知识点

setWindowOpacity(0.5);//设置窗体透明度
QTimer* Timer = new QTimer(this);//新建一个定时器
connect(Timer,SIGNAL(timeout()),this,SLOT(ShowTime()));
Timer->start(1000);//启动定时器

digiclock.h

#ifndef DIGICLOCK_H
#define DIGICLOCK_H

#include <QDialog>
#include <QLCDNumber>

class DigiClock : public QLCDNumber
{
    Q_OBJECT

public:
    DigiClock(QWidget *parent = nullptr);
    ~DigiClock();
    void mousePressEvent(QMouseEvent*);
    void mouseMoveEvent(QMouseEvent*);
public slots:
    void ShowTime();
private:
    QPoint DragPosition;//保存鼠标相对电子时钟窗体左上角的偏移值
    bool ShowColon;//用于显示事件是否显示":"

};
#endif // DIGICLOCK_H

digiclock.cpp

#include "digiclock.h"
#include <QTimer>
#include <QTime>
#include <QMouseEvent>

DigiClock::DigiClock(QWidget *parent)
    : QLCDNumber(parent)
{
   
    QPalette p = palette();
    p.setColor(QPalette::Window,Qt::blue);
    setPalette(p);
    setWindowFlags(Qt::FramelessWindowHint);//设置一个没有面板和窗体栏的窗体
    setWindowOpacity(0.5);//设置窗体透明度
    QTimer* Timer = new QTimer(this);//新建一个定时器
    connect(Timer,SIGNAL(timeout()),this,SLOT(ShowTime()));
    Timer->start(1000);//启动定时器
    ShowTime();//初始化时间显示
    resize(150,60);//设置窗口大小
    ShowColon = true;
}

DigiClock::~DigiClock() {}

void DigiClock::mousePressEvent(QMouseEvent *Event)
{
    if(Event->button() == Qt::LeftButton)
    {
        DragPosition = Event->globalPos() - frameGeometry().topLeft();
        Event->accept();
    }
    if(Event->button() == Qt::RightButton)
    {
        close();
    }
}

void DigiClock::mouseMoveEvent(QMouseEvent *Event)
{
    if(Event->buttons()&Qt::LeftButton)
    {
        move(Event->globalPos()-DragPosition);
        Event->accept();
    }
}

void DigiClock::ShowTime()
{
    QTime Time = QTime::currentTime();
    QString Text = Time.toString("hh:mm");
    if(ShowColon)
    {
        Text[2] = ':';
        ShowColon = false;
    }
    else
    {
        Text[2] = ' ';
        ShowColon = true;
    }

    display(Text);

}

main.cpp

#include "digiclock.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    DigiClock w;
    w.show();
    return a.exec();
}

运行图