QTextEdit *mpContentTextEdit = new QTextEdit(this);

//设置占位符文本
mpContentTextEdit->setPlaceholderText(tr("Placeholder text"));

//设置关联控件，用于QTextEdit控件捕获MouseButtonPress等事件
QWidget *mpContentWidget = mpContentTextEdit->viewport();

//设置事件过滤器
mpContentTextEdit->installEventFilter(this);
mpContentWidget->installEventFilter(this);

//QTextEdit处理占位符及事件
bool eventFilter(QObject *obj, QEvent *pEvent)
{
    if (obj == mpContentTextEdit)
    {
        if (pEvent->type() == QEvent::FocusIn)
        {
            //清空编辑框默认占位符
            mpContentTextEdit->setPlaceholderText("");
        }
        else if (pEvent->type() == QEvent::FocusOut)
        {
            //设置编辑框默认占位符
            mpContentTextEdit->setPlaceholderText(tr("Placeholder text"));
        }
    }

    if ((((QKeyEvent *)pEvent)->key() == Qt::Key_Tab || pEvent->type() == QEvent::MouseButtonPress)
        && mpContentWidget->hasFocus())
    {
        //通过关联控件，可间接拦截编辑框鼠标等事件操作
        mpContentTextEdit->setText("");
    }
    
    return QDialog::eventFilter(obj, pEvent);
}