187 lines
7.3 KiB
Plaintext
187 lines
7.3 KiB
Plaintext
/****************************************************************************
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**
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** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies).
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** Contact: http://www.qt-project.org/
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**
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** This file is part of the documentation of the Qt Toolkit.
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**
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** $QT_BEGIN_LICENSE:FDL$
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** GNU Free Documentation License
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** Alternatively, this file may be used under the terms of the GNU Free
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** Documentation License version 1.3 as published by the Free Software
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** Foundation and appearing in the file included in the packaging of
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** this file.
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**
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** Other Usage
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** Alternatively, this file may be used in accordance with the terms
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** and conditions contained in a signed written agreement between you
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** and Nokia.
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**
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**
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**
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**
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**
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** $QT_END_LICENSE$
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**
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****************************************************************************/
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/*!
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\example qws/dbscreen
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\title Double Buffered Graphics Driver Example
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The Double Buffered Graphics Driver example shows how to write your own
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double buffered graphics driver and add it to Qt for Embedded Linux.
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Similar to the \l{Accelerated Graphics Driver Example}, there are three steps
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to writing and implementing this graphics driver:
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\list 1
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\li \l {Step 1: Creating a Custom Graphics Driver}
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{Creating a Custom Graphics Driver}
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\li \l {Step 2: Implementing the Back Buffer}
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{Implementing the Back Buffer}
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\li \l {Step 3: Creating the Driver Plugin}
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{Creating the Driver Plugin}
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\endlist
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After compiling the example code, install the graphics driver plugin with
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the command \c {make install}. To start an application using the graphics
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driver, you can either set the environment variable \l QWS_DISPLAY and
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then run the application, or you can just run the application using the
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\c -display switch.
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Note that this is a minimal example and this driver will not work well
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with widgets painting themself directly to the screen (e.g. widgets with
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the Qt::WA_PaintOnScreen window attribute set). Also, the example requires
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the Linux framebuffer to be set up correctly and with the correct device
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permissions. For further information, refer to
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\l{Testing the Linux Framebuffer}.
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\section1 Step 1: Creating a Custom Graphics Driver
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Usually, a custom graphics driver is created by subclassing the QScreen
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class, the base class for implementing screen or graphics drivers in
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Qt for Embedded Linux. In this example, however, we subclass the QLinuxFbScreen
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class instead, to ensure that our driver uses the Linux framebuffer.
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For our graphics driver, the \c DBScreen class, we reimplement five
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functions belonging to QScreen:
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\list
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\li \l{QScreen::initDevice()}{initDevice()},
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\li \l{QScreen::shutdownDevice()}{shutdownDevice()},
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\li \l{QScreen::blit()}{blit()},
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\li \l{QScreen::solidFill()}{solidFill()}, and
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\li \l{QScreen::exposeRegion()}{exposeRegion()}.
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\endlist
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\snippet examples/qws/dbscreen/dbscreen.h 0
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In addition to the abovementioned functions, there is a private instance
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of QPainter and QImage - \c painter, used for drawing operations on
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the back buffer, and \c image, the back buffer itself.
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\section1 Step 2: Implementing the Back Buffer
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The graphics driver must carry out three main functions:
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\list 1
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\li Allocate the back buffer on startup and deallocate it on shutdown.
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\li Draw to the back buffer instead of directly to the screen
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(which is what QLinuxFbScreen does).
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\li Copy the back buffer to the screen whenever a screen update is
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done.
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\endlist
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\section2 Device initializing and shutdown
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We first reimplement \c initDevice() and \c shutdownDevice().
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The \c initDevice() function initializes the framebuffer. We reimplement
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this function to enable accelerated drivers to set up the graphic card.
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For this example, we first call the super class' implementation to set up
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the Linux framebuffer. If this call returns \c false, we return \c false.
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Otherwise, we initialize the screen cursor with
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QScreenCursor::initSoftwareCursor() as well as instantiate \c image and
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\c painter. Then, we return \c true.
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\snippet examples/qws/dbscreen/dbscreen.cpp 0
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The \c shutdownDevice() function's default implementation only hides the
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mouse cursor. Hence, we reimplement it to carry out the necessary cleanup
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before the Qt for Embedded Linux server exits.
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\snippet examples/qws/dbscreen/dbscreen.cpp 1
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Again, we call the super class implementation to shutdown the Linux
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framebuffer prior to deleting \c image and \c painter.
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\section2 Drawing to the back buffer
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We move on to the drawing functions - \c solidFill() and \c blit(). In
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QLinuxFbScreen, these functions draw directly to the Linux framebuffer;
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but in our driver we reimplement them to draw to the back buffer instead.
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\snippet examples/qws/dbscreen/dbscreen.cpp 2
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The \c solidFill() function is called from \c exposeRegion() to fill the
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given \c region of the screen with the specified \c color. In this
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example, we use \c painter to fill rectangles in \c image, the back
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buffer, according to the given region.
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\snippet examples/qws/dbscreen/dbscreen.cpp 3
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The \c blit() function is also called from \c exposeRegion() to copy the
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given QRegion object, \c region, in the given QImage object, \c image, to
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the QPoint object specified by \c topLeft. Once again we use \c painter
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to draw in the back buffer, \c image.
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\section2 Displaying the buffer on the screen
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The \c exposeRegion() function is called by the Qt for Embedded Linux server
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whenever a screen update is required. The given \c region is the screen
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region that needs to be updated and \c changing is is the index into
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QWSServer::clientWindows() of the window that caused the update.
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\snippet examples/qws/dbscreen/dbscreen.cpp 4
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In our implementation, we first call the super class implementation to
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ensure that \c solidFill() and \c blit() will be called correctly. This
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causes the changed areas to be updated in the back buffer. We then call
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the super class' implementation of \c blit() to copy the updated region
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from the back buffer into the Linux framebuffer.
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\section1 Step 3: Creating the Driver Plugin
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Qt provides a high level API for writing Qt extensions. One of the plugin
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base classes provided is QScreenDriverPlugin, which we use in this example
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to create our screen driver plugin.
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\snippet examples/qws/dbscreen/dbscreendriverplugin.cpp 0
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There are only two functions to reimplement:
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\list
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\li \l{QScreenDriverPlugin::create()}{create()} - creates a driver
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matching the given key
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\li \l{QScreenDriverPlugin::create()}{keys()} - returns a list of
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valid keys representing the drivers supported by the plugin
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\endlist
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\snippet examples/qws/dbscreen/dbscreendriverplugin.cpp 1
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\codeline
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\snippet examples/qws/dbscreen/dbscreendriverplugin.cpp 2
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Our plugin will only support one driver, \c dbscreen.
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Lastly, we export the plugin.
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\snippet examples/qws/dbscreen/dbscreendriverplugin.cpp 3
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For detailed information about the Qt plugin system see
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\l{How to Create Qt Plugins.}
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*/
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