Symbian3/PDK/Source/GUID-C6E9D609-E82C-4FAC-9265-F6A4FF61049C.dita
author Dominic Pinkman <dominic.pinkman@nokia.com>
Fri, 11 Jun 2010 15:24:34 +0100
changeset 9 59758314f811
parent 5 f345bda72bc4
permissions -rw-r--r--
Week 23 contribution of PDK documentation content. See release notes for details. Fixes bugs Bug 2714, Bug 462.
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<?xml version="1.0" encoding="utf-8"?>
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<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
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<!-- This component and the accompanying materials are made available under the terms of the License 
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"Eclipse Public License v1.0" which accompanies this distribution, 
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and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
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<!-- Initial Contributors:
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    Nokia Corporation - initial contribution.
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Contributors: 
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-->
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<!DOCTYPE concept
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  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
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<concept id="GUID-C6E9D609-E82C-4FAC-9265-F6A4FF61049C" xml:lang="en"><title>Drawing
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in the view architecture</title><prolog><metadata><keywords/></metadata></prolog><conbody>
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<p>The Symbian platform calls <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-63295719-90A0-3D53-A643-0C52BF4068A1"><apiname>CCoeControl::Draw()</apiname></xref> when
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a control area needs to be updated on the display. Controls may implement <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-63295719-90A0-3D53-A643-0C52BF4068A1"><apiname>CCoeControl::Draw()</apiname></xref> or
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leave the drawing to their child controls. For more information on control
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hierarchies, see <xref href="GUID-E244744F-4837-5B46-8E37-4666A28BF0B7.dita">The
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run-time control hierarchy</xref>. </p>
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<p>The Symbian platform calls <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-63295719-90A0-3D53-A643-0C52BF4068A1"><apiname>CCoeControl::Draw()</apiname></xref> for
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the parent control first, and then recursively for each control.</p>
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<p>Controls should override <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-63295719-90A0-3D53-A643-0C52BF4068A1"><apiname>CCoeControl::Draw()</apiname></xref> to draw
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their content. The override should do nothing else and should be as fast as
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possible. For example, it is bad design to create fonts dynamically, and read
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any bitmaps or resources while drawing. A good rule of thumb is that there
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should not be trap handlers in the method override; any time-consuming functionality
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that can be done beforehand should be cached.</p>
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<p>In most cases controls are drawn on the display using the screen device
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graphics context, accessed with <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-6586DB74-AF5C-330B-9D0A-E4637396A04E"><apiname>CCoeControl::SystemGc()</apiname></xref>.
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The graphics context provides a wide set of <xref href="GUID-E89F034F-C807-5FF9-B06B-F7CCD2441041.dita">GDI</xref> (Graphics
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Device Interface  - common Symbian platform graphics API) drawing primitives
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that can be used for drawing virtually anything on screen.</p>
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<p>An example of a basic override of <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-63295719-90A0-3D53-A643-0C52BF4068A1"><apiname>CCoeControl::Draw()</apiname></xref> for
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a control that is a top-level window in an application is as follows:</p>
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<codeblock id="GUID-9EF298F3-9B92-48C1-BA7C-0C91143D075C" xml:space="preserve">void CMyAppView::Draw( const TRect&amp; /*aRect*/ ) const
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    {
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    // Get the standard graphics context
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    CWindowGc&amp; gc = SystemGc();
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    gc.SetPenStyle( CGraphicsContext::ENullPen );
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    gc.SetBrushColor( KRgbWhite);
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    gc.SetBrushStyle( CGraphicsContext::ESolidBrush );
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    // Gets the control's extent
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    TRect rect( Rect());
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        {
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        gc.Clear( rect );
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        }
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    }
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</codeblock>
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<p>, where</p>
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<ul>
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<li><p><parmname>CWindowGc&amp; gc = SystemGc();</parmname> gets
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the graphics context that is used when drawing the control.</p></li>
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<li><p><xref href="GUID-0AEE5955-C530-35F1-A904-69183331B294.dita#GUID-0AEE5955-C530-35F1-A904-69183331B294/GUID-026A1015-0D79-3A66-91BA-5FB343387EE0"><apiname>CWindowGc::SetPenStyle()</apiname></xref>, <xref href="GUID-0AEE5955-C530-35F1-A904-69183331B294.dita#GUID-0AEE5955-C530-35F1-A904-69183331B294/GUID-0A923CAA-7A89-3ED2-A844-2F4147B62FEC"><apiname>CWindowGc::SetBrushColor()</apiname></xref>,
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and <xref href="GUID-0AEE5955-C530-35F1-A904-69183331B294.dita#GUID-0AEE5955-C530-35F1-A904-69183331B294/GUID-363A9FB3-75F7-3EB7-A783-91BBAEBCE670"><apiname>CWindowGc::SetBrushStyle()</apiname></xref> are used to set the drawing
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primatives for the context.</p></li>
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<li><p><xref href="GUID-101762DC-E498-3325-88AB-B0FF17DC62B6.dita"><apiname>TRect</apiname></xref> gets the size of the control rectangle.</p>
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</li>
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<li><p><xref href="GUID-D22FD07E-59E0-346A-9BFA-8D109F509DB1.dita"><apiname>CWindowGc:Clear(rect)</apiname></xref> clears the control
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rectangle.</p></li>
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</ul>
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<section id="GUID-DBACBFCC-F260-4ABE-B55C-BBB23F332E42"><title>Drawing with
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caches</title>
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<p>For controls that perform intensive drawing operations, the drawing
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should be cached: a process also known as double-buffering. Here the drawing
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is done to a memory context first and then in the <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-63295719-90A0-3D53-A643-0C52BF4068A1"><apiname>CCoeControl::Draw()</apiname></xref> method
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only the context's bitmap is passed to screen. In the Symbian platform, the
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easiest way to implement a double buffer is to create a <xref href="GUID-683A1D42-2764-3EB7-BD19-9E12559199AB.dita"><apiname>CFbsBitmap</apiname></xref> and
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then bind a graphics context to that  - this makes it possible to use the
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same GDI interface for drawing as the display context. The drawing is done
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to a memory bitmap buffer, and when the Symbian platform updates the invalidated
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screen area, the memory buffer is copied to it. Double-buffering is a common
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paradigm used in games, but can also be utilized in any application when performance
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of drawing controls is important.</p>
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<p>The following is a short example of how a double buffer is created and
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used:</p>
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<codeblock id="GUID-995DBACE-EE60-4666-BDCF-975EE98B01FD" xml:space="preserve">iGcBmp = new (ELeave) CWsBitmap(iEikonEnv-&gt;WsSession());
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User::LeaveIfError(iGcBmp-&gt;Create(aClientRect.Size(), iEikonEnv-&gt;ScreenDevice()-&gt;DisplayMode()));
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iGcDevice = CFbsBitmapDevice::NewL(iGcBmp);
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User::LeaveIfError(iGcDevice-&gt;CreateBitmapContext(iGc));
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</codeblock>
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<p><parmname>iGcBmp</parmname> is a pointer to <xref href="GUID-17150D76-BB82-3A4B-8B1A-8BA93CB1A9EF.dita"><apiname>CWsBitmap</apiname></xref>,
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the bitmap memory buffer, that is created with the same width and height as
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the top-level window and with the same color bit depth as the display.<parmname> iGcDevice</parmname> is
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a pointer to the <xref href="GUID-2DEFEC47-F36E-3133-A08D-55F7C2534CC0.dita"><apiname>CBitmapDevice</apiname></xref> device class and the context <parmname>iGc</parmname> holds
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the <xref href="GUID-FC746873-0570-3900-AD89-42B205FDC0D3.dita"><apiname>CBitmapContext</apiname></xref> instance. <parmname>iGc</parmname> is
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then used instead of <parmname>CScreenGc</parmname>, obtained from the <parmname>CCoeControl::SystemGc()</parmname> method,
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when the control draws itself. The double-buffer drawing should be done outside
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of the <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-63295719-90A0-3D53-A643-0C52BF4068A1"><apiname>CCoeControl::Draw()</apiname></xref> method and can be called directly
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when needed. Only at the end of the off-screen drawing is the memory buffer
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flushed to the screen by calling <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-9FB682AC-0209-302A-83F3-7BCB1162B998"><apiname>CCoeControl::DrawDeferred()</apiname></xref></p>
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<codeblock id="GUID-13A1E553-7A78-422C-85AC-7C89696B2D18" xml:space="preserve">void CMyDrawingExample::Draw(const TRect&amp; /*aRect*/) const
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    {
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    SystemGc().BitBlt(TPoint(0, 0), iGcBmp);
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    }</codeblock>
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</section>
1
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</conbody></concept>