关键技术：

1、使用QPropertyAnimation对象，实现动画效果，逐渐缩小窗体尺寸，以及透明度；

2、在对话框缩小时，要将界面中的控件都隐藏起来，并且将对话框布局的Margin修改成0

代码如下：

dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include <QLabel>
#include <QPushButton>

class Dialog : public QDialog
{
    Q_OBJECT    
public:
    Dialog(QWidget *parent = nullptr);
    ~Dialog();
    
private:
    void init();
    void hideAllWidget(QLayout *lay);
    
private slots:
    void onCloseButtonClicked();
    
private:
    QLabel          *m_lbInfo = nullptr;
    QPushButton     *m_btnClose = nullptr;
};
#endif // DIALOG_H

dialog.cpp

#include "dialog.h"
#include <QVBoxLayout>
#include <QPropertyAnimation>
#include <QParallelAnimationGroup>

Dialog::Dialog(QWidget *parent)
    : QDialog(parent)
{
    init();
}

Dialog::~Dialog()
{
}

void Dialog::init()
{
    this->setMaximumSize(500, 400);
    //  去掉标题栏
    this->setWindowFlags(Qt::FramelessWindowHint);
    
    m_lbInfo = new QLabel(this);
    m_btnClose = new QPushButton("关闭", this);
    
    m_lbInfo->setWordWrap(true);
    m_lbInfo->setText("秦时明月汉时关，\n万里长征人未还。\n但使龙城飞将在，\n不教胡马度阴山。");
    m_btnClose->setFixedSize(100, 32);
    
    QVBoxLayout *vLay = new QVBoxLayout(this);
    vLay->setMargin(50);
    vLay->setSpacing(60);
    vLay->addWidget(m_lbInfo, 0, Qt::AlignCenter);
    vLay->addWidget(m_btnClose, 0, Qt::AlignRight);
    
    connect(m_btnClose, &QPushButton::clicked, this, &Dialog::onCloseButtonClicked);
}

void Dialog::hideAllWidget(QLayout *lay)
{
    if (!lay)
        return;
    
    lay->setMargin(0);
    
    for (int i = 0; i < lay->count(); ++i) {
        QLayout *subLay = lay->itemAt(i)->layout();
        if (subLay) {
            hideAllWidget(subLay);
        }
        
        QWidget *wgt = lay->itemAt(i)->widget();
        if (wgt) {
            wgt->hide();
        }
    }
}

void Dialog::onCloseButtonClicked()
{
    // 隐藏所有的组件，因为这些组件若显示的话，就会占据一定的位置，对话框就不能缩小到0
    hideAllWidget(this->layout());
    
    const QRect &currRect = this->geometry();
    
    // 实现对话框逐渐缩小的动画
    QPropertyAnimation *pAnim1 = new QPropertyAnimation(this, "geometry");
    pAnim1->setStartValue(currRect);
    pAnim1->setEndValue(QRect(currRect.x(), currRect.y(), 0, 0));
    pAnim1->setEasingCurve(QEasingCurve::Linear);
    pAnim1->setDuration(600);
    
    // 实现对话框逐渐隐退的动画
    QPropertyAnimation *pAnim2 = new QPropertyAnimation(this, "windowOpacity");
    pAnim2->setStartValue(1);
    pAnim2->setEndValue(0.5);
    pAnim2->setDuration(600);
    
    QParallelAnimationGroup *animGrp = new QParallelAnimationGroup(this);
    animGrp->addAnimation(pAnim1);
    animGrp->addAnimation(pAnim2);
    animGrp->start(QAbstractAnimation::DeleteWhenStopped);
    
    connect(animGrp, &QParallelAnimationGroup::finished, this, &Dialog::close);
}

效果图如下：