I want to create a console application in Qt which handles Close, Minimize and Maximize buttons of the console window. My goal is just to show some message before the application quits - i.e. Close button is clicked.
Further, I want the application to be minimized to the system tray instead of task bar. However, it seems there are no signals or events which I can process when user clicks on one of the system buttons.
Is it even possible?
I do not think you can handle such "signals" (minimize, maximize, and close the terminal window running a QCoreApplication) through the APIs provided by Qt.
But QCoreApplication sends a signal called aboutToQuit(). Probably you can use it to do what you want (write in the terminal, for example), just do not know if the user will be able to read in time.
About minimize the application to the tray: Again, probably not possible to do it in a terminal application using the Qt APIs. But it is perfectly possible in a QApplication (which has a window). See this answer to a similar SO question.
You can use POSIX signals both in Linux and Windows
#include
void quit_handle( int ) {
qApp->quit();
}
int main( int argc, char *argv[] ) {
QCoreApplication a(argc, argv);
...
signal( SIGINT, quit_handle );
return a.exec();
}
I tested it only in Windows+MinGW, but I think it would work in Linux too
Are you using a Unix / Linux or a Windows?
If you are using Unix or Linux you might want to look into the Posix / Unix Signals.
They do not offer any solution to the maximize / minimize buttons,
but they give you the possibility to at least catch the System Signal when you hit the close button. From my experience, the aboutToQuit() Signal is not as fast as a custom override of the Signal Handler.
As soon as you catch the Signal you can process it yourself.
Here's a good tutorial for the Custom Signal Handlers:
http://doc.qt.io/qt-5/unix-signals.html
Related
For example:
void MainWidget::testThreadTask()
{
qDebug() << "On test task";
}
void MainWidget::onBtnClick()
{
QThread *thread = new QThread;
connect(thread, QThread::started, this, testThreadTask);
thread->start();
qDebug() << "Thread START, now we wait 5s";
QElapsedTimer timer;
timer.start();
while (timer.elapsed() < 5000)
{
}
qDebug() << "END";
}
The program output is:
START wait 5s
END
On test task
I want to create a task to handle something after the button is pressed, and then the function will wait for the task to complete before returning.
In fact, it may not be necessary to create a new task and wait for it to execute, because since you have to wait and get stuck there, why not run it directly in the function.
But this is actually a problem when I deal with QT serial data. I want to send the data to the serial port after pressing the button, and then wait for the data (by constantly reading), but I find that when I have been waiting, the serial port can not read the data at all, only when I exit the function the serial port can read the data.
Is there any way to deal with serial data sending and receiving synchronization?
void MainWidget::onBtnClick()
{
serial->write("Test");
if (serial->bytesAvailable())
{
QByteArray data = serialIo->readAll();
// handle the data
}
}
You are mistaken with what is happening in your application. I suggest you read Threads and QObjects (the entire page), Qt::ConnectionType and the detailed description of QThread.
What is happening to you is:
MainWidget does not live in thread. For the slot of a regular object to be called from thread, it first needs to be moved to that thread.Note that subclasses of QWidget cannot be moved to another thread. Because some OS supported by Qt limit where windows can live, they made the choice to force all QWidget to stay in the main thread, in all OS Qt can execute on.
When you connect thread to this (which BTW is incorrect in your question, it should have been with ampersands connect(thread, &QThread::started, this, &MainWidget::testThreadTask);), you create a queued connection, even though the thread has not technically started yet.
When you start the thread:
It fires its started signal.
Because the connection is a Qt::QueuedConnection, the slot will only be executed after returning to the main thread's event loop, i.e. some time after returning from onBtnClick.
Notes:
You would have more useful information in qDebug() about the threads running your code by using QThread::currentThread().Even better than that, your IDE should provide you a window specifically to see what thread has reached a breakpoint (Ctrl+Alt+H on Visual Studio).
At the risk of insisting, keep in mind this warning from the Qt help:
Be aware that using direct connections when the sender and receiver live in different threads is unsafe if an event loop is running in the receiver's thread, for the same reason that calling any function on an object living in another thread is unsafe.
With that said, because you wait 5 seconds before returning to the event loop and because it is only test code (= there should be no bug + it does not matter even if there is one), you should try to create a Qt::DirectConnection, just to see the slot be invoked from the worker thread.
The detailed description of QThread (link above) shows a complete working example of a worker object being moved to the new thread before it is started. The point is:
A worker object is created, then moved to the worker thread.
Connections are created for the controller to send QString to the worker object via signal/slot and for the worker object to return result to the controller via signal/slot too.
All these connections are Qt::QueuedConnection by default since the worker object was moved.
The worker thread is started. Since run was not overriden, it starts an event loop (in exec).
And there you have it.
Remember 1 things: widgets cannot be moved!!! Create your own worker object.
I'm using a qt5.9 app on imx6slevk board with yocto bsp,
Before running the app I see a virtual terminal on the display and this screen blanks after 10 minutes as specified in the driver file found at Kernel_source > drivers/tty/vt/vt.c
static int blankinterval = 10*60;
Just as this file is used for virtual terminal the file qlinuxfbscreen.cpp is used by qt applications running on linuxfb.
I didn't find any option of do_screen_blank or do_screen_unblank in this file
All I found was this function:
static void blankScreen(int fd, bool on)
{
ioctl(fd, FBIOBLANK, on ? VESA_POWERDOWN : VESA_NO_BLANKING);
}
on passing on = 1 the screen goes off completely with no display at all
I tried setting Idleaction in systemd/logind.conf
IdleAction=suspend
IdleActionSec=30Sec
after this the app turns off after 30 sec but it does so even if I press keys on the matrix keypad, systemd doesn't seem to register key press as active events and the screen turns off.
Is there any patch that I can apply to enable screen timeout? Or could I at least register key press as active event in systemd?
You'd need to modify your qt app.
The algorithm would be something like
Start timer
If key pressed reset timer to max value
If no key pressed go to suspend mode, you may use:
system("echo mem > /sys/power/state")
Wakeup on power key press.
I need to detect the application get exit as normal or crash. QProcess have the finished() signal and can get the exit code. But i need this exit code for QApplication when the application get crash or close.
When your process crashes, it's gone. The crash means that the process has finished because of an unhandled exception. Your job should be to prevent the crash from happening. In other words: handle the exceptions. Note that the exceptions may not be C++ exceptions, they may be low-level platform-specific mechanisms, such as native exceptions on Windows or signals on UNIX. You'd have to handle those, but recognize that the underlying issue is not fixed merely because you catch such an exception. You must assume that the state of your application has been corrupted, and the only safe thing to do is to exit ASAP anyway. For example, do not try to modify any files: you're likely to corrupt them.
I don't think this is something you can do just like that. Reading the value returned by QApplication::exec() is related to the Qt infrastructure:
Enters the main event loop and waits until exit() is called, then
returns the value that was set to exit() (which is 0 if exit() is
called via quit()).
Usually your main looks like this:
#include <QApplication>
int main( int argc, char **argv )
{
QApplication a( argc, argv );
// Initialize your widget(s)
return a.exec(); // You can store this and check its value
}
However if I'm not mistaken this doesn't include handling a crash of your application (segmentation fault, unhandled exception etc.). In Linux people usually use a script which starts the application and then reads its exit code after the application quits or crashes. If you use Linux you can use echo $? to read the exit code from the bash scrip (or its equivalent for a different shell) and then do something based on its value.
Note also that you can at least do some exception handling since some crashes result in exactly that - an exception that has been thrown for some reason and has not been processed properly. Unhandled exceptions in Qt get propagated to the top level (that is QCoreApplication).
I am developing an application on Qt symbian, in which I have to restart my application within my application, have used:
qApp->quit();
QProcess::startDetached(qApp->arguments()[0],qApp->arguments());
from a method in mainWindow. It is working fine on simulator but not on device, it closes but not restarting by itself, I have to restart it by myself, is there anything else I have to do to make it work on device.
One solution would be to create small console process that you can launch from your main program before closing it. Then this console process would just launch your program and close.
I have been using this kind of processes to keep track of my apps and restart them when they crash.
One minor but fundamental thing: on Symbian there is an emulator and not a simulator. The difference is that the later simulates the device on the assembly level while the former does it only on API support level. For example iPhone simulator simulates the phone on assembly level. Contrarily in Symbian the underlying API implementation might be and is completely different for the ARM and for the WINS architecture. Especially in such cases when you interact with the OS like exiting the application.
The application quit operation on Symbian is eventually implemented by throwing a special exception (I don't remember it's name, something like KExitException) that is caught by the main Active Scheduler loop that tells the kernel to shut down the process. In other words it means that it is a synchronous call. If you first call quit then startProcess then the later will be never executed. It is not that clear why does not it work if you first call startProcess and then quit: this might be an asynchronous call that can not complete before you exit, or you simple can not start the same (GUI) application in two instances. Anyway check the return value of startProcess to see whether it succeeded or not.
Your ultimate solution will be to create a watchdog process as #Riho suggested. You start the watchdog process before you call quit, in the watchdog main function you wait some seconds and restart your application. You will need SwEvent capability for your watchdog.
I have tried it with Qprocess() and it seems to be working fine (still testing for memory and thread issues)
in main.cpp I write this code (which I got from other link )
int main(int argc, char *argv[])
{
#define RESTART_CODE 1000
int return_from_event_loop_code;
QPointer<QApplication> app;
QPointer<MainWindow> main_window;
do
{
if(main_window) delete main_window;
if(app) delete app;
app = new QApplication(argc, argv);
main_window = new MainWindow;
QList<QString> lang = AppStatus::getCurrentLanguage();
QTranslator translator;
translator.load(lang.at(0));
app->installTranslator(&translator);
main_window->setOrientation(MainWindow::ScreenOrientationLockPortrait);
#if defined(Q_OS_SYMBIAN)
main_window->showMaximized();
#else
main_window->show();
#endif
return_from_event_loop_code = app->exec();
}
while(return_from_event_loop_code==RESTART_CODE);
return return_from_event_loop_code;
}
and in my method from where I have to restart my app I have written this.
QProcess::startDetached(qApp->applicationFilePath(),qApp->arguments());
qApp->exit(RESTART_CODE);
And my app is restarting like I wanted.. If any changes nedded plese let me know.
I have a Qt application that runs on a linux machine that displays UI windows on a Windows machine running an X server. There's a strong possibility that the users of my application will logout of Windows or kill the X server without quitting my application first.
Is there a way to handle the unexpected shutdown of the X server, so that the application can gracefully shutdown, save stored data, etc? So far I've tried tying QApplication's aboutToQuit signal to a slot in my app, plus catching all exceptions in main. Neither worked as I expected.
I think your best bet is to attempt to handle it via signal handling
depending on how application is being killed by the OS, exiting gracefully may or may not be possible at all.
I'm not sure, but maybe this QCoreApplication::aboutToQuit() signal can help you.
If connection with the X server is lost then libX11 inside the application calls _XIOError() function that calls _XDefaultError handler that calls exit(). It is possible to override that with a custom handler:
#include <X11/Xlib.h>
static int handler(Display *dpy)
{
/*
* cleanup
* ...
*/
exit(1);
return 0;
}
...
int main(int argc, char** argv)
{
QApplication app(argc, argv);
XSetIOErrorHandler(handler);
...
It's better to still call exit() in the handler because Qt will segfault anyway without a proper X connection.