(fAngle) * 40;
                qreal y = 50 + sin(fAngle) * 40;
                p.drawPoint(QPointF(x, y));
                        i++;
        }
        p.end();
    }
}

在第二个例子中,QPainter::begin(this)和QPainter::end()调用是必不可少的.在第一个示例中,您可以考虑QPainter::begin(this)在构造函数和QPainter::end()析构函数中调用

出于这个原因,我猜测:由于QPaintDevices在QT4中通常是双缓冲的,因此QPainter::end()可能是图像传输到图形内存的地方.