实现背景：
Qt本身有自己的QSlider，为什么我们还要自定义实现呢，因为Qt自带的QSlider存在一个问题，当首尾为圆角时，滑动滚动条到首尾时会出现圆角变成矩形的问题。当然如果QSS之间的margin和滑动条的圆角控制的好的话是不会出现这个问题的，但是我们一般都是按照美工设计来完成工作的，如果她的设计是必须一摸一样的话，这个margin和圆角配合不了出现以上问题的话，那我们就需要实现一个自定义的QSlider了。

实现思路：
1、继承QWidget或者QSlider都可以，当然如果我们继承QSlider的话，那还不如使用重写QStyle的方式来重绘。
2、使用paintevent绘制事件来进行重绘。
3、配合mouse鼠标事件实现拖动功能。
4、配合resizeEvent事件来实现自适应大小。

实现效果：

实现代码：
头文件：

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QPainter>
#include <QMouseEvent>

class Widget : public QWidget
{
    Q_OBJECT

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

    void reset();   //复位
    void setDirection(int dire){m_direction = dire;}
protected:
    void mousePressEvent(QMouseEvent *event);
    void mouseMoveEvent(QMouseEvent *event);
    void mouseReleaseEvent(QMouseEvent *event);
    void resizeEvent(QResizeEvent *event);
    void paintEvent(QPaintEvent *event);
signals:
    void sig_Run();
private:
    QRect m_handleRect;
    bool m_pressFlag = false;
    bool m_autoFg = false;
    int m_lastX = 0;
    int m_lastY = 0;
    int m_direction = 0; //0:水平，1:垂直
    QPixmap m_handlepix;
};


#endif // WIDGET_H

cpp文件：

#include "widget.h"
#include <QDebug>
Widget::Widget(QWidget *parent) :
    QWidget(parent)
{

}

Widget::~Widget()
{
}

void Widget::resizeEvent(QResizeEvent *event)
{
    if (m_direction == 0)
        m_handleRect = QRect(1,1,height(),height() - 2);
    else
        m_handleRect = QRect(1,height() - width(),width(),width() - 2);
}
void Widget::paintEvent(QPaintEvent *event)
{
    QPainter p(this);


    p.setRenderHint(QPainter::Antialiasing);
    p.setPen(QColor("#ffffff"));
    p.setBrush(QColor("#ffffff"));
    if (m_direction == 0)
        p.drawRoundedRect(this->rect(),height()/2,height()/2);
    else
        p.drawRoundedRect(this->rect(),width()/2,width()/2);

    //滑轨前半部分
    QLinearGradient linearGradient(QPoint(0,0),QPoint(0,height()));
    linearGradient.setColorAt(0,QColor("#56B478"));
    linearGradient.setColorAt(0.7,QColor("#006009"));
    linearGradient.setColorAt(1,QColor("#56B478"));
    QBrush brush(linearGradient);
    p.setBrush(brush);
    if (m_direction == 0)
        p.drawRoundedRect(QRect(1,1,m_handleRect.right(),height()-2),height()/2,height()/2);
    else
        p.drawRoundedRect(QRect(1,1,width(),m_handleRect.bottom()-2),width()/2,width()/2);

    //滑轨后半部分
    QBrush brush1(QColor("#606060"));
    p.setBrush(brush1);
    if (m_direction == 0)
        p.drawRoundedRect(QRect(m_handleRect.left(),1,width() - m_handleRect.left() - 2,height()-2),height()/2,height()/2);
    else
        p.drawRoundedRect(QRect(1,m_handleRect.top(),width()-2,height() - m_handleRect.top()),width()/2,width()/2);

    //文本
    p.setPen(QColor("#ffffff"));
    p.setBrush(QColor("#ffffff"));
    p.setFont(QFont("Microsoft YaHei",8));
    p.drawText(2*width()/3,0,width()/3,height(),Qt::AlignCenter,"进度条");

    //滑动块
    //    p.drawPixmap(m_handleRect,m_handlepix);
    p.setPen(QColor("#ffffff"));
    p.setBrush(QColor("#842245"));
    p.drawEllipse(m_handleRect);
}


void Widget::mousePressEvent(QMouseEvent *event)
{
    qDebug()<<m_handleRect<<event->pos();
    if (m_handleRect.contains(event->pos()))
    {
        m_pressFlag = true;
    }

}

void Widget::mouseMoveEvent(QMouseEvent *event)
{
    if (m_pressFlag)
    {
        if (m_direction == 0)
        {
            int x = event->x();
            if (event->x() >= this->rect().right())
                x = this->rect().right() - 1;
            qDebug()<<x<<m_lastX<<width();
            if (x > this->rect().right() - m_handleRect.width())
                x = this->rect().right() - m_handleRect.width();
            if (x < 0)
                x = 0;
            m_handleRect = QRect(x,m_handleRect.y(),m_handleRect.width(),m_handleRect.height());
            if (m_autoFg)
            {
                if (x > m_lastX && m_handleRect.right() >= this->rect().right() - width()/3)
                {
                    m_handleRect = QRect(this->rect().width() - m_handleRect.width() - 1,m_handleRect.y(),m_handleRect.width(),m_handleRect.height());
                }
                else if (m_handleRect.left() <= 0)
                    m_handleRect = QRect(1,m_handleRect.y(),m_handleRect.width(),m_handleRect.height());
                m_lastX = x;
            }
        }
        else
        {
            int  y = event->y();
            if (event->y() >= this->rect().bottom())
                y = this->rect().bottom() - 1;
            if (y > this->rect().bottom() - m_handleRect.height())
                y = this->rect().bottom() - m_handleRect.height();
            if (y < 0)
                y = 0;
            m_handleRect = QRect(m_handleRect.x(),y,m_handleRect.width(),m_handleRect.height());
            if (m_autoFg)
            {
                if (y < m_lastY && m_handleRect.top() <= height()/3)
                {
                    m_handleRect = QRect(m_handleRect.x(),1,m_handleRect.width(),m_handleRect.height());
                }
                m_lastY = y;
            }
        }

        update();
    }
}

void Widget::mouseReleaseEvent(QMouseEvent *event)
{
    //auto
    if (m_autoFg)
    {
        if (m_handleRect.right() != this->rect().right() - 1)
        {
            m_handleRect = QRect(1,m_handleRect.y(),m_handleRect.width(),m_handleRect.height());
            update();
        }
        else {
            emit sig_Run();
        }
    }
    else
        emit sig_Run();
    m_pressFlag = false;

}

void Widget::reset()
{
    m_handleRect = QRect(1,m_handleRect.y(),m_handleRect.width(),m_handleRect.height());
    update();
}