imageviewer.cpp Example File
^{widgets/imageviewer/imageviewer.cpp}

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    ** This file is part of the documentation of the Qt Toolkit.
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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)));
    }