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# A Trivial Example

The application in this example has a log-view that displays messages sent by further instances of the same application.

The example demonstrates the use of the QtSingleApplication class to detect and communicate with a running instance of the application using the sendMessage() API. The messageReceived() signal is used to display received messages in a QTextEdit log.

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     #include <qtsingleapplication.h>
     #include <QtGui/QTextEdit>

     class TextEdit : public QTextEdit
     {
         Q_OBJECT
     public:
         TextEdit(QWidget *parent = 0)
             : QTextEdit(parent)
         {}
     public slots:
         void append(const QString &str)
         {
             QTextEdit::append(str);
         }
     };

     #include "main.moc"

     int main(int argc, char **argv)
     {
         QtSingleApplication instance(argc, argv);

The example has only the `main` entry point function. A QtSingleApplication object is created immediately.

         if (instance.sendMessage("Wake up!"))
             return 0;

If another instance of this application is already running, sendMessage() will succeed, and this instance just exits immediately.

         TextEdit logview;
         logview.setReadOnly(true);
         logview.show();

Otherwise the instance continues as normal and creates the user interface.

         instance.setActivationWindow(&logview);

         QObject::connect(&instance, SIGNAL(messageReceived(const QString&)),
                          &logview, SLOT(append(const QString&)));

         return instance.exec();

The `logview` object is also set as the application's activation window. Every time a message is received, the window will be raised and activated automatically.

The messageReceived() signal is also connected to the QTextEdit's append() slot. Every message received from further instances of this application will be displayed in the log.

Finally the event loop is entered.