设置QListView中的QScrollBar样式表

刚开始我只是用简单的qss语句设置QScrollBar，居然完全不起作用！

代码如下：

    m_view = new QListView(this);    m_view->setGeometry(20,25,w-40,h-50);    m_view->setVerticalScrollBarPolicy(Qt::ScrollBarAsNeeded);    m_view->setFrameShape(QFrame::NoFrame);    // 没有效果    //m_view->setStyleSheet("QScrollArea{background-color:transparent;}"                                                           //                 "QScrollBar::add-line:vertical, QScrollBar::sub-line:vertical{width:0;height:0;}"                      //                  "QScrollBar::handle:vertical{image:url(:/ClassBlackBoard/Resources/ClassBlackBoard/Slider.png)}"); m_view->verticalScrollBar()->setStyleSheet("QScrollBar::handle:vertical{image:url(:/ClassBlackBoard/Resources/ClassBlackBoard/Slider.png)}");//  没有效果
苦思冥想都不知道该怎么办！准备自己重写QListView的时候，在别人指导下，重新写了一下qss代码：
 m_view->setStyleSheet("QScrollBar:vertical/    {                                         /        width:8px;                            /        background:rgba(0,0,0,0%);            /        margin:0px,0px,0px,0px;               /        padding-top:9px;                      /        padding-bottom:9px;                   /    }                                         /    QScrollBar::handle:vertical               /    {                                         /        width:8px;                            /        background:rgba(0,0,0,15%);           /        border-radius:4px;                    /        min-height:20;                        /    }                                         /    QScrollBar::handle:vertical:hover         /    {                                         /        width:8px;                            /        background:rgba(0,0,0,25%);           /        border-radius:4px;                    /        min-height:20;                        /    }                                         /    QScrollBar::add-line:vertical               /    {                                                                  /        height:9px;width:8px;                                          /        border-image:url(:/images/a/3.png);                            /        subcontrol-position:bottom;                                    /    }                                                                  /    QScrollBar::sub-line:vertical                                      /    {                                                                  /        height:9px;width:8px;                                          /        border-image:url(:/images/a/1.png);                            /        subcontrol-position:top;                                       /    }                                                                  /    QScrollBar::add-line:vertical:hover                                /    {                                                                  /        height:9px;width:8px;                                          /        border-image:url(:/images/a/4.png);                            /        subcontrol-position:bottom;                                    /    }                                                                  /    QScrollBar::sub-line:vertical:hover                                /    {                                                                  /        height:9px;width:8px;                                          /        border-image:url(:/images/a/2.png);                            /        subcontrol-position:top;                                       /    }                                                                  /    QScrollBar::add-page:vertical,QScrollBar::sub-page:vertical        /    {                                                                  /        background:rgba(0,0,0,5%);                                     /        border-radius:4px;                                             /    }");
居然有效果了！
然后逐句删掉多余的qss语句，发现原来是没有给定宽高值。。。
最后代码如下：
m_view = new QListView(this);    m_view->setGeometry(20,25,w-40,h-50);    m_view->setVerticalScrollBarPolicy(Qt::ScrollBarAsNeeded);    m_view->setFrameShape(QFrame::NoFrame);    m_view->setItemDelegate(new ItemDelegate(this));    m_view->setFocusPRoxy(this);    m_view->setStyleSheet("QScrollBar:vertical/    {/        width:15px;/        background:rgba(0,0,0,0);/        padding-bottom:9px;                        /        background-position: right;/    }                                              /    QScrollBar::handle:vertical                    /    {                                              /        width:15px;                                 /        border-image:url(:/ClassBlackBoard/Resources/ClassBlackBoard/Slider.png);                       /    }                                              /    QScrollBar::add-line:vertical/    {/        height:0px;width:0px;/        subcontrol-position:bottom;/    }/    QScrollBar::sub-line:vertical/    {/        height:0px;width:0px;/        subcontrol-position:top;/    }");