The Qt Tray application closes by itself after performing any action in the menu - qt

Question

I'm new in QT and I ran into a problem.
I decided to try writing a small application using Qt and QSystemTrayIcon. To start, I decided to reproduce the example from the official site of Qt.
https://doc.qt.io/qt-5/qtwidgets-desktop-systray-example.html?
This is my header file:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QSystemTrayIcon>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = nullptr);
~MainWindow();
private:
Ui::MainWindow *ui;
QMenu *trayIconMenu;
QAction *launchAction;
QAction *quitAction;
QSystemTrayIcon *trayIcon;
public slots:
void changeEvent(QEvent*);
void trayIconActivated(QSystemTrayIcon::ActivationReason reason);
void trayActionExecute();
void setTrayIconActions();
void showTrayIcon();
};
#endif // MAINWINDOW_H
This is my source code file:
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QMessageBox>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
this->setTrayIconActions();
this->showTrayIcon();
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::showTrayIcon() {
trayIcon = new QSystemTrayIcon(this);
QIcon trayImage(":/images/trayIcon.png");
trayIcon->setIcon(trayImage);
trayIcon->setContextMenu(trayIconMenu);
connect(trayIcon,
SLOT(activated(QSystemTrayIcon::ActivationReason)),
this,
SLOT(trayIconActivated(QSystemTrayIcon::ActivationReason)));
trayIcon->show();
}
void MainWindow::trayActionExecute() {
QMessageBox::information(this, "TrayIcon", "Info text");
}
void MainWindow::trayIconActivated(QSystemTrayIcon::ActivationReason
reason) {
switch (reason) {
case QSystemTrayIcon::Trigger:
case QSystemTrayIcon::DoubleClick:
this->trayActionExecute();
break;
default:
break;
}
}
void MainWindow::showMsg() {
QMessageBox::information(this, "Tray message", "Hello world!");
}
void MainWindow::setTrayIconActions() {
launchAction = new QAction("Launch", this);
quitAction = new QAction("Exit", this);
connect (launchAction, SIGNAL(triggered()), this, SLOT(showMsg()));
connect (quitAction, SIGNAL(triggered()), qApp, SLOT(quit()));
// Setting system tray's icon menu
trayIconMenu = new QMenu(this);
trayIconMenu -> addAction (launchAction);
trayIconMenu -> addAction (quitAction);
}
void MainWindow::changeEvent(QEvent *event) {
QMainWindow::changeEvent(event);
if (event->type() == QEvent::WindowStateChange) {
if (isMinimized()) {
this->hide();
}
}
}
When I perform any action in the context menu, the application simply closes.
I thought that the point was to override the method QCloseEvent and that would fix the situation. So, i reimplent it:
void MainWindow::closeEvent(QCloseEvent *event) {
#ifdef Q_OS_OSX
if (!event->spontaneous() || !isVisible()) {
return;
}
#endif
if (trayIcon->isVisible()) {
QMessageBox::information(this, tr("Systray"),
tr("The program will keep running in the "
"system tray"));
hide();
event->ignore();
}
}
What can be wrong?
Thanks.

Answer 1

The issue can be reproduced by minimizing the main window, which hides it as per your changeEvent() implementation, and subsequently clicking "Launch" in the system tray icon menu. The message box is displayed, and afterwards the application simply closes.
This is caused by a "feature" of QT called QuitOnLastWindowClosed which is enabled by default. If it is enabled, the application quits when the last non-hidden window is closed - in your example, this is the message box spawned by the "Launch" command (which counts as a window)!
You can solve the issue by calling QApplication::setQuitOnLastWindowClosed(false); at some point after creating the QApplication, e.g. in the main.cpp which likely sets up your application:
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
a.setQuitOnLastWindowClosed(false);
MainWindow w;
w.show();
return a.exec();
}
On a side note, your connect() syntax in showTrayIcon() is wrong: It should spell SIGNAL(activated(QSystemTrayIcon::ActivationReason)) instead of SLOT(activated(QSystemTrayIcon::ActivationReason)).
But I would highly recommend to use the new signal/slot syntax (https://wiki.qt.io/New_Signal_Slot_Syntax) instead:
connect(trayIcon, &QSystemTrayIcon::activated, this, &MainWindow::trayIconActivated);
This has the advantage of showing you any mistakes (like unknown signals/slots or incompatible parameters) at compile-time instead of runtime.