How to Set QMainWindow Size?

[Cobra91151]

@bsomervi

I have tried your code, fixed a few compile errors, but for me it works well.

Result:

What are you trying to achieve? By the way, you can use QSplitter https://doc.qt.io/qt-5/qsplitter.html with layout to hide the hideable_edit_ (QTextEdit).

Also, there are two options you can try:

1. Use QResizeEvent https://doc.qt.io/qt-5/qresizeevent.html
2. Use nativeEvent https://doc.qt.io/qt-5/qwidget.html#nativeEvent to intercept and handle the resize/restore window messages (https://docs.microsoft.com/en-us/windows/win32/winmsg/wm-size, https://docs.microsoft.com/en-us/windows/win32/winmsg/wm-sizing)

[bsomervi]

The problem is when the single widget rendering is shrunk using the mouse to the minimum size toggling the action twice does not restore the main window size. I can find no way to set the size of the main window as small as it was settable using the mouse. This is a problem (a) because there seems to be no way to do in code what can be done with the mouse, and (b) the whole point is to have a "view" of the main window set by the user that can be returned to using the code. Users simply cannot believe we can't do this and their chosen size is ignored.

[bsomervi]

native events are no use to us as our application is portable. How do i use resizeEvents to do this? Note that my real case has a whole bunch of widgets and layouts in the hideable part, I don't think a splitter is appropriate as the bottom part is not resizable. I just used another QTextEdit in my example above as the header was already included.

[bsomervi]

@Cobra91151 where are the compile errors in my code example?

You second animation above clearly shows the issue I am trying to solve.

How do I make use of an override resizeEvent() to fix this issue? I don't see how it can help, a resize Event is triggered by some external action, how can I make it fix the specific when a call to MainWindow::resize() does not do what it is asked to do?

[Cobra91151]

@bsomervi

What do you mean portable? I suppose your app must run on different operating systems? If so, then check out this link: https://forum.qt.io/topic/104533/how-to-ignore-the-event-when-i-click-on-the-button-to-maximize-the-window-windowmaximizebuttonhint/9

You can use nativeEvent with defines for different operating systems and add the appropriate code.

[Cobra91151]

@bsomervi

As for compile errors: you forgot to add & in two connects. Example below:

Your code:
connect(resize_action_, QAction::toggled, this, &MainWindow::do_resize);
QApplication::connect(&app, QApplication::lastWindowClosed, &app, QApplication::quit);

Fixed:
connect(resize_action_, &QAction::toggled, this, &MainWindow::do_resize);
QApplication::connect(&app, &QApplication::lastWindowClosed, &app, QApplication::quit);

[bsomervi]

Ha! OK on compile errors. My compiler I used for test doesn't pick that up and works (MinGW 8.1). They are indeed errors.

[bsomervi]

OK, so here's another related frustration. My real code persists the state of the checkable action into a settings file. On startup it builds the UI and then if the action is checked hides the lower widget(s). This is all in the main window constructor. It then goes on to restore the reduced size main window using MainWindow::restoreGeometry(geometry) also from the settings file stored at shutdown with QMainWindow::saveGeometry(). This shows the correctly sized main window. If I then toggle the action twice the main window size then is bigger. This is really annoying. Why does MainWindow::restoreGeometry() work in the constructor before the first QMainWindow::show() and yet after that neither MainWindow::restoreGeometry(), nor QMainWindow::resize(), nor any other way of trying to set the size, I have tried so far, work correctly.

I have tried removing the lower widget(s) from the layout. I have tried hiding the whole main window before making the size adjustments and showing again afterwards (although I guess this is a no-op as i don't get into the event loop between the calls).

I must be missing the necessary incantation to make these sizing member functions work correctly.

[bsomervi]

@Cobra91151 Note also that this misbehaviour is the same on Windows, Linux, and macOS, so I don't think it is anything to do with native control renderings. It is something in Qt that is causing hidden widgets to influence the minimum size of a window when they should not.

[Christian Ehrlicher]

Hidden widgets are not used by the sizeHint() calculation. Still no example which shows the opposite.

[bsomervi]

@Christian-Ehrlicher why is no one seeing this issue?

• Build my example above.
• Run it.
• Click the checkable menu item to move to the single widget display.
• Shrink the window to its minimum size using the mouse.
• Click the checkable menu item to move to the dual widget display.
• Click it again to move back to the smaller single widget display.

At this point the window is larger that it was shrunk to before. Why? The code calls resize with the smaller size but Qt still somehow accounts for the hidden widget. MainWIndow::restoreGeometry() does the same.

[Christian Ehrlicher]

Ok, it took a while to understand what's going on here.
When you hide the widget, the main window as it's parent still has the old minimum height set which includes the (now hidden) widget. Therefore you can't resize the main window to the smaller saved size. You have to wait until the main window is notified about the hidden widget and it's minimum size is adjusted.

void do_resize (bool checked)
{
  auto s = size ();
  hideable_edit_->setVisible (!checked);
  qDebug() << "Current minimum size:" << minimumHeight() << saved_size_;
  if (saved_size_.isValid ())
  {
      desired_size_ = saved_size_;
  }
  saved_size_ = s;          // save for when we flip back
}

bool event(QEvent *e) override
{
  auto ret = QMainWindow::event(e);
  if (e->type() == QEvent::LayoutRequest) {
    if (desired_size_.isValid()) {
      auto s = desired_size_;
      desired_size_ = QSize();
      resize(s);
    }
  }
  return ret;
}
QSize desired_size_;

[bsomervi]

Thanks for taking the time to work that out @Christian-Ehrlicher, it works perfectly with restoring the size and also with restoring the whole window geometry which was my actual use case. I knew there would be an incantation that did this but finding it would have taken me a lot longer than the attempts I had made so far.

Looks to me that the Qt logic should be either giving higher priority to resize invocations, or it should be deferring the actual resize until after the layout change has been done. I'll bet it is a nasty chicken and egg, which comes first, situation.

[Christian Ehrlicher]

The problem is the OS - until the OS does not tell us that the widget is really hidden or resized we have to wait. And all this stuff is async.