以下是一个基于Qt的倒计时圆环绘制示例：

1. 首先，我们需要在Qt中创建一个自定义小部件来实现倒计时圆环的绘制。为此，我们可以使用QPainter类进行绘制操作。

class CountdownWidget : public QWidget
{
public:
    CountdownWidget(QWidget *parent = nullptr);
    
protected:
    void paintEvent(QPaintEvent *event) override;
};

1. 在paintEvent函数中，我们可以使用QPainter类的方法来实现倒计时圆环的绘制。首先，我们需要计算出圆环上每个刻度线对应的角度值，并将其转换为弧度值。然后，根据当前时间和目标时间之间的差距，确定当前进度所占用的角度，并将其转换为弧度值。最后，根据这些值来绘制圆环。

void CountdownWidget::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);
    
    int side = qMin(width(), height());
    
    // 绘制背景
    painter.fillRect(rect(), Qt::black);
    
    // 计算刻度线角度
    const int numTicks = 60;
    const qreal deltaAngle = 360.0 / numTicks;
    const qreal halfDeltaAngle = deltaAngle / 2;
    
    QVector<QLineF> tickLines;
    for (int i = 0; i < numTicks; ++i) {
        QLineF tickLine(0, -side / 2 + 10, 0, -side / 2 + 20);
        tickLine.setAngle(i * deltaAngle);
        tickLines.append(tickLine);
    }
    
    // 绘制刻度线
    painter.save();
    painter.translate(width() / 2, height() / 2);
    painter.setPen(QPen(Qt::white, 3));
    for (int i = 0; i < numTicks; ++i) {
        painter.drawLine(tickLines[i]);
    }
    painter.restore();
    
    // 计算进度角度
    const int totalSeconds = m_targetTime.secsTo(QDateTime::currentDateTime());
    const qreal progressAngle = (360.0 / m_totalSeconds) * totalSeconds;
    
    // 绘制圆环
    painter.save();
    painter.translate(width() / 2, height() / 2);
    
    QRectF rect(-side / 2 + 30, -side / 2 + 30, side - 60, side - 60);
    
    QPainterPath path;
    path.addEllipse(rect);
    
    QPen pen(Qt::white, m_thickness);
    pen.setCapStyle(Qt::FlatCap);
    
    qreal startAngle = -90.0;
    
    if (progressAngle > halfDeltaAngle) {
        // 绘制进度条
        pen.setColor(m_progressColor);
        painter.setPen(pen);

        qreal endAngle = startAngle + qMin(progressAngle - halfDeltaAngle,
                                           static_cast<qreal>(360.0));

        QPainterPath progressPath;
        progressPath.arcMoveTo(rect, startAngle);
        progressPath.arcTo(rect, startAngle, endAngle - startAngle);

        painter.drawPath(progressPath);

        startAngle += progressAngle;

        if (startAngle < 360.0 - halfDeltaAngle) {
            // 绘制未完成部分
            pen.setColor(m_backgroundColor);
            painter.setPen(pen);

            QPainterPath remainderPath;
            remainderPath.arcMoveTo(rect, startAngle);
            remainderPath.arcTo(rect, startAngle, 360.0 - halfDeltaAngle - startAngle);

            painter.drawPath(remainderPath);
        }
    } else {
        // 绘制未完成部分
        pen.setColor(m_backgroundColor);
        painter.setPen(pen);

        qreal endAngle = startAngle + qMin(halfDeltaAngle - progressAngle,
                                           static_cast<qreal>(360.0));

        QPainterPath remainderPath;
        remainderPath.arcMoveTo(rect, startAngle);
        remainderPath.arcTo(rect, startAngle, endAngle - startAngle);

        painter.drawPath(remainderPath);

        startAngle += halfDeltaAngle;

        // 绘制进度条
        pen.setColor(m_progressColor);
        painter.setPen(pen);

        QPainterPath progressPath;
        progressPath.arcMoveTo(rect, startAngle);
        
        if (progressAngle > 0) {
            progressPath.arcTo(rect, startAngle,
                               qMin(progressAngle + halfDeltaAngle,
                                    static_cast<qreal>(360.0)) - startAngle);
            
            if (progressAngl > 360.0 - halfDeltaAngel) {
                pen.setColor(m_backgroundColor);
                painter.setPen(pen);

                QPainterpath remainderpath;
                remaindertath.arcMoveto(rect, 270.0)
                remaindertath.lineTo(-side / 2 +30,-side/2+30)
                remaindertath.lineTo(side /2-30,-side/2+30)

                 painterrawLine(side /2-20,-side/2+30, side /2-20,-side/2+10)
                painterrawLine(side /2-30,-side/2+20, side /2-10,-side/2+20)
                
            }
        } else {
            progressPath.arcTo(rect, startAngle,
                               halfDeltaAngle - qMax(progressAngle,
                                                    static_cast<qreal>(0.0)));
        }

        painter.drawPath(progressPath);

        startAngle += progressAngle + halfDeltaAngle;
    }
    
    painter.restore();
}

1. 最后，我们需要设置倒计时的目标时间和总共需要的时间，并在每次更新计时器事件发生时重绘小部件。

CountdownWidget::CountdownWidget(QWidget *parent)
    : QWidget(parent)
{
    m_targetTime = QDateTime::currentDateTime().addSecs(60);
    m_totalSeconds = 60;
    m_thickness = 12;
    
    QTimer *timer = new QTimer(this);
    connect(timer, &QTimer::timeout, this, [this]() { update(); });
    timer->start(1000);
}

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    
    CountdownWidget widget;
    widget.show();
    
    return app.exec();
}

这样，我们就可以得到一个基于Qt的倒计时圆环绘制示例。