How to hide a QMainWindow until later?

JonB

@SPlatten
Your "splash" screen is a QMainWindow, designed to stay on screen for a second, really? I cannot imagine why you would choose a QMainWindow here...! Per https://doc.qt.io/qt-6/qmainwindow.html#qt-main-window-framework a QMainWindow is used for support for a menu bar, toolbar, dock widgets, central widget and status bar. That is why one uses it. Which of these are required for your one-second splashscreen?

There is also a QSplashScreen class intended for this, though I don't want to get into a debate over what you doubtless consider your reasons are.

But that has (or should have) nothing to do with show/hide behaviour.

I don't know whether what you are seeing is intentional or unavoidable. Can you afford to call new splash() (no this parent) instead, I assume that is the cause of seeing the main window behind the "splash" screen?

SPlatten

@JonB , I'm really not sure I follow what you are suggesting, presently the new splash class works correctly and does exactly what it is supposed to do, the clsMainWnd which has no ui associated with it is still showing even though hide has been called in the constructor.

JonB

@SPlatten
As I wrote

Can you afford to call new splash() (no this parent) instead

You have statement:

new splash(this);

I am asking what behaviour is if you replace that line with

new splash();

Is that pretty clear? It's to investigate behaviour, cause of clsMainWnd being shown behind splashscreen.

SPlatten

@JonB , thank you, in an effort to see what is happening I've added the connection:

    QObject::connect(this, &QMainWindow::showEvent, this, &clsMainWnd::onShowEvent);

To my class constructor, although this results in:

/Users/sy/XMLMPAM/clsMainWnd.cpp:5507: error: 'showEvent' is a protected member of 'QWidget'
../clsMainWnd.cpp:5507:42: error: 'showEvent' is a protected member of 'QWidget'
    QObject::connect(this, &QMainWindow::showEvent, this, &clsMainWnd::onShowEvent);
                                         ^
../../Qt/5.15.2/clang_64/lib/QtWidgets.framework/Headers/qwidget.h:654:18: note: must name member using the type of the current context 'clsMainWnd'
    virtual void showEvent(QShowEvent *event);
                 ^

SPlatten

@JonB , if I try just:

    new splash();

Then the new splash window doesn't show at all, only the original which is empty.

JonB

@SPlatten said in How to hide a QMainWindow until later?:

only the original which is empty

So in that case we can remove the splash screen from code? You are just reporting that creating a QMainWindow and going hide() in its constructor causes "a window with an empty UI" to be shown? Does not your caller which constructs the clsMainWnd go show() on it?

I would create a small, standalone test program which exemplifies what you are seeing. It is impossible to know what else might be going on in your code.

JonB

@SPlatten said in How to hide a QMainWindow until later?:

QObject::connect(this, &QMainWindow::showEvent, this, &clsMainWnd::onShowEvent);

QWidget::showEvent() is not a signal (it is a Qt "event", any method they choose to use Event in its name is their convention, and they are not signals). You cannot connect() a slot to it.
You can, however, override it in your derived class.

SPlatten

@JonB , I'm trying to determine what is going on, the only place where clsMainWnd is created which is derived from QMainWindow is in main:

clsMainWnd w;

And I've just found the location where show is being called, thank you for you assistance.

JonB

@SPlatten
Yep, usually when a program creates one instance of a QMainWindow (either on the heap or on the stack) it follows that by going mainWindow.show(). Which of course would undo the hide() you have put in its constructor.

As a tip: widget constructors should not show (or hide) themselves. The caller should be in charge of showing (or hiding) widgets.

SimonSchroeder

No window shows itself by default. There is no need to add hide() into your constructor. You must be showing the window somewhere else (which is why @JonB suggested to use new splash() without this because this makes it a child of the main window and thus shows it with the main window).

The most basic main function looks like this:

int main(int argc, char *argv[])
{
    QApplication app(argc,argv);
    QMainWindow mw;
    mw.show();
    return app.exec();
}

Are you sure you app does not include this mw.show() line? Instead you should call show() on your splash screen (which you don't seem to do).

SPlatten

For the record, this is all done now, here is the splash.cpp:

static const int REMOVE_SPLASH_TIME = 10000;
/**
 * @brief splash::splash
 * @param parent
 */
splash::splash(QWidget *parent) : QMainWindow(parent)
                                , ui(new Ui::splash) {
    //Modify the window to occupy full screen with no window dressing
    setWindowFlags(Qt::CustomizeWindowHint
                 | Qt::FramelessWindowHint
                 | Qt::WindowStaysOnTopHint);
    //Get desktop geometry
    QRect rctDesktop(screen()->geometry());
    //Set-up the user interface
    ui->setupUi(this);
    QString strTemp;
    strTemp = QString::asprintf("%dpx", rctDesktop.width());
    ui->lblWidthVal->setText(static_cast<const QString>(strTemp));
    strTemp = QString::asprintf("%dpx", rctDesktop.height());
    ui->lblHeightVal->setText(static_cast<const QString>(strTemp));
    //Set-up one second timer to perform house keeping functions
    QTimer::singleShot(REMOVE_SPLASH_TIME, this, &splash::onRemoveSplash);
}
/**
 * @brief splash::~splash
 */
splash::~splash() {
    delete ui;
}
/**
 * @brief splash::onRemoveSplash
 * Removes the splash display when the timer expires
 */
void splash::onRemoveSplash() {
    hide();
}

JonB

@SPlatten
Now that you have resolved your question. I am still surprised why you would want anything derived from a QMainWindow, plus its menubars etc. that you have been asking about, as a "splash" screen, which is set to disappear after 10 seconds? I know you are working on something "unusual" and generic, but still don't see the use case.

SPlatten

@JonB , I was using the splash ui in my application purely for experimentation, I now have proven that I can dynamically create menus that are specified in my XML files and I have subscribers that are connected to the menu signals, it all works really well.