效果：

代码： 

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QTimer>

QT_BEGIN_NAMESPACE
namespace Ui { class Widget; }
QT_END_NAMESPACE

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();
protected:
    void paintEvent(QPaintEvent *event)override;
    void mousePressEvent(QMouseEvent *event)override;
    void mouseReleaseEvent(QMouseEvent* event)override;
private:
    bool isOn{false};
    QTimer timer;
    int x;
    void onTimer();
    bool runing{false};
};
#endif // WIDGET_H

#include "widget.h"
#include <QPainter>
#include <QMouseEvent>
#include <math.h>
#include <QDebug>

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    connect(&timer,&QTimer::timeout,this,&Widget::onTimer);
    timer.setInterval(40);

    QTimer::singleShot(100,[this]
    {
        QRect rect = this->rect();
        QRect outRoundRect = rect.adjusted(3,3,-3,-3);
        x = outRoundRect.x() + 2;
        this->update();
    });
}

Widget::~Widget()
{
}

void Widget::mousePressEvent(QMouseEvent *event)
{
    if(event->button() == Qt::LeftButton)
    {
        isOn = !isOn;
        timer.start();
    }
    return QWidget::mousePressEvent(event);
}

void Widget::mouseReleaseEvent(QMouseEvent* event)
{
    return QWidget::mouseReleaseEvent(event);
}

void Widget::paintEvent(QPaintEvent *event)
{
    Q_UNUSED(event);
    QPainter painter(this);
    painter.save();
    painter.setRenderHint(QPainter::Antialiasing,true);
    painter.setRenderHint(QPainter::SmoothPixmapTransform,true);
    painter.setRenderHint(QPainter::TextAntialiasing,true);
    painter.setPen(Qt::transparent);

    QRect rect = this->rect();
    QRect outRoundRect = rect.adjusted(3,3,-3,-3);

    if(x > outRoundRect.x() + outRoundRect.width() * 0.45 - 2)
    {
        x = outRoundRect.x() + outRoundRect.width() * 0.45 - 2;
        if(runing)
        {
            timer.stop();
            runing = false;
        }
    }
    if(x < outRoundRect.x() + 2)
    {
        x = outRoundRect.x() + 2;
        if(runing)
        {
            timer.stop();
            runing = false;
        }
    }
    if(isOn)
    {
        if(runing)
            painter.setBrush(QColor("#E7E7E7"));
        else
            painter.setBrush(QColor("#D93152"));

        painter.drawRoundRect(outRoundRect,6,6);
        painter.setBrush(Qt::white);

        QRect innerRect;
        if(runing)
            innerRect = QRect(x,outRoundRect.y() + 2,outRoundRect.width() * 0.55,outRoundRect.height() - 4);
        else
            innerRect = QRect(outRoundRect.x() + outRoundRect.width() * 0.45 - 2 ,outRoundRect.y() + 2,outRoundRect.width() * 0.55,outRoundRect.height() - 4);
        painter.drawRoundRect(innerRect,8,8);
    }
    else
    {
        if(runing)
            painter.setBrush(QColor("#D93152"));
        else
            painter.setBrush(QColor("#E7E7E7"));

        painter.drawRoundRect(outRoundRect,6,6);
        painter.setBrush(Qt::white);

        QRect innerRect;
        if(runing)
            innerRect = QRect(x,outRoundRect.y() + 2,outRoundRect.width() * 0.55,outRoundRect.height() - 4);
        else
            innerRect = QRect(outRoundRect.x() + 2,outRoundRect.y() + 2,outRoundRect.width() * 0.55,outRoundRect.height() - 4);
        painter.drawRoundRect(innerRect,8,8);
    }
    painter.restore();
}

void Widget::onTimer()
{
    runing = true;
    QRect rect = this->rect();
    if(isOn)
        x += rect.width() * 0.072;
    else
        x -= rect.width() * 0.072;
    update();
}