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     7     Nokia Corporation - initial contribution.

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    10 <!DOCTYPE concept

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

    12 <concept id="GUID-DC59BEAD-0047-4D7A-96D1-C5E3EC4F982B" xml:lang="en"><title>Drawing

    13 in traditional architecture</title><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <p>The application framework 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

    15 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

    16 leave the drawing to their child controls. For more information on control

    17 hierarchies, see <xref href="GUID-E244744F-4837-5B46-8E37-4666A28BF0B7-GENID-1-8-1-3-1-1-7-1-7-1-5-1.dita">The

    18 run-time control hierarchy</xref>. The 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

    19 the parent control first, and then recursively for each control.</p>

    20 <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

    21 their content. The override should do nothing else and should be as fast as

    22 possible. For example, it is bad design to create fonts dynamically, and read

    23 any bitmaps or resources while drawing. A good rule of thumb is that there

    24 should not be trap handlers in the method override; any time-consuming functionality

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

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

    27 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>.

    28 The graphics context provides a wide set of <xref href="GUID-E89F034F-C807-5FF9-B06B-F7CCD2441041.dita">GDI</xref> (Graphics

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

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

    31 <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

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

    33 <codeblock id="GUID-27363E40-E10A-4399-BF1A-A48305A1DC1B" xml:space="preserve">void CMyAppView::Draw( const TRect&amp; /*aRect*/ ) const

    34     {

    35     // Get the standard graphics context

    36     CWindowGc& gc = SystemGc();

    37     gc.SetPenStyle( CGraphicsContext::ENullPen );

    38     gc.SetBrushColor( KRgbWhite);

    39     gc.SetBrushStyle( CGraphicsContext::ESolidBrush );

    40 

    41     // Gets the control's extent

    42     TRect rect( Rect());

    43 

    44         {

    45         gc.Clear( rect );

    46         }

    47     }

    48 </codeblock>

    49 <p>, where</p>

    50 <ul>

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

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

    53 <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>,

    54 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

    55 primatives for the context</p></li>

    56 <li><p><xref href="GUID-101762DC-E498-3325-88AB-B0FF17DC62B6.dita"><apiname>TRect</apiname></xref> gets the size of the control rectangle</p>

    57 </li>

    58 <li><p><xref href="GUID-D22FD07E-59E0-346A-9BFA-8D109F509DB1.dita"><apiname>CWindowGc:Clear(rect)</apiname></xref> clears the control

    59 rectangle</p></li>

    60 </ul>

    61 <section id="GUID-F52B945E-F63A-40FC-B7EF-CBA4CFD415E8"><title>Double buffering</title>

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

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

    64 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

    65 only the context's bitmap is passed to screen. In the Symbian platform, the

    66 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

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

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

    69 to a memory bitmap buffer. Double-buffering is a common paradigm used in games,

    70 but can also be utilized in any application when performance of drawing controls

    71 is important.</p>

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

    73 used:</p>

    74 <codeblock id="GUID-2FA4ADA3-E8F5-4BEE-86C4-40DE748BB91B" xml:space="preserve">iGcBmp = new (ELeave) CWsBitmap(iEikonEnv-&gt;WsSession());

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

    76 iGcDevice = CFbsBitmapDevice::NewL(iGcBmp);

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

    78 </codeblock>

    79 <p><parmname>iGcBmp</parmname> is a pointer to <xref href="GUID-17150D76-BB82-3A4B-8B1A-8BA93CB1A9EF.dita"><apiname>CWsBitmap</apiname></xref>,

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

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

    82 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

    83 the <xref href="GUID-FC746873-0570-3900-AD89-42B205FDC0D3.dita"><apiname>CBitmapContext</apiname></xref> instance. <parmname>iGc</parmname> is

    84 then used instead of <parmname>CScreenGc</parmname>, obtained from the <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> method,

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

    86 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

    87 when needed. Only at the end of the off-screen drawing is the memory buffer

    88 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>

    89 <codeblock id="GUID-9474C692-0E2E-4B2C-A218-C4DF99E85C31" xml:space="preserve">void CMyDrawingExample::Draw(const TRect&amp; /*aRect*/) const

    90     {

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

    92     }</codeblock>

    93 </section>

    94 </conbody></concept>