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/*!

    \example widgets/imageviewer

    \title Image Viewer Example

    The example shows how to combine QLabel and QScrollArea to

    display an image. QLabel is typically used for displaying 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.

    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}).

*/