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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.}
+*/