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/*!
    \example desktop/systray
    \title System Tray Icon Example


    The System Tray Icon example shows how to add an icon with a menu
    and popup messages to a desktop environment's system tray.

    \image systemtray-example.png Screenshot of the System Tray Icon.

    Modern operating systems usually provide a special area on the
    desktop, called the system tray or notification area, where
    long-running applications can display icons and short messages.

    This example consists of one single class, \c Window, providing
    the main application window (i.e., an editor for the system tray
    icon) and the associated icon.

    \image systemtray-editor.png

    The editor allows the user to choose the preferred icon as well as
    set the balloon message's type and duration. The user can also
    edit the message's title and body. Finally, the editor provide a
    checkbox controlling whether the icon is actually shown in the
    system tray, or not.

    \section1 Window Class Definition

    The \c Window class inherits QWidget:

    \snippet examples/desktop/systray/window.h 0

    We implement several private slots to respond to user
    interaction. The other private functions are only convenience
    functions provided to simplify the constructor.

    The tray icon is an instance of the QSystemTrayIcon class. To
    check whether a system tray is present on the user's desktop, call
    the static QSystemTrayIcon::isSystemTrayAvailable()
    function. Associated with the icon, we provide a menu containing
    the typical \gui minimize, \gui maximize, \gui restore and \gui
    quit actions. We reimplement the QWidget::setVisible() function to
    update the tray icon's menu whenever the editor's appearance
    changes, e.g., when maximizing or minimizing the main application
    window.

    Finally, we reimplement QWidget's \l {QWidget::}{closeEvent()}
    function to be able to inform the user (when closing the editor
    window) that the program will keep running in the system tray
    until the user chooses the \gui Quit entry in the icon's context
    menu.

    \section1 Window Class Implementation

    When constructing the editor widget, we first create the various
    editor elements before we create the actual system tray icon:

    \snippet examples/desktop/systray/window.cpp 0

    We ensure that the application responds to user input by
    connecting most of the editor's input widgets (including the
    system tray icon) to the application's private slots. But note the
    visibility checkbox; its \l {QCheckBox::}{toggled()} signal is
    connected to the \e {icon}'s \l {QSystemTrayIcon::}{setVisible()}
    function instead.

    \snippet examples/desktop/systray/window.cpp 3

    The \c setIcon() slot is triggered whenever the current index in
    the icon combobox changes, i.e., whenever the user chooses another
    icon in the editor. Note that it is also called when the user
    activates the tray icon with the left mouse button, triggering the
    icon's \l {QSystemTrayIcon::}{activated()} signal. We will come
    back to this signal shortly.

    The QSystemTrayIcon::setIcon() function sets the \l
    {QSystemTrayIcon::}{icon} property that holds the actual system
    tray icon. On Windows, the system tray icon size is 16x16; on X11,
    the preferred size is 22x22. The icon will be scaled to the
    appropriate size as necessary.

    Note that on X11, due to a limitation in the system tray
    specification, mouse clicks on transparent areas in the icon are
    propagated to the system tray. If this behavior is unacceptable,
    we suggest using an icon with no transparency.

    \snippet examples/desktop/systray/window.cpp 4

    Whenever the user activates the system tray icon, it emits its \l
    {QSystemTrayIcon::}{activated()} signal passing the triggering
    reason as parameter. QSystemTrayIcon provides the \l
    {QSystemTrayIcon::}{ActivationReason} enum to describe how the
    icon was activated.

    In the constructor, we connected our icon's \l
    {QSystemTrayIcon::}{activated()} signal to our custom \c
    iconActivated() slot: If the user has clicked the icon using the
    left mouse button, this function changes the icon image by
    incrementing the icon combobox's current index, triggering the \c
    setIcon() slot as mentioned above. If the user activates the icon
    using the middle mouse button, it calls the custom \c
    showMessage() slot:

    \snippet examples/desktop/systray/window.cpp 5

    When the \e showMessage() slot is triggered, we first retrieve the
    message icon depending on the currently chosen message type. The
    QSystemTrayIcon::MessageIcon enum describes the icon that is shown
    when a balloon message is displayed. Then we call
    QSystemTrayIcon's \l {QSystemTrayIcon::}{showMessage()} function
    to show the message with the title, body, and icon for the time
    specified in milliseconds.
    
    Mac OS X users note: The Growl notification system must be
    installed for QSystemTrayIcon::showMessage() to display messages.

    QSystemTrayIcon also has the corresponding, \l {QSystemTrayIcon::}
    {messageClicked()} signal, which is emitted when the user clicks a
    message displayed by \l {QSystemTrayIcon::}{showMessage()}.

    \snippet examples/desktop/systray/window.cpp 6

    In the constructor, we connected the \l
    {QSystemTrayIcon::}{messageClicked()} signal to our custom \c
    messageClicked() slot that simply displays a message using the
    QMessageBox class.

    QMessageBox provides a modal dialog with a short message, an icon,
    and buttons laid out depending on the current style. It supports
    four severity levels: "Question", "Information", "Warning" and
    "Critical". The easiest way to pop up a message box in Qt is to
    call one of the associated static functions, e.g.,
    QMessageBox::information().

    As we mentioned earlier, we reimplement a couple of QWidget's
    virtual functions:

    \snippet examples/desktop/systray/window.cpp 1

    Our reimplementation of the QWidget::setVisible() function updates
    the tray icon's menu whenever the editor's appearance changes,
    e.g., when maximizing or minimizing the main application window,
    before calling the base class implementation.

    \snippet examples/desktop/systray/window.cpp 2

    We have reimplemented the QWidget::closeEvent() event handler to
    receive widget close events, showing the above message to the
    users when they are closing the editor window.

    In addition to the functions and slots discussed above, we have
    also implemented several convenience functions to simplify the
    constructor: \c createIconGroupBox(), \c createMessageGroupBox(),
    \c createActions() and \c createTrayIcon(). See the \l
    {desktop/systray/window.cpp}{window.cpp} file for details.
*/