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diff --git a/doc/src/examples/dbscreen.qdoc b/doc/src/examples/dbscreen.qdoc new file mode 100644 index 0000000000..cfcdecd68f --- /dev/null +++ b/doc/src/examples/dbscreen.qdoc @@ -0,0 +1,186 @@ +/**************************************************************************** +** +** Copyright (C) 2011 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. +** +** $QT_BEGIN_LICENSE:FDL$ +** No Commercial Usage +** This file contains pre-release code and may not be distributed. +** You may use this file in accordance with the terms and conditions +** contained in the Technology Preview License Agreement accompanying +** this package. +** +** 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. +** +** If you have questions regarding the use of this file, please contact +** Nokia at qt-info@nokia.com. +** $QT_END_LICENSE$ +** +****************************************************************************/ + +/*! + \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.} +*/ |