Usually a user wants an application to be run as a single instance. This means that if the launched application is already running, the existing application window is activated and no new application processes are started.

Note: This functionality is currently included in the Qt Quick application template provided by the Qt SDK. To enable single instance support for your application, select Make application boostable checkbox when creating a new project with the template.

When not using the template provided by the Qt SDK, this can be achieved by adding --single-instance to the invoker command in the .desktop file:

[Desktop Entry]
Type=Application
Name=My Application
Exec=/usr/bin/invoker --single-instance --type=e /usr/bin/myApp

As a result, a lock file /var/run/single-instance-locks/usr/bin/myApp/instance.lock is created. If applauncherd cannot acquire the lock, it tries to find the corresponding window and activates it.

Using single instance support requires that the shown window belongs to the invoked application binary. For example, if the invoked application starts a new application as a plug-in and the plug-in shows the window, you must manually set the plug-in window XProperty WM_COMMAND. To use single instance support, set the property to correspond with the application name used in the single instance lock file.

Consider using --single-instance instead of the single instance functionality provided by D-Bus, because --single-instance is much faster in most cases.