怎么让滚动条随着控件更新

如何让滚动条随着控件更新
我做了个滚动画面，画面里有20个按钮，现在我拖动滚动条按钮跟随着滚动，但是我移动按钮的焦点左边的滚动条并不跟着动，怎么设定一下

    QVBoxLayout *scrolllayout = new QVBoxLayout;
    scrolllayout->setAlignment(Qt::AlignHCenter);
    scrolllayout->setSpacing(0);
    scrolllayout->setMargin(0);

    QWidget *widget = new QWidget(label);
    QStringListIterator list(strlist);
    for(int i = 0; i < 20; i++){
        QToolButton *button = new QToolButton(widget);
        if(list.hasNext()){
            button->setText(list.next().toLocal8Bit().constData());
        }
        button->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Fixed);
        button->setFixedHeight(30);
        scrolllayout->addWidget(button);
    }
    widget->setLayout(scrolllayout);
    widget->setFocusPolicy(Qt::NoFocus);
    widget->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);

    scroll = new QScrollArea(label);
    scroll->setWidget(widget);
    scroll->setFrameStyle(QFrame::NoFrame);
    scroll->setWidgetResizable(true);