path: root/doc/src/platforms/emb-architecture.qdoc

/****************************************************************************
**
** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** GNU Free Documentation License
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file.
**
** Other Usage
** Alternatively, this file may be used in accordance with the terms
** and conditions contained in a signed written agreement between you
** and Nokia.
**
**
**
**
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \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}.
*/