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

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/*!
    \example qws/dbscreen
    \title Double Buffered Graphics Driver Example

    The Double Buffered Graphics Driver example shows how to write your own
    double buffered graphics driver and add it to Qt for Embedded Linux.

    Similar to the \l{Accelerated Graphics Driver Example}, there are three steps
    to writing and implementing this graphics driver:

    \list 1
        \o \l {Step 1: Creating a Custom Graphics Driver}
        {Creating a Custom Graphics Driver}

        \o \l {Step 2: Implementing the Back Buffer}
        {Implementing the Back Buffer}

        \o \l {Step 3: Creating the Driver Plugin}
        {Creating the Driver Plugin}

    \endlist

    After compiling the example code, install the graphics driver plugin with
    the command \c {make install}. To start an application using the graphics
    driver, you can either set the environment variable \l QWS_DISPLAY and
    then run the application, or you can just run the application using the
    \c -display switch.

    Note that this is a minimal example and this driver will not work well
    with widgets painting themself directly to the screen (e.g. widgets with
    the Qt::WA_PaintOnScreen window attribute set). Also, the example requires
    the Linux framebuffer to be set up correctly and with the correct device
    permissions. For further information, refer to
    \l{Testing the Linux Framebuffer}.

    \section1 Step 1: Creating a Custom Graphics Driver

    Usually, a custom graphics driver is created by subclassing the QScreen
    class, the base class for implementing screen or graphics drivers in
    Qt for Embedded Linux. In this example, however, we subclass the QLinuxFbScreen
    class instead, to ensure that our driver uses the Linux framebuffer.

    For our graphics driver, the \c DBScreen class, we reimplement five
    functions belonging to QScreen:

    \list
        \o \l{QScreen::initDevice()}{initDevice()},
        \o \l{QScreen::shutdownDevice()}{shutdownDevice()},
        \o \l{QScreen::blit()}{blit()},
        \o \l{QScreen::solidFill()}{solidFill()}, and
        \o \l{QScreen::exposeRegion()}{exposeRegion()}.
    \endlist

    \snippet examples/qws/dbscreen/dbscreen.h 0

    In addition to the abovementioned functions, there is a private instance
    of QPainter and QImage - \c painter, used for drawing operations on
    the back buffer, and \c image, the back buffer itself.

    \section1 Step 2: Implementing the Back Buffer

    The graphics driver must carry out three main functions:

    \list 1
        \o Allocate the back buffer on startup and deallocate it on shutdown.
        \o Draw to the back buffer instead of directly to the screen
           (which is what QLinuxFbScreen does).
        \o Copy the back buffer to the screen whenever a screen update is
           done.
    \endlist

    \section2 Device initializing and shutdown 

    We first reimplement \c initDevice() and \c shutdownDevice().

    The \c initDevice() function initializes the framebuffer. We reimplement
    this function to enable accelerated drivers to set up the graphic card.
    For this example, we first call the super class' implementation to set up
    the Linux framebuffer. If this call returns \c false, we return \c false.
    Otherwise, we initialize the screen cursor with
    QScreenCursor::initSoftwareCursor() as well as instantiate \c image and
    \c painter. Then, we return \c true.

    \snippet examples/qws/dbscreen/dbscreen.cpp 0

    The \c shutdownDevice() function's default implementation only hides the
    mouse cursor. Hence, we reimplement it to carry out the necessary cleanup
    before the Qt for Embedded Linux server exits.

    \snippet examples/qws/dbscreen/dbscreen.cpp 1

    Again, we call the super class implementation to shutdown the Linux
    framebuffer prior to deleting \c image and \c painter.

    \section2 Drawing to the back buffer

    We move on to the drawing functions - \c solidFill() and \c blit(). In
    QLinuxFbScreen, these functions draw directly to the Linux framebuffer;
    but in our driver we reimplement them to draw to the back buffer instead.

    \snippet examples/qws/dbscreen/dbscreen.cpp 2

    The \c solidFill() function is called from \c exposeRegion() to fill the
    given \c region of the screen with the specified \c color. In this
    example, we use \c painter to fill rectangles in \c image, the back
    buffer, according to the given region.

    \snippet examples/qws/dbscreen/dbscreen.cpp 3

    The \c blit() function is also called from \c exposeRegion() to copy the
    given QRegion object, \c region, in the given QImage object, \c image, to
    the QPoint object specified by \c topLeft. Once again we use \c painter
    to draw in the back buffer, \c image.

    \section2 Displaying the buffer on the screen

    The \c exposeRegion() function is called by the Qt for Embedded Linux server
    whenever a screen update is required. The given \c region is the screen
    region that needs to be updated and \c changing is is the index into
    QWSServer::clientWindows() of the window that caused the update.

    \snippet examples/qws/dbscreen/dbscreen.cpp 4

    In our implementation, we first call the super class implementation to
    ensure that \c solidFill() and \c blit() will be called correctly. This
    causes the changed areas to be updated in the back buffer. We then call
    the super class' implementation of \c blit() to copy the updated region
    from the back buffer into the Linux framebuffer.

    \section1 Step 3: Creating the Driver Plugin

    Qt provides a high level API for writing Qt extentions. One of the plugin
    base classes provided is QScreenDriverPlugin, which we use in this example
    to create our screen driver plugin.

    \snippet examples/qws/dbscreen/dbscreendriverplugin.cpp 0

    There are only two functions to reimplement:

    \list
        \o \l{QScreenDriverPlugin::create()}{create()} - creates a driver
           matching the given key
        \o \l{QScreenDriverPlugin::create()}{keys()} - returns a list of
           valid keys representing the drivers supported by the plugin
    \endlist

    \snippet examples/qws/dbscreen/dbscreendriverplugin.cpp 1
    \codeline
    \snippet examples/qws/dbscreen/dbscreendriverplugin.cpp 2

    Our plugin will only support one driver, \c dbscreen.

    Lastly, we export the plugin.

    \snippet examples/qws/dbscreen/dbscreendriverplugin.cpp 3

    For detailed information about the Qt plugin system see
    \l{How to Create Qt Plugins.}
*/