QT realizes window topping, topping state switching, and multi window topping priority relationship

We use QT During interface development , You may encounter the need to put the window on the top . The most common is , You need to make a floating toolbar , Suspension menu , Even a floating drawing board . This means that we need to put this window on the top “ System ” And ourselves “ Software ” On top of the window .
In fact, the implementation method is very simple , When creating this window class , Add one in the constructor Qt::WindowFlags
Enumeration identification

Widget::Widget(QWidget *parent)
    : QWidget(parent,Qt::Widget | Qt::WindowStaysOnTopHint)
    , ui(new Ui::Widget)
{
    
    ui->setupUi(this);
}

Qt::WindowStaysOnTopHint It means that the window is on the top , After setting, it can be suspended on the desktop .
For other enumeration identifiers , You can refer to this .Qt::WindowFlags Enumeration type resolution Like canceling the top toolbar 、 no border 、 Pop up or something , Can be realized .

Now there is a situation , We need to flexibly switch the top state of the window , Then you can directly call the relevant setting interface to realize .setWindowFlags（）;
To be specific ：

void Widget::on_pushButton_clicked()
{
    
    if(!istop)
    {
    
        Qt::WindowFlags m_flags = windowFlags();
        setWindowFlags(m_flags | Qt::WindowStaysOnTopHint);
        show();
        qDebug()<<"always top";
    }else{
    
        //Qt::WindowFlags m_flags = windowFlags();
        //setWindowFlags(Qt::Widget);
        setWindowFlags(NULL);
        show();
        qDebug()<<"no always top";
    }
    istop=!istop;
}

setWindowFlags(Qt::Widget); and setWindowFlags(NULL); Can cancel the top status , Just reset it . But after this setting , The window is not displayed by default , So we need to show once . but show When , The window will flash obviously . This has not been well solved .

Another question , The priority between two top windows cannot be set and determined , That is to say, set two top windows in the same way , The drag between them , It can cover each other , It depends on which window the current focus clicks in .
This problem , On some occasions, it is mutually exclusive and contradictory .
At present, one method is , hold a Object set to b The parent of the object , that b The object is always at the top and a object

Widget::Widget(QWidget *parent)
    : QWidget(parent,Qt::Widget | Qt::WindowStaysOnTopHint)
    , ui(new Ui::Widget)
{
    
    ui->setupUi(this);
    son_widget_1 = new QWidget(this);
    son_widget_2 = new QWidget(son_widget_1);

    son_widget_1->setWindowFlags(Qt::Dialog | Qt::WindowStaysOnTopHint);
    son_widget_2->setWindowFlags(Qt::Dialog | Qt::WindowStaysOnTopHint);
    son_widget_1->setWindowTitle("son_widget_1");
    son_widget_2->setWindowTitle("son_widget_2");

    son_widget_1->show();
    son_widget_2->show();
}

These three top windows are superimposed on each other , It's related to the relationship between father and son .（son_widget_1/2 Set up Qt::Dialog, If it is Qt::Widget, When the parent class is set , Is a child part of the parent class ）.

2022.5.25 Add
Thank the friends in the comment area for providing better solutions , It solves the problem that you must show Caused by flickering , The following code has been verified to be feasible ：

void Widget::on_pushButton_clicked()
{
    
    if(!istop)
    {
    
// Qt::WindowFlags m_flags = windowFlags();
// setWindowFlags(m_flags | Qt::WindowStaysOnTopHint);
// show();
        qDebug()<<"always top";

        QWindow* pWin = this->windowHandle();
        pWin->setFlags(Qt::Widget | Qt::WindowStaysOnTopHint);
    }else{
    
        //Qt::WindowFlags m_flags = windowFlags();
// setWindowFlags(Qt::Widget);
// //setWindowFlags(NULL);
// show();

        QWindow* pWin = this->windowHandle();
        pWin->setFlags(Qt::Widget);
        qDebug()<<"no always top";
    }
    istop=!istop;
}