Detecting when a Qt application has been switched to the background and when resumed
This code snippet shows how to create an event filter in Qt C++ to detect when an app has been switched to the background and when it is resumed. Note that for apps using Qt Quick Components for Symbian, the Symbian Element foreground property can be used instead.
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Introduction
In many Qt supported platforms it is possible to switch the application to the background and resume to it when needed. For example in Harmattan the applications can be swiped to the background, and in Symbian^3 the switching can be done via the menu button. It is common that applications need to run some operations when these kinds of deactivation and activation events happen. For example games usually need to be paused when they are switched to the background. Also in other kinds of applications the unnecessary operations can be paused to free processing and memory resources.
Summary
In Qt this can be handled by listening the ApplicationDeactivate and ApplicationActivate events and reacting to them accordingly. The QObject class has an eventFilter method, which can be overridden in some class derived from QObject. Those earlier mentioned events can be listened in this reimplemented method. The class that reimplements the eventFilter method is then installed as an event filter for the QApplication object.
This methodology is used in Qt GameEnabler.
Sources
eventfilter.cpp
EventFilter derived from QObject
bool EventFilter::eventFilter(QObject *obj, QEvent *event)
{
if (event->type() == QEvent::ApplicationDeactivate) {
// The application deactivation can be handled here
return true; // The event is handled
}
if (event->type() == QEvent::ApplicationActivate) {
// The application activation can be handled here
return true;
}
return QObject::eventFilter(obj, event); // Unhandled events are passed to the base class
}
main.cpp
#include "eventfilter.h"
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
EventFilter eventFilter;
app.installEventFilter(&eventFilter); // Installing the event filter
...
}
Postconditions
This code snippet demonstrated how to detect when a Qt application has been switched to the background and when it is resumed.
