source: trunk/doc/src/examples/screenshot.qdoc@ 651

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/*!
    \example desktop/screenshot
    \title Screenshot Example

    The Screenshot example shows how to take a screenshot of the
    desktop using QApplication and QDesktopWidget. It also shows how
    to use QTimer to provide a single-shot timer, and how to
    reimplement the QWidget::resizeEvent() event handler to make sure
    that an application resizes smoothly and without data loss.

    \image screenshot-example.png

    With the application the users can take a screenshot of their
    desktop. They are provided with a couple of options:

    \list
    \o Delaying the screenshot, giving them time to rearrange
       their desktop.
    \o Hiding the application's window while the screenshot is taken.
    \endlist

    In addition the application allows the users to save their
    screenshot if they want to.

    \section1 Screenshot Class Definition

    \snippet examples/desktop/screenshot/screenshot.h 0

    The \c Screenshot class inherits QWidget and is the application's
    main widget. It displays the application options and a preview of
    the screenshot.

    We reimplement the QWidget::resizeEvent() function to make sure
    that the preview of the screenshot scales properly when the user
    resizes the application widget. We also need several private slots
    to facilitate the options:

    \list
    \o The \c newScreenshot() slot prepares a new screenshot.
    \o The \c saveScreenshot() slot saves the last screenshot.
    \o The \c shootScreen() slot takes the screenshot.
    \o The \c updateCheckBox() slot enables or disables the
       \gui {Hide This Window} option.
    \endlist

    We also declare some private functions: We use the \c
    createOptionsGroupBox(), \c createButtonsLayout() and \c
    createButton() functions when we construct the widget. And we call
    the private \c updateScreenshotLabel() function whenever a new
    screenshot is taken or when a resize event changes the size of the
    screenshot preview label.

    In addition we need to store the screenshot's original pixmap. The
    reason is that when we display the preview of the screenshot, we
    need to scale its pixmap, storing the original we make sure that
    no data are lost in that process.

    \section1 Screenshot Class Implementation

    \snippet examples/desktop/screenshot/screenshot.cpp 0

    In the constructor we first create the QLabel displaying the
    screenshot preview.

    We set the QLabel's size policy to be QSizePolicy::Expanding both
    horizontally and vertically. This means that the QLabel's size
    hint is a sensible size, but the widget can be shrunk and still be
    useful. Also, the widget can make use of extra space, so it should
    get as much space as possible. Then we make sure the QLabel is
    aligned in the center of the \c Screenshot widget, and set its
    minimum size.

    We create the applications's buttons and the group box containing
    the application's options, and put it all into a main
    layout. Finally we take the initial screenshot, and set the inital
    delay and the window title, before we resize the widget to a
    suitable size.

    \snippet examples/desktop/screenshot/screenshot.cpp 1

    The \c resizeEvent() function is reimplemented to receive the
    resize events dispatched to the widget. The purpose is to scale
    the preview screenshot pixmap without deformation of its content,
    and also make sure that the application can be resized smoothly.

    To achieve the first goal, we scale the screenshot pixmap using
    Qt::KeepAspectRatio. We scale the pixmap to a rectangle as large
    as possible inside the current size of the screenshot preview
    label, preserving the aspect ratio. This means that if the user
    resizes the application window in only one direction, the preview
    screenshot keeps the same size.

    To reach our second goal, we make sure that the preview screenshot
    only is repainted (using the private \c updateScreenshotLabel()
    function) when it actually changes its size.

    \snippet examples/desktop/screenshot/screenshot.cpp 2

    The private \c newScreenshot() slot is called when the user
    requests a new screenshot; but the slot only prepares a new
    screenshot.

    First we see if the \gui {Hide This Window} option is checked, if
    it is we hide the \c Screenshot widget. Then we disable the \gui
    {New Screenshot} button, to make sure the user only can request
    one screenshot at a time.

    We create a timer using the QTimer class which provides repetitive
    and single-shot timers. We set the timer to time out only once,
    using the static QTimer::singleShot() function. This function
    calls the private \c shootScreen() slot after the time interval
    specified by the \gui {Screenshot Delay} option. It is \c
    shootScreen() that actually performs the screenshot.

    \snippet examples/desktop/screenshot/screenshot.cpp 3

    The \c saveScreenshot() slot is called when the user push the \gui
    Save button, and it presents a file dialog using the QFileDialog
    class.

    QFileDialog enables a user to traverse the file system in order to
    select one or many files or a directory. The easiest way to create
    a QFileDialog is to use the convenience static
    functions.

    We define the default file format to be png, and we make the file
    dialog's initial path the path the application is run from. We
    create the file dialog using the static
    QFileDialog::getSaveFileName() function which returns a file name
    selected by the user. The file does not have to exist. If the file
    name is valid, we use the QPixmap::save() function to save the
    screenshot's original pixmap in that file.

    \snippet examples/desktop/screenshot/screenshot.cpp 4

    The \c shootScreen() slot is called to take the screenshot. If the
    user has chosen to delay the screenshot, we make the application
    beep when the screenshot is taken using the static
    QApplication::beep() function.

    The QApplication class manages the GUI application's control flow
    and main settings. It contains the main event loop, where all
    events from the window system and other sources are processed and
    dispatched.

    \snippet examples/desktop/screenshot/screenshot.cpp 5

    We take the screenshot using the static QPixmap::grabWindow()
    function. The function grabs the contents of the window passed as
    an argument, makes a pixmap out of it and returns that pixmap.

    We identify the argument window using the QWidget::winID()
    function which returns the window system identifier. Here it
    returns the identifier of the current QDesktopWidget retrieved by
    the QApplication::desktop() function. The QDesktopWidget class
    provides access to screen information, and inherits
    QWidget::winID().

    We update the screenshot preview label using the private \c
    updateScreenshotLabel() function. Then we enable the \gui {New
    Screenshot} button, and finally we make the \c Screenshot widget
    visible if it was hidden during the screenshot.

    \snippet examples/desktop/screenshot/screenshot.cpp 6

    The \gui {Hide This Window} option is enabled or disabled
    depending on the delay of the screenshot. If there is no delay,
    the application window cannot be hidden and the option's checkbox
    is disabled.

    The \c updateCheckBox() slot is called whenever the user changes
    the delay using the \gui {Screenshot Delay} option.

    \snippet examples/desktop/screenshot/screenshot.cpp 7

    The private \c createOptionsGroupBox() function is called from the
    constructor.

    First we create a group box that will contain all of the options'
    widgets. Then we create a QSpinBox and a QLabel for the \gui
    {Screenshot Delay} option, and connect the spinbox to the \c
    updateCheckBox() slot. Finally, we create a QCheckBox for the \gui
    {Hide This Window} option, add all the options' widgets to a
    QGridLayout and install the layout on the group box.

    Note that we don't have to specify any parents for the widgets
    when we create them. The reason is that when we add a widget to a
    layout and install the layout on another widget, the layout's
    widgets are automatically reparented to the widget the layout is
    installed on.

    \snippet examples/desktop/screenshot/screenshot.cpp 8

    The private \c createButtonsLayout() function is called from the
    constructor. We create the application's buttons using the private
    \c createButton() function, and add them to a QHBoxLayout.

    \snippet examples/desktop/screenshot/screenshot.cpp 9

    The private \c createButton() function is called from the \c
    createButtonsLayout() function. It simply creates a QPushButton
    with the provided text, connects it to the provided receiver and
    slot, and returns a pointer to the button.

    \snippet examples/desktop/screenshot/screenshot.cpp 10

    The private \c updateScreenshotLabel() function is called whenever
    the screenshot changes, or when a resize event changes the size of
    the screenshot preview label. It updates the screenshot preview's
    label using the QLabel::setPixmap() and QPixmap::scaled()
    functions.

    QPixmap::scaled() returns a copy of the given pixmap scaled to a
    rectangle of the given size according to the given
    Qt::AspectRatioMode and Qt::TransformationMode.

    We scale the original pixmap to fit the current screenshot label's
    size, preserving the aspect ratio and giving the resulting pixmap
    smoothed edges.
*/