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imageviewer.cpp Example File
widgets/imageviewer/imageviewer.cpp

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 #include <QtGui>

 #include "imageviewer.h"

 ImageViewer::ImageViewer()
 {
     imageLabel = new QLabel;
     imageLabel->setBackgroundRole(QPalette::Base);
     imageLabel->setSizePolicy(QSizePolicy::Ignored, QSizePolicy::Ignored);
     imageLabel->setScaledContents(true);

     scrollArea = new QScrollArea;
     scrollArea->setBackgroundRole(QPalette::Dark);
     scrollArea->setWidget(imageLabel);
     setCentralWidget(scrollArea);

     createActions();
     createMenus();

     setWindowTitle(tr("Image Viewer"));
     resize(500, 400);
 }

 void ImageViewer::open()
 {
     QString fileName = QFileDialog::getOpenFileName(this,
                                     tr("Open File"), QDir::currentPath());
     if (!fileName.isEmpty()) {
         QImage image(fileName);
         if (image.isNull()) {
             QMessageBox::information(this, tr("Image Viewer"),
                                      tr("Cannot load %1.").arg(fileName));
             return;
         }
         imageLabel->setPixmap(QPixmap::fromImage(image));
         scaleFactor = 1.0;

         printAct->setEnabled(true);
         fitToWindowAct->setEnabled(true);
         updateActions();

         if (!fitToWindowAct->isChecked())
             imageLabel->adjustSize();
     }
 }

 void ImageViewer::print()
 {
     Q_ASSERT(imageLabel->pixmap());
     QPrintDialog dialog(&printer, this);
     if (dialog.exec()) {
         QPainter painter(&printer);
         QRect rect = painter.viewport();
         QSize size = imageLabel->pixmap()->size();
         size.scale(rect.size(), Qt::KeepAspectRatio);
         painter.setViewport(rect.x(), rect.y(), size.width(), size.height());
         painter.setWindow(imageLabel->pixmap()->rect());
         painter.drawPixmap(0, 0, *imageLabel->pixmap());
     }
 }

 void ImageViewer::zoomIn()
 {
     scaleImage(1.25);
 }

 void ImageViewer::zoomOut()
 {
     scaleImage(0.8);
 }

 void ImageViewer::normalSize()
 {
     imageLabel->adjustSize();
     scaleFactor = 1.0;
 }

 void ImageViewer::fitToWindow()
 {
     bool fitToWindow = fitToWindowAct->isChecked();
     scrollArea->setWidgetResizable(fitToWindow);
     if (!fitToWindow) {
         normalSize();
     }
     updateActions();
 }

 void ImageViewer::about()
 {
     QMessageBox::about(this, tr("About Image Viewer"),
             tr("<p>The <b>Image Viewer</b> example shows how to combine QLabel "
                "and QScrollArea to display an image. QLabel is typically used "
                "for displaying a text, but it can also display an image. "
                "QScrollArea provides a scrolling view around another widget. "
                "If the child widget exceeds the size of the frame, QScrollArea "
                "automatically provides scroll bars. </p><p>The example "
                "demonstrates how QLabel's ability to scale its contents "
                "(QLabel::scaledContents), and QScrollArea's ability to "
                "automatically resize its contents "
                "(QScrollArea::widgetResizable), can be used to implement "
                "zooming and scaling features. </p><p>In addition the example "
                "shows how to use QPainter to print an image.</p>"));
 }

 void ImageViewer::createActions()
 {
     openAct = new QAction(tr("&Open..."), this);
     openAct->setShortcut(tr("Ctrl+O"));
     connect(openAct, SIGNAL(triggered()), this, SLOT(open()));

     printAct = new QAction(tr("&Print..."), this);
     printAct->setShortcut(tr("Ctrl+P"));
     printAct->setEnabled(false);
     connect(printAct, SIGNAL(triggered()), this, SLOT(print()));

     exitAct = new QAction(tr("E&xit"), this);
     exitAct->setShortcut(tr("Ctrl+Q"));
     connect(exitAct, SIGNAL(triggered()), this, SLOT(close()));

     zoomInAct = new QAction(tr("Zoom &In (25%)"), this);
     zoomInAct->setShortcut(tr("Ctrl++"));
     zoomInAct->setEnabled(false);
     connect(zoomInAct, SIGNAL(triggered()), this, SLOT(zoomIn()));

     zoomOutAct = new QAction(tr("Zoom &Out (25%)"), this);
     zoomOutAct->setShortcut(tr("Ctrl+-"));
     zoomOutAct->setEnabled(false);
     connect(zoomOutAct, SIGNAL(triggered()), this, SLOT(zoomOut()));

     normalSizeAct = new QAction(tr("&Normal Size"), this);
     normalSizeAct->setShortcut(tr("Ctrl+S"));
     normalSizeAct->setEnabled(false);
     connect(normalSizeAct, SIGNAL(triggered()), this, SLOT(normalSize()));

     fitToWindowAct = new QAction(tr("&Fit to Window"), this);
     fitToWindowAct->setEnabled(false);
     fitToWindowAct->setCheckable(true);
     fitToWindowAct->setShortcut(tr("Ctrl+F"));
     connect(fitToWindowAct, SIGNAL(triggered()), this, SLOT(fitToWindow()));

     aboutAct = new QAction(tr("&About"), this);
     connect(aboutAct, SIGNAL(triggered()), this, SLOT(about()));

     aboutQtAct = new QAction(tr("About &Qt"), this);
     connect(aboutQtAct, SIGNAL(triggered()), qApp, SLOT(aboutQt()));
 }

 void ImageViewer::createMenus()
 {
     fileMenu = new QMenu(tr("&File"), this);
     fileMenu->addAction(openAct);
     fileMenu->addAction(printAct);
     fileMenu->addSeparator();
     fileMenu->addAction(exitAct);

     viewMenu = new QMenu(tr("&View"), this);
     viewMenu->addAction(zoomInAct);
     viewMenu->addAction(zoomOutAct);
     viewMenu->addAction(normalSizeAct);
     viewMenu->addSeparator();
     viewMenu->addAction(fitToWindowAct);

     helpMenu = new QMenu(tr("&Help"), this);
     helpMenu->addAction(aboutAct);
     helpMenu->addAction(aboutQtAct);

     menuBar()->addMenu(fileMenu);
     menuBar()->addMenu(viewMenu);
     menuBar()->addMenu(helpMenu);
 }

 void ImageViewer::updateActions()
 {
     zoomInAct->setEnabled(!fitToWindowAct->isChecked());
     zoomOutAct->setEnabled(!fitToWindowAct->isChecked());
     normalSizeAct->setEnabled(!fitToWindowAct->isChecked());
 }

 void ImageViewer::scaleImage(double factor)
 {
     Q_ASSERT(imageLabel->pixmap());
     scaleFactor *= factor;
     imageLabel->resize(scaleFactor * imageLabel->pixmap()->size());

     adjustScrollBar(scrollArea->horizontalScrollBar(), factor);
     adjustScrollBar(scrollArea->verticalScrollBar(), factor);

     zoomInAct->setEnabled(scaleFactor < 3.0);
     zoomOutAct->setEnabled(scaleFactor > 0.333);
 }

 void ImageViewer::adjustScrollBar(QScrollBar *scrollBar, double factor)
 {
     scrollBar->setValue(int(factor * scrollBar->value()
                             + ((factor - 1) * scrollBar->pageStep()/2)));
 }


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Qt 4.3.5