代码：

widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

QT_BEGIN_NAMESPACE
namespace Ui { class Widget; }
QT_END_NAMESPACE

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();

    void paintEvent(QPaintEvent *event) override;

public slots:
    void hslideValueChangedSlot(int);
private:
    Ui::Widget *ui;
};
#endif // WIDGET_H

widget.cpp

#include "widget.h"
#include "ui_widget.h"
#include <QPainter>
#include <QPen>
#include <QtCore/qmath.h>
Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    ui->horizontalSlider->setMaximum(300);
    ui->horizontalSlider->setMinimum(0);
    connect(ui->horizontalSlider,SIGNAL(valueChanged(int)),this,SLOT(hslideValueChangedSlot(int)));
}

Widget::~Widget()
{
    delete ui;
}


void Widget::paintEvent(QPaintEvent *event)
{
    QPainter p(this);

    //Qpainter:: translate()
    p.translate(this->width()/2,this->height()/2);
    p.setPen(Qt::NoPen);
    QBrush brush; brush.setColor(QColor(192,192,192)); brush.setStyle(Qt::SolidPattern);
    p.setBrush(brush);
    //外圆
    p.drawEllipse(-200,-200,400,400);

    QBrush brush1; brush1.setColor(QColor(0,0,0)); brush1.setStyle(Qt::SolidPattern);
    p.setBrush(brush1);
    //内圆
    p.drawEllipse(-170,-170,340,340);

    //直线
    p.rotate(ui->horizontalSlider->value());
    QPen pen; pen.setColor(QColor(255,255,255)); pen.setWidth(4); pen.setStyle(Qt::SolidLine);
    p.setPen(pen);
    p.drawLine(0,0,-120,120*sqrt(2));






}

void Widget::hslideValueChangedSlot(int)
{
    this->update();
}

运行截图：