我做的仪表盘控件是个功能简单的显示控件。该控件在VS2015和Qt5.9上测试可用。它默认的显示范围是[0, 100]，不能修改它的显示范围，因为不同的数值显示在前界面上的效果不同，可能导致显示重叠。成员函数只有一个setValue方法用来设置当前显示值。

代码如下，头文件：

class MDashboard : public QWidget
{
    Q_OBJECT

public:
    MDashboard(QWidget* parent = nullptr);
    void setValue(qreal ivalue);

private:
    void paintEvent(QPaintEvent *event) override;

private:
    const static int radius;
    const static int maxv;
    const static int minv;
    qreal value;
};

CPP文件：

const int MDashboard::radius = 80;
const int MDashboard::minv = 0;
const int MDashboard::maxv = 100;

MDashboard::MDashboard(QWidget* parent) :
    QWidget(parent)
{
    value = 0;
}

void MDashboard::setValue(qreal ivalue)
{
    value = ivalue;
    update();
}

void MDashboard::paintEvent(QPaintEvent *)
{
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);
    QPoint center(width() / 2, height() / 2);
    painter.save();

    /* 绘制背景圆形 */
    painter.translate(center);
    painter.setPen(Qt::NoPen);
    painter.setBrush(QColor(143, 143, 143));
    const int bkRadius = radius + 3;
    painter.drawEllipse(QPoint(), bkRadius, bkRadius);

    /* 绘制长短刻度线 */
    painter.rotate(-135);
    const int longStep = 10;
    const int shortStep = 50;
    for (int i = 0; i <= shortStep; i++)
    {
        /* 前80%绿色刻度线，后20%红色 */
        painter.setPen((i < shortStep * 0.8) ? QColor(32, 243, 32) : QColor(243, 32, 32));
        if (i % (shortStep / longStep) != 0)
        {
            QPoint p1(0, -(radius - 8));
            QPoint p2(0, -radius);
            painter.drawLine(p1, p2);
        }
        else
        {
            QPoint pl(0, -(radius - 12));
            QPoint p2(0, -radius);
            painter.drawLine(pl, p2);
        }
        painter.rotate(270.0 / shortStep);
    }

    /* 绘制表盘名字和它的背景矩形 */
    painter.restore();
    QFontMetrics fm = painter.fontMetrics();
    int tx = center.x();
    int ty = int(center.y() + (radius / 1.4142));
    QString title = u8"仪表盘";
    QSize tsz = fm.size(0, title);
    QRect trect(QPoint(tx - tsz.width() / 2, ty - tsz.height() / 2), tsz);
    painter.setPen(QColor(127, 127, 127));
    painter.setBrush(QColor(173, 163, 163));
    painter.drawRoundedRect(trect.adjusted(-6, -4, 6, 4), 3, 3);
    painter.setPen(QColor(243, 243, 243));
    painter.setBrush(Qt::NoBrush);
    painter.drawText(trect, title);/* 绘制每个长刻度线对应的文字 */
    painter.setPen(QColor(243, 243, 243));
    const int hand = radius - 21;
    for (int i = 0; i <= longStep; i++)
    {
        qreal angle = qDegreesToRadians(-135.0 + i * (270.0 / longStep));
        QString etext = QString::number(minv + i * (maxv - minv) / longStep); /* 注意这里整形运算 */
        QPointF dirVec(qSin(angle), -qCos(angle));
        QPointF ecenter(center.x() + hand * dirVec.x(), center.y() + hand * dirVec.y());
        QSize esz = fm.size(0, etext);
        QRectF erect(QPointF(ecenter.x() - esz.width() / 2, ecenter.y() - esz.height() / 2), esz);
        painter.drawText(erect, etext);
    }

    /* 绘制表盘指针 */
    painter.save();
    QPoint triangle[] = /* 向上的三角形 */
    {
        { -5, 0 },
        { 0, 25 - radius },
        { 5, 0 },
    };
    qreal degree = -135.0 + 270.0 * (value - minv) / (maxv - minv);
    painter.translate(center);
    painter.rotate(degree);
    painter.setPen(Qt::NoPen);
    painter.setBrush(Qt::red);
    painter.drawPolygon(triangle, 3);
    painter.setPen(QPen(QColor(213, 0, 0), 2));
    painter.drawEllipse(QPoint(), 7, 7);
    painter.restore();
}