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/*!

    \page qt-embedded-architecture.html

    \title Qt for Embedded Linux Architecture

    \ingroup qt-embedded-linux

    A \l{Qt for Embedded Linux} application requires a server

    application to be running, or to be the server application itself.

    Any \l{Qt for Embedded Linux} application can act as the server.

    When more than one application is running, the subsequent

    applications connect to the existing server application as clients.

    The server and client processes have different responsibilities:

    The server process manages pointer handling, character input, and

    screen output. In addition, the server controls the appearance of

    the screen cursor and the screen saver. The client process

    performs all application specific operations.

    The server application is represented by an instance of the

    QWSServer class, while the client applications are represented by

    instances of the QWSClient class. On each side, there are several

    classes performing the various operations.

    \image qt-embedded-architecture2.png

    All system generated events, including keyboard and mouse events,

    are passed to the server application which then propagates the

    event to the appropriate client.

    When rendering, the default behavior is for each client to render

    its widgets into memory while the server is responsible for

    putting the contents of the memory onto the screen. But when the

    hardware is known and well defined, as is often the case with

    software for embedded devices, it may be useful for the clients to

    manipulate and control the underlying hardware directly.

    \l{Qt for Embedded Linux} provides two different approaches to

    achieve this behavior, see the graphics rendering section below for

    details.

    \tableofcontents

    \section1 Client/Server Communication

    The running applications continuously alter the appearance of the

    screen by adding and removing widgets. The server maintains

    information about each top-level window in a corresponding

    QWSWindow object.

    Whenever the server receives an event, it queries its stack of

    top-level windows to find the window containing the event's

    position. Each window can identify the client application that

    created it, and returns its ID to the server upon

    request. Finally, the server forwards the event, encapsulated by

    an instance of the QWSEvent class, to the appropriate client.

    \image qt-embedded-clientservercommunication.png

    If an input method is installed, it is used as a filter between

    the server and the client application. Derive from the

    QWSInputMethod class to implement custom input methods, and use

    the server's \l {QWSServer::}{setCurrentInputMethod()} function to

    install it. In addition, it is possible to implement global,

    low-level filters on key events using the

    QWSServer::KeyboardFilter class; this can be used to implement

    things like advanced power management suspended from a button

    without having to filter for it in all applications.

    \table 100%

    \header \o UNIX Domain Socket

    \row

    \o

    \image qt-embedded-client.png

    The server communicates with the client applications over the UNIX

    domain socket. You can retrieve direct access to all the events a

    client receives from the server, by reimplementing QApplication's

    \l {QApplication::}{qwsEventFilter()} function.

    \endtable

    The clients (and the server) communicate with each other using the

    QCopChannel class. QCOP is a many-to-many communication protocol

    for transferring messages on various channels. A channel is

    identified by a name, and anyone who wants to can listen to

    it. The QCOP protocol allows clients to communicate both within

    the same address space and between different processes.

    \section1 Pointer Handling Layer

    \list

        \o QWSMouseHandler

        \o QMouseDriverPlugin

        \o QMouseDriverFactory

    \endlist

    The mouse driver (represented by an instance of the

    QWSMouseHandler class) is loaded by the server application when it

    starts running, using Qt's \l {How to Create Qt Plugins}{plugin

    system}.

    \image qt-embedded-pointerhandlinglayer.png

    A mouse driver receives mouse events from the device and

    encapsulates each event with an instance of the QWSEvent class

    which it then passes to the server.

    \l{Qt for Embedded Linux} provides ready-made drivers for several mouse

    protocols, see the \l{Qt for Embedded Linux Pointer Handling}{pointer

    handling} documentation for details. Custom mouse drivers can be

    implemented by subclassing the QWSMouseHandler class and creating

    a mouse driver plugin. The default implementation of the

    QMouseDriverFactory class will automatically detect the plugin,

    loading the driver into the server application at runtime.

    In addition to the generic mouse handler, \l{Qt for Embedded Linux}

    provides a calibrated mouse handler. Use the

    QWSCalibratedMouseHandler class as the base class when the system

    device does not have a fixed mapping between device and screen

    coordinates and/or produces noisy events, e.g. a touchscreen.

    See also: \l{Qt for Embedded Linux Pointer Handling} and

    \l{How to Create Qt Plugins}.

    \section1 Character Input Layer

    \list

        \o QWSKeyboardHandler

        \o QKbdDriverPlugin

        \o QKbdDriverFactory

    \endlist

    The keyboard driver (represented by an instance of the

    QWSKeyboardHandler class) is loaded by the server application when

    it starts running, using Qt's \l {How to Create Qt Plugins}{plugin

    system}.

    \image qt-embedded-characterinputlayer.png

    A keyboard driver receives keyboard events from the device and

    encapsulates each event with an instance of the QWSEvent class

    which it then passes to the server.

    \l{Qt for Embedded Linux} provides ready-made drivers for several keyboard

    protocols, see the \l {Qt for Embedded Linux Character Input}{character

    input} documentation for details. Custom keyboard drivers can be

    implemented by subclassing the QWSKeyboardHandler class and

    creating a keyboard driver plugin. The default implementation of the

    QKbdDriverFactory class will automatically detect the plugin, loading the

    driver into the server application at run-time.

    See also: \l{Qt for Embedded Linux Character Input} and \l {How to Create

    Qt Plugins}.

    \section1  Graphics Rendering

    \list

        \o QApplication

        \o QDecoration

        \o QDecorationPlugin

        \o QDecorationFactory

    \endlist

    The default behavior is for each client to render its widgets as well

    as its decorations into memory, while the server copies the memory content

    to the device's framebuffer.

    Whenever a client receives an event that alters any of its

    widgets, the application updates the relevant parts of its memory

    buffer:

    \image qt-embedded-clientrendering.png

    The decoration is loaded by the client application when it starts

    running (using Qt's \l {How to Create Qt Plugins}{plugin system}),

    and can be customized by deriving from the QDecoration class and

    creating a decoration plugin. The default implementation of

    the QDecorationFactory class will automatically detect the plugin,

    loading the decoration into the application at runtime. Call the

    QApplication::qwsSetDecoration() function to actually apply the

    given decoration to an application.

    \table 100%

    \header \o Direct Painting \target Direct Painting

    \row

    \o

    It is possible for the clients to manipulate and control the

    underlying hardware directly. There are two ways of achieving

    this: The first approach is to set the Qt::WA_PaintOnScreen window

    attribute for each widget, the other is to use the QDirectPainter

    class to reserve a region of the framebuffer.

    \image qt-embedded-setwindowattribute.png

    By setting the Qt::WA_PaintOnScreen attribute, the application

    renders the widget directly onto the screen and the affected

    region will not be modified by the screen driver \e unless another

    window with a higher focus requests (parts of) the same

    region. Note that if you want to render all of an application's

    widgets directly on screen, it might be easier to set the

    QT_ONSCREEN_PAINT environment variable.

    \image qt-embedded-reserveregion.png

    Using QDirectPainter, on the other hand, provides a complete

    control over the reserved region, i.e., the screen driver will

    never modify the given region.

    To draw on a region reserved by a QDirectPainter instance, the

    application must get hold of a pointer to the framebuffer. In

    general, a pointer to the framebuffer can be retrieved using the

    QDirectPainter::frameBuffer() function. But note that if the

    current screen has subscreens, you must query the screen driver

    instead to identify the correct subscreen. A pointer to the

    current screen driver can always be retrieved using the static

    QScreen::instance() function. Then use QScreen's \l

    {QScreen::}{subScreenIndexAt()} and \l {QScreen::}{subScreens()}

    functions to access the correct subscreen, and the subscreen's \l

    {QScreen::}{base()} function to retrieve a pointer to the

    framebuffer.

    Note that \l{Qt for Embedded Linux} also provides the QWSEmbedWidget class,

    making it possible to embed the reserved region (i.e., the

    QDirectPainter object) in a regular widget.

    \endtable

    \section1 Drawing on Screen

    \list

        \o QScreen

        \o QScreenDriverPlugin

        \o QScreenDriverFactory

    \endlist

    When a screen update is required, the server runs through all the

    top-level windows that intersect with the region that is about to

    be updated, and ensures that the associated clients have updated

    their memory buffer. Then the server uses the screen driver

    (represented by an instance of the QScreen class) to copy the

    content of the memory to the screen.

    The screen driver is loaded by the server application when it

    starts running, using Qt's plugin system. \l{Qt for Embedded Linux}

    provides ready-made drivers for several screen protocols, see the

    \l{Qt for Embedded Linux Display Management}{display management}

    documentation for details. Custom screen drivers can be

    implemented by subclassing the QScreen class and creating a screen

    driver plugin. The default implementation of the QScreenDriverFactory

    class will automatically detect the plugin, loading the driver into

    the server application at run-time.

    \image qt-embedded-drawingonscreen.png

    To locate the relevant parts of memory, the driver is provided

    with the list of top-level windows that intersect with the given

    region. Associated with each of the top-level windows there is an

    instance of the QWSWindowSurface class representing the drawing

    area of the window. The driver uses these objects to retrieve

    pointers to the various memory blocks. Finally, the screen driver

    composes the surface images before copying the updated region to

    the framebuffer.

    \table 100%

    \header \o Accelerated Graphics

    \row

    \o

    In \l{Qt for Embedded Linux}, painting is a pure software implementation,

    but (starting with Qt 4.2) it is possible to add an accelerated

    graphics driver to take advantage of available hardware resources.

    \image qt-embedded-accelerateddriver.png

    The clients render each window onto a corresponding window surface

    object using Qt's paint system, and then store the surface in

    memory. The screen driver accesses the memory and composes the

    surface images before it copies them to the screen as explained

    above.

    To add an accelerated graphics driver you must create a custom

    screen and implement a custom raster paint engine

    (\l{Qt for Embedded Linux} uses a raster-based paint engine to

    implement the painting operations). Then you must create a custom

    paint device that is aware of your paint engine, a custom window

    surface that knows about your paint device, and make your screen

    able to recognize your window surface.

    See the \l{Adding an Accelerated Graphics Driver to Qt for Embedded Linux}

    {accelerated graphics driver} documentation for details.

    \endtable

    See also: \l{Qt for Embedded Linux Display Management} and

    \l{How to Create Qt Plugins}.

*/