How to coordinate foreground/background windows between 2 processes?

Maitre Bart

@SGaist : Thank you for your help. Sorry to have omitted this essential info:

This is the QtCore library version 4.8.5
Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
Contact: http://www.qt-project.org/legal

Build key:           i386 linux g++-4 full-config
Compat build key:    | i686 Linux g++-4 full-config |
Build date:          2014-03-10
Installation prefix: /opt/qt-4.8.5
Library path:        /opt/qt-4.8.5/lib
Include path:        /opt/qt-4.8.5/include
Processor features:  mmx sse sse2 cmov sse3 ssse3 sse4.1 sse4.2

SGaist

Are you using a X server or Qt for Embedded Linux ?

Maitre Bart

@SGaist : As far as I can see, it is a X server, running on Yocto.

SGaist

Ok, because your setup looks like you could use Qt for Embedded Linux and QWS which would give you a simple window manager that would likely handle your use case in a simpler manner.

kshegunov

How is the splash screens shown full screen? If you use QWidget::showFullScreen then I think that sets a number of window flags (stay on top comes to mind so it won't release the focus to any other widget/window. I suggest trying a "pseudo" full screen by manually moving and resizing the splash to the correct rect. E.g. (untested):

QWidget * splash;  // The splash screen

splash->setWindowFlags(Qt::WindowStaysOnBottomHint);   // So it's under stuff
splash->setGeometry(QDesktopWidget::screenGeometry());
splash->show();   // Show regular window

Kind regards.

Maitre Bart

@SGaist : I understand. But I don't own the project and it involves a very big framework that encapsulates P2's main view (the instrument panel). You see, the tests on the product have all passed, and only few minor issues still irritate the product owner. One of it is the absence of the splash screen (without it the screen is white for too long, and it is not acceptable). So replacing the X server by something else would be considered a high risk at this phase.

That being said, with all the attempts I did until now, I think I'm fighting against the window manager. Here's why I say that: When P2 wants to display the instrument panel (after its initialization), I always see a glitch on the screen for a fraction of a second. I believe this glitch is P2 that wants to show/raise its view. I believe something else prevents P2's view to be moved on top or get the focus (despite the multiple raise() calls I placed everywhere in P2's code). I believe this entity is the window manager that forces the splash screen to stay on top.

SGaist

Understandable

Then maybe a silly question: wouldn't it be simpler to just set a background on your X session ?

Maitre Bart

@SGaist : Exactly what I thought at the beginning of my investigation! But the splash screen is not static: there is some text animation (rolling dots). So even if the image is in background, I'll still need a "transparent" window for the text animation, which I fear will still be maintained up-front by the wm. Not an easy one!

At this point, I'll start looking at a simple and portable peer-to-peer IPC mechnanism where P2 will notify "something" and that something will be P1. And once P1 notified, it will hide or close the splash screen. As far as I can see, a boost message_queue should do it.

This is basically what @Wieland suggested at the beginning, but in the mean time I could not find an appropriate call back in the big framework and have to conclude at this point that the problem I have is most probably caused by the window manager.

SGaist

QLocalSocket and QLocalServer are portable and lightweight.

Maitre Bart

I just finished implementing the hide-and-seek communications between P1 and P2 (I used a boost's message queue).
However I have another problem when P1's window is up. Normally P1's window is hidden. But it can be activated via a P2's secret menu (known by the maintenance team). This window is supposed to be always on top.

In that window, one thing that is possible to do is to stop (kill) and restart P2 via a button. But when P2 is killed, the splash screen comes up and hide everything, including P1's window. But since P1 killed P2 on purpose, it waits for the user to click the button again (that is a use-case). So it is impossible to restart P2 because the user cannot see/access P1's window... It's a nightmare.

What I can understand from QSplashScreen is that it works well when the main window is in the same process. Indeed, calling finish(mainWindow) seems to solve a bunch of problems.

Is it possible to (easily) replace QSplashScreen or at least remove some flags that makes it appear "always on top"?

Also, looking at the Qt documentation, I can read that a QWidget that has a parent can become a window by setting the Qt::WA_Window flag. Maybe that's the way to go in my case? I don't know however if I will have to invent the wheel again in order to manage such a bare object...

Or maybe as @SGaist suggested, moving the splash screen image to the screen background suddenly becomes an viable alternative.

Any thoughts?