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

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

<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

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

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

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

run-time control hierarchy</xref>. </p>

<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

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

<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

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

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

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

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

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

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

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>.

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

Device Interface  - common Symbian platform graphics API) drawing primitives

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

<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

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

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

    {

    // Get the standard graphics context

    CWindowGc& gc = SystemGc();

    gc.SetPenStyle( CGraphicsContext::ENullPen );

    gc.SetBrushColor( KRgbWhite);

    gc.SetBrushStyle( CGraphicsContext::ESolidBrush );

    // Gets the control's extent

    TRect rect( Rect());

        {

        gc.Clear( rect );

        }

    }

</codeblock>

<p>, where</p>

<ul>

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

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

<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>,

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

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

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

</li>

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

rectangle.</p></li>

</ul>

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

caches</title>

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

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

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

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

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

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

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

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

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

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

of drawing controls is important.</p>

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

used:</p>

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

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

iGcDevice = CFbsBitmapDevice::NewL(iGcBmp);

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

</codeblock>

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

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

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

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

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

then used instead of <parmname>CScreenGc</parmname>, obtained from the <parmname>CCoeControl::SystemGc()</parmname> method,

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

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

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

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>

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

    {

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

    }</codeblock>

</section>