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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.