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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.}
+*/