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If the child widget exceeds the size of the frame, QScrollArea automatically provides scroll bars. 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. In addition the example shows how to use QPainter to print an image. \image imageviewer-example.png Screenshot of the Image Viewer example With the Image Viewer application, the users can view an image of their choice. The \gui File menu gives the user the possibility to: \list \o \gui{Open...} - Open an image file \o \gui{Print...} - Print an image \o \gui{Exit} - Exit the application \endlist Once an image is loaded, the \gui View menu allows the users to: \list \o \gui{Zoom In} - Scale the image up by 25% \o \gui{Zoom Out} - Scale the image down by 25% \o \gui{Normal Size} - Show the image at its original size \o \gui{Fit to Window} - Stretch the image to occupy the entire window \endlist In addition the \gui Help menu provides the users with information about the Image Viewer example in particular, and about Qt in general. \section1 ImageViewer Class Definition \snippet examples/widgets/imageviewer/imageviewer.h 0 The \c ImageViewer class inherits from QMainWindow. We reimplement the constructor, and create several private slots to facilitate the menu entries. In addition we create four private functions. We use \c createActions() and \c createMenus() when constructing the \c ImageViewer widget. We use the \c updateActions() function to update the menu entries when a new image is loaded, or when the \gui {Fit to Window} option is toggled. The zoom slots use \c scaleImage() to perform the zooming. In turn, \c scaleImage() uses \c adjustScrollBar() to preserve the focal point after scaling an image. \section1 ImageViewer Class Implementation \snippet examples/widgets/imageviewer/imageviewer.cpp 0 In the constructor we first create the label and the scroll area. We set \c {imageLabel}'s size policy to \l {QSizePolicy::Ignored}{ignored}, making the users able to scale the image to whatever size they want when the \gui {Fit to Window} option is turned on. Otherwise, the default size polizy (\l {QSizePolicy::Preferred}{preferred}) will make scroll bars appear when the scroll area becomes smaller than the label's minimum size hint. We ensure that the label will scale its contents to fill all available space, to enable the image to scale properly when zooming. If we omitted to set the \c {imageLabel}'s \l {QLabel::scaledContents}{scaledContents} property, zooming in would enlarge the QLabel, but leave the pixmap at its original size, exposing the QLabel's background. We make \c imageLabel the scroll area's child widget, and we make \c scrollArea the central widget of the QMainWindow. At the end we create the associated actions and menus, and customize the \c {ImageViewer}'s appearance. \snippet examples/widgets/imageviewer/imageviewer.cpp 1 \snippet examples/widgets/imageviewer/imageviewer.cpp 2 In the \c open() slot, we show a file dialog to the user. The easiest way to create a QFileDialog is to use the static convenience functions. QFileDialog::getOpenFileName() returns an existing file selected by the user. If the user presses \gui Cancel, QFileDialog returns an empty string. Unless the file name is a empty string, we check if the file's format is an image format by constructing a QImage which tries to load the image from the file. If the constructor returns a null image, we use a QMessageBox to alert the user. The QMessageBox class provides a modal dialog with a short message, an icon, and some buttons. As with QFileDialog the easiest way to create a QMessageBox is to use its static convenience functions. QMessageBox provides a range of different messages arranged along two axes: severity (question, information, warning and critical) and complexity (the number of necessary response buttons). In this particular example an information message with an \gui OK button (the default) is sufficient, since the message is part of a normal operation. \snippet examples/widgets/imageviewer/imageviewer.cpp 3 \snippet examples/widgets/imageviewer/imageviewer.cpp 4 If the format is supported, we display the image in \c imageLabel by setting the label's \l {QLabel::pixmap}{pixmap}. Then we enable the \gui Print and \gui {Fit to Window} menu entries and update the rest of the view menu entries. The \gui Open and \gui Exit entries are enabled by default. If the \gui {Fit to Window} option is turned off, the QScrollArea::widgetResizable property is \c false and it is our responsibility (not QScrollArea's) to give the QLabel a reasonable size based on its contents. We call \{QWidget::adjustSize()}{adjustSize()} to achieve this, which is essentially the same as \snippet doc/src/snippets/code/doc_src_examples_imageviewer.qdoc 0 In the \c print() slot, we first make sure that an image has been loaded into the application: \snippet examples/widgets/imageviewer/imageviewer.cpp 5 \snippet examples/widgets/imageviewer/imageviewer.cpp 6 If the application is built in debug mode, the \c Q_ASSERT() macro will expand to \snippet doc/src/snippets/code/doc_src_examples_imageviewer.qdoc 1 In release mode, the macro simply disappear. The mode can be set in the application's \c .pro file. One way to do so is to add an option to \gui qmake when building the appliction: \snippet doc/src/snippets/code/doc_src_examples_imageviewer.qdoc 2 or \snippet doc/src/snippets/code/doc_src_examples_imageviewer.qdoc 3 Another approach is to add this line directly to the \c .pro file. \snippet examples/widgets/imageviewer/imageviewer.cpp 7 \snippet examples/widgets/imageviewer/imageviewer.cpp 8 Then we present a print dialog allowing the user to choose a printer and to set a few options. We construct a painter with a QPrinter as the paint device. We set the painter's window and viewport in such a way that the image is as large as possible on the paper, but without altering its \l{Qt::KeepAspectRatio}{aspect ratio}. In the end we draw the pixmap at position (0, 0). \snippet examples/widgets/imageviewer/imageviewer.cpp 9 \snippet examples/widgets/imageviewer/imageviewer.cpp 10 We implement the zooming slots using the private \c scaleImage() function. We set the scaling factors to 1.25 and 0.8, respectively. These factor values ensure that a \gui {Zoom In} action and a \gui {Zoom Out} action will cancel each other (since 1.25 * 0.8 == 1), and in that way the normal image size can be restored using the zooming features. The screenshots below show an image in its normal size, and the same image after zooming in: \table \row \o \inlineimage imageviewer-original_size.png \o \inlineimage imageviewer-zoom_in_1.png \o \inlineimage imageviewer-zoom_in_2.png \endtable \snippet examples/widgets/imageviewer/imageviewer.cpp 11 \snippet examples/widgets/imageviewer/imageviewer.cpp 12 When zooming, we use the QLabel's ability to scale its contents. Such scaling doesn't change the actual size hint of the contents. And since the \l {QLabel::adjustSize()}{adjustSize()} function use those size hint, the only thing we need to do to restore the normal size of the currently displayed image is to call \c adjustSize() and reset the scale factor to 1.0. \snippet examples/widgets/imageviewer/imageviewer.cpp 13 \snippet examples/widgets/imageviewer/imageviewer.cpp 14 The \c fitToWindow() slot is called each time the user toggled the \gui {Fit to Window} option. If the slot is called to turn on the option, we tell the scroll area to resize its child widget with the QScrollArea::setWidgetResizable() function. Then we disable the \gui {Zoom In}, \gui {Zoom Out} and \gui {Normal Size} menu entries using the private \c updateActions() function. If the \l {QScrollArea::widgetResizable} property is set to \c false (the default), the scroll area honors the size of its child widget. If this property is set to \c true, the scroll area will automatically resize the widget in order to avoid scroll bars where they can be avoided, or to take advantage of extra space. But the scroll area will honor the minimum size hint of its child widget independent of the widget resizable property. So in this example we set \c {imageLabel}'s size policy to \l {QSizePolicy::Ignored}{ignored} in the constructor, to avoid that scroll bars appear when the scroll area becomes smaller than the label's minimum size hint. The screenshots below shows an image in its normal size, and the same image with the \gui {Fit to window} option turned on. Enlarging the window will stretch the image further, as shown in the third screenshot. \table \row \o \inlineimage imageviewer-original_size.png \o \inlineimage imageviewer-fit_to_window_1.png \o \inlineimage imageviewer-fit_to_window_2.png \endtable If the slot is called to turn off the option, the {QScrollArea::setWidgetResizable} property is set to \c false. We also restore the image pixmap to its normal size by adjusting the label's size to its content. And in the end we update the view menu entries. \snippet examples/widgets/imageviewer/imageviewer.cpp 15 \snippet examples/widgets/imageviewer/imageviewer.cpp 16 We implement the \c about() slot to create a message box describing what the example is designed to show. \snippet examples/widgets/imageviewer/imageviewer.cpp 17 \snippet examples/widgets/imageviewer/imageviewer.cpp 18 In the private \c createAction() function, we create the actions providing the application features. We assign a short-cut key to each action and connect them to the appropiate slots. We only enable the \c openAct and \c exitAxt at the time of creation, the others are updated once an image has been loaded into the application. In addition we make the \c fitToWindowAct \l {QAction::checkable}{checkable}. \snippet examples/widgets/imageviewer/imageviewer.cpp 19 \snippet examples/widgets/imageviewer/imageviewer.cpp 20 In the private \c createMenu() function, we add the previously created actions to the \gui File, \gui View and \gui Help menus. The QMenu class provides a menu widget for use in menu bars, context menus, and other popup menus. The QMenuBar class provides a horizontal menu bar that consists of a list of pull-down menu items. So at the end we put the menus in the \c {ImageViewer}'s menu bar which we retrieve with the QMainWindow::menuBar() function. \snippet examples/widgets/imageviewer/imageviewer.cpp 21 \snippet examples/widgets/imageviewer/imageviewer.cpp 22 The private \c updateActions() function enables or disables the \gui {Zoom In}, \gui {Zoom Out} and \gui {Normal Size} menu entries depending on whether the \gui {Fit to Window} option is turned on or off. \snippet examples/widgets/imageviewer/imageviewer.cpp 23 \snippet examples/widgets/imageviewer/imageviewer.cpp 24 In \c scaleImage(), we use the \c factor parameter to calculate the new scaling factor for the displayed image, and resize \c imageLabel. Since we set the \l{QLabel::scaledContents}{scaledContents} property to \c true in the constructor, the call to QWidget::resize() will scale the image displayed in the label. We also adjust the scroll bars to preserve the focal point of the image. At the end, if the scale factor is less than 33.3% or greater than 300%, we disable the respective menu entry to prevent the image pixmap from becoming too large, consuming too much resources in the window system. \snippet examples/widgets/imageviewer/imageviewer.cpp 25 \snippet examples/widgets/imageviewer/imageviewer.cpp 26 Whenever we zoom in or out, we need to adjust the scroll bars in consequence. It would have been tempting to simply call \snippet doc/src/snippets/code/doc_src_examples_imageviewer.qdoc 4 but this would make the top-left corner the focal point, not the center. Therefore we need to take into account the scroll bar handle's size (the \l{QScrollBar::pageStep}{page step}). */