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# Loading Documents

The application in this example loads or prints the documents passed as commandline parameters to further instances of this application.

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     #include <qtsingleapplication.h>
     #include <QtCore/QFile>
     #include <QtGui/QMainWindow>
     #include <QtGui/QPrinter>
     #include <QtGui/QPainter>
     #include <QtGui/QTextEdit>
     #include <QtGui/QMdiArea>
     #include <QtCore/QTextStream>

     class MainWindow : public QMainWindow
     {
         Q_OBJECT
     public:
         MainWindow();

     public slots:
         void handleMessage(const QString& message);

     signals:
         void needToShow();

     private:
         QMdiArea *workspace;
     };

The user interface in this application is a QMainWindow subclass with a QMdiArea as the central widget. It implements a slot `handleMessage()` that will be connected to the messageReceived() signal of the QtSingleApplication class.

     MainWindow::MainWindow()
     {
         workspace = new QMdiArea(this);

         setCentralWidget(workspace);
     }

The MainWindow constructor creates a minimal user interface.

     void MainWindow::handleMessage(const QString& message)
     {
         enum Action {
             Nothing,
             Open,
             Print
         } action;

         action = Nothing;
         QString filename = message;
         if (message.toLower().startsWith("/print ")) {
             filename = filename.mid(7);
             action = Print;
         } else if (!message.isEmpty()) {
             action = Open;
         }
         if (action == Nothing) {
             emit needToShow();
             return;
         }

         QFile file(filename);
         QString contents;
         if (file.open(QIODevice::ReadOnly))
             contents = file.readAll();
         else
             contents = "[[Error: Could not load file " + filename + "]]";

         QTextEdit *view = new QTextEdit;
         view->setPlainText(contents);

         switch(action) {

The handleMessage() slot interprets the message passed in as a filename that can be prepended with /print to indicate that the file should just be printed rather than loaded.

         case Print:
             {
                 QPrinter printer;
                 view->print(&printer);
                 delete view;
             }
             break;

         case Open:
             {
                 workspace->addSubWindow(view);
                 view->setWindowTitle(message);
                 view->show();
                 emit needToShow();
             }
             break;
         default:
             break;
         };
     }

Loading the file will also activate the window.

     #include "main.moc"

     int main(int argc, char **argv)
     {
         QtSingleApplication instance("File loader QtSingleApplication example", argc, argv);
         QString message;
         for (int a = 1; a < argc; ++a) {
             message += argv[a];
             if (a < argc-1)
                 message += " ";
         }

         if (instance.sendMessage(message))
             return 0;

The `main` entry point function creates a QtSingleApplication object, and creates a message to send to a running instance of the application. If the message was sent successfully the process exits immediately.

         MainWindow mw;
         mw.handleMessage(message);
         mw.show();

         QObject::connect(&instance, SIGNAL(messageReceived(const QString&)),
                          &mw, SLOT(handleMessage(const QString&)));

         instance.setActivationWindow(&mw, false);
         QObject::connect(&mw, SIGNAL(needToShow()), &instance, SLOT(activateWindow()));

         return instance.exec();
     }

If the message could not be sent the application starts up. Note that `false` is passed to the call to setActivationWindow() to prevent automatic activation for every message received, e.g. when the application should just print a file. Instead, the message handling function determines whether activation is requested, and signals that by emitting the needToShow() signal. This is then simply connected directly to QtSingleApplication's activateWindow() slot.