1 <?xml version="1.0" encoding="utf-8"?>

     2 <!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->

     3 <!-- This component and the accompanying materials are made available under the terms of the License 

     4 "Eclipse Public License v1.0" which accompanies this distribution, 

     5 and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->

     6 <!-- Initial Contributors:

     7     Nokia Corporation - initial contribution.

     8 Contributors: 

     9 -->

    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-C6E9D609-E82C-4FAC-9265-F6A4FF61049C" xml:lang="en"><title>Drawing

    13 in the view architecture</title><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <p>The Symbian platform calls <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Control_Environment/CCoeControlClass.html#%3a%3aCCoeControl%3a%3aDraw%28const%20TRect%20%26amp%3b%29const" format="application/java-archive"><parmname>CCoeControl::Draw</parmname></xref> when a control area needs

    15 to be updated on the display. Controls may implement <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Control_Environment/CCoeControlClass.html#%3a%3aCCoeControl%3a%3aDraw%28const%20TRect%20%26amp%3b%29const" format="application/java-archive"><parmname>CCoeControl::Draw</parmname></xref> or leave the drawing

    16 to their child controls. For more information on control hierarchies, see <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/guide/Application-Framework-subsystem-guide/UIControlFrameworkGuide/UIControlFrameworkGuide2/RunTimeControlHierarchy.guide.html" format="application/java-archive">The run-time control hierarchy</xref>. </p>

    17 <p>The Symbian platform calls <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Control_Environment/CCoeControlClass.html#%3a%3aCCoeControl%3a%3aDraw%28const%20TRect%20%26amp%3b%29const" format="application/java-archive"><parmname>CCoeControl::Draw</parmname></xref> for the parent control

    18 first, and then recursively for each control.</p>

    19 <p>Controls should override <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Control_Environment/CCoeControlClass.html#%3a%3aCCoeControl%3a%3aDraw%28const%20TRect%20%26amp%3b%29const" format="application/java-archive"><parmname>CCoeControl::Draw</parmname></xref> to draw their content.

    20 The override should do nothing else and should be as fast as possible. For

    21 example, it is bad design to create fonts dynamically, and read any bitmaps

    22 or resources while drawing. A good rule of thumb is that there should not

    23 be trap handlers in the method override; any time-consuming functionality

    24 that can be done beforehand should be cached.</p>

    25 <p>In most cases controls are drawn on the display using the screen device

    26 graphics context, accessed with <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Control_Environment/CCoeControlClass.html#%3a%3aCCoeControl%3a%3aSystemGc%28%29const" format="application/java-archive"><parmname>CCoeControl::SystemGc()</parmname></xref>. The graphics context provides a wide set of <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/GDI/index.html" format="application/java-archive">GDI</xref> (Graphics

    27 Device Interface  - common Symbian platform graphics API) drawing primitives

    28 that can be used for drawing virtually anything on screen.</p>

    29 <p>An example of a basic override of <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Control_Environment/CCoeControlClass.html#%3a%3aCCoeControl%3a%3aDraw%28const%20TRect%20%26amp%3b%29const" format="application/java-archive"><parmname>CCoeControl::Draw</parmname></xref> for a control that is

    30 a top-level window in an application is as follows:</p>

    31 <codeblock id="GUID-9EF298F3-9B92-48C1-BA7C-0C91143D075C" xml:space="preserve">void CMyAppView::Draw( const TRect&amp; /*aRect*/ ) const

    32     {

    33     // Get the standard graphics context

    34     CWindowGc& gc = SystemGc();

    35     gc.SetPenStyle( CGraphicsContext::ENullPen );

    36     gc.SetBrushColor( KRgbWhite);

    37     gc.SetBrushStyle( CGraphicsContext::ESolidBrush );

    38 

    39     // Gets the control's extent

    40     TRect rect( Rect());

    41 

    42         {

    43         gc.Clear( rect );

    44         }

    45     }

    46 </codeblock>

    47 <p>, where</p>

    48 <ul>

    49 <li><p><parmname>CWindowGc&amp; gc = SystemGc();</parmname> gets

    50 the graphics context that is used when drawing the control.</p></li>

    51 <li><p><xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Window_Server/CWindowGcClass.html#%3a%3aCWindowGc%3a%3aSetPenStyle%28TPenStyle%29" format="application/java-archive"><parmname>CWindowGc::SetPenStyle</parmname></xref>, <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Window_Server/CWindowGcClass.html#%3a%3aCWindowGc%3a%3aSetBrushColor%28const%20TRgb%20%26amp%3b%29" format="application/java-archive"><parmname>CWindowGc::SetBrushColor</parmname></xref>, and <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Window_Server/CWindowGcClass.html#%3a%3aCWindowGc%3a%3aSetBrushStyle%28TBrushStyle%29" format="application/java-archive"><parmname>CWindowGc::SetBrushStyle</parmname></xref> are used to set the drawing primatives for the context.</p>

    52 </li>

    53 <li><p><xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Kernel_Architecture_2/TRectClass.html" format="application/java-archive"><parmname>TRect</parmname></xref> gets

    54 the size of the control rectangle.</p></li>

    55 <li><p><xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Window_Server/CWindowGcClass.html#%3a%3aCWindowGc%3a%3aClear%28const%20TRect%20%26amp%3b%29" format="application/java-archive"><parmname>CWindowGc:Clear(rect)</parmname></xref> clears the control rectangle.</p>

    56 </li>

    57 </ul>

    58 <section id="GUID-DBACBFCC-F260-4ABE-B55C-BBB23F332E42"><title>Drawing with

    59 caches</title>

    60 <p>For controls that perform intensive drawing operations, the drawing

    61 should be cached: a process also known as double-buffering. Here the drawing

    62 is done to a memory context first and then in the <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Control_Environment/CCoeControlClass.html#%3a%3aCCoeControl%3a%3aDraw%28const%20TRect%20%26amp%3b%29const" format="application/java-archive"><parmname>CCoeControl::Draw</parmname></xref> method only the context's

    63 bitmap is passed to screen. In the Symbian platform, the easiest way to implement

    64 a double buffer is to create a <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Font_and_Bitmap_Server/CFbsBitmapClass.html" format="application/java-archive"><parmname>CFbsBitmap</parmname></xref> and

    65 then bind a graphics context to that  - this makes it possible to use the

    66 same GDI interface for drawing as the display context. The drawing is done

    67 to a memory bitmap buffer, and when the Symbian platform updates the invalidated

    68 screen area, the memory buffer is copied to it. Double-buffering is a common

    69 paradigm used in games, but can also be utilized in any application when performance

    70 of drawing controls is important.</p>

    71 <p>The following is a short example of how a double buffer is created and

    72 used:</p>

    73 <codeblock id="GUID-995DBACE-EE60-4666-BDCF-975EE98B01FD" xml:space="preserve">iGcBmp = new (ELeave) CWsBitmap(iEikonEnv-&gt;WsSession());

    74 User::LeaveIfError(iGcBmp->Create(aClientRect.Size(), iEikonEnv->ScreenDevice()->DisplayMode()));

    75 iGcDevice = CFbsBitmapDevice::NewL(iGcBmp);

    76 User::LeaveIfError(iGcDevice->CreateBitmapContext(iGc));

    77 </codeblock>

    78 <p><parmname>iGcBmp</parmname> is a pointer to <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Window_Server/CWsBitmapClass.html" format="application/java-archive"><parmname>CWsBitmap</parmname></xref>,

    79 the bitmap memory buffer, that is created with the same width and height as

    80 the top-level window and with the same color bit depth as the display.<parmname> iGcDevice</parmname> is

    81 a pointer to the <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/GDI/CBitmapDeviceClass.html" format="application/java-archive"><parmname>CBitmapDevice</parmname></xref> device

    82 class and the context <parmname>iGc</parmname> holds the <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/GDI/CBitmapContextClass.html" format="application/java-archive"><parmname>CBitmapContext</parmname></xref> instance. <parmname>iGc</parmname> is then used instead of <parmname>CScreenGc</parmname>, obtained from the <parmname>CCoeControl::SystemGc()</parmname> method,

    83 when the control draws itself. The double-buffer drawing should be done outside

    84 of the <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Control_Environment/CCoeControlClass.html#%3a%3aCCoeControl%3a%3aDraw%28const%20TRect%20%26amp%3b%29const" format="application/java-archive"><parmname>CCoeControl::Draw</parmname></xref> method and can be called

    85 directly when needed. Only at the end of the off-screen drawing is the memory

    86 buffer flushed to the screen by calling <xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Control_Environment/CCoeControlClass.html#%3a%3aCCoeControl%3a%3aDrawDeferred%28%29const" format="application/java-archive"><parmname>CCoeControl::DrawDeferred()</parmname></xref></p>

    87 <codeblock id="GUID-13A1E553-7A78-422C-85AC-7C89696B2D18" xml:space="preserve">void CMyDrawingExample::Draw(const TRect&amp; /*aRect*/) const

    88     {

    89     SystemGc().BitBlt(TPoint(0, 0), iGcBmp);

    90     }</codeblock>

    91 </section>

    92 </conbody></concept>