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<concept id="GUID-8DB1C618-597C-560C-95A2-C0AB2CEBB027" xml:lang="en"><title>Redraw Drawing</title><shortdesc>This topic provides background information about redraw

drawing and tips for updating old code to comply with the recommendation

that all drawing is migrated to redraw drawing. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p> <b>Variant</b>: Both (ScreenPlay and non-ScreenPlay). <b>Target

audience</b>: Application developers. </p>

<section id="GUID-71199005-1F00-434D-B130-47886DD53E92"><title>About

Redraw Drawing</title> <p>Symbian recommends that all <xref href="GUID-0AEE5955-C530-35F1-A904-69183331B294.dita"><apiname>CWindowGc</apiname></xref> drawing is <b>redraw drawing</b>, which means that it takes place

between <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita#GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79/GUID-9337538E-7A53-3153-A330-968B5E4F2FF2"><apiname>RWindow::BeginRedraw()</apiname></xref> and <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita#GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79/GUID-3DE16607-AD3B-3946-BEB3-88512EAAB9CE"><apiname>RWindow::EndRedraw()</apiname></xref> calls. <xref href="GUID-6C16417B-5B37-5310-B59A-750D971AA6D4.dita">The

UI Control Framework (CONE)</xref> automatically takes care of this

for you if you use the <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-250ED45A-76F3-3623-BA4B-22D31613A2EF"><apiname>CCoeControl::DrawNow(const TRect &amp;)const</apiname></xref> and <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-33A19096-F8D1-373C-862F-EF627981C782"><apiname>CCoeControl::DrawDeferred()const</apiname></xref> methods. </p> <p>For example, suppose an application wants to write the text "Hello!"

to a window and then add a red diagonal line below it (A in following

diagram). When this is performed as redraw drawing, the application

calculates the bounding rectangles of the "Hello!" text and the red

line (B). For each one, the application passes the rectangle to <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita#GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79/GUID-F5E85A7B-0DAE-32C1-AF71-8FC2F5F01509"><apiname>RWindow::BeginRedraw(const TRect &amp;)</apiname></xref>, then calls the

draw commands and afterwards calls <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita#GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79/GUID-3DE16607-AD3B-3946-BEB3-88512EAAB9CE"><apiname>RWindow::EndRedraw()</apiname></xref>. </p> <fig id="GUID-53B962F7-D425-5D72-8148-F822A3462243">

<image href="GUID-1DDFFB60-BBA8-500E-A9B1-F048EE5BFBD0_d0e236781_href.png" placement="inline"/>

</fig> <p>This has the advantage that the Window Server knows that

the area of the window that has the "Hello!" text is not affected

by the drawing of the red line. There are also advantages when some

or all of the window needs to be repainted. For example, suppose another

window (such as an OK dialog box) appears above the window (as shown

in C). When the dialog box is closed, the Window Server repaints the

screen behind the dialog box. Because in this example the OK dialog

box was entirely within the red line's bounding rectangle, the Window

Server simply replays that rectangle's drawing operations (D). This

is more efficient than repainting the whole window. </p> <p>However,

in earlier versions of Symbian you could, for example, draw the red

line without bracketing it within <codeph>BeginRedraw()</codeph> and <codeph>EndRedraw()</codeph> calls. This is called <b>non-redraw drawing</b>. In ScreenPlay, each non-redraw drawing operation triggers the Window

Server to invalidate the entire window. This means that the client

application must then perform a full window redraw, as shown in the

next diagram. </p> <fig id="GUID-A36DF80C-6B53-5D9D-878B-0854BB36E44A">

<title>              Non-redraw drawing sequence            </title>

<image href="GUID-7C51BC7A-CB97-5A7E-A5EF-BB3623DFFFED_d0e236804_href.png" placement="inline"/>

</fig> <p>This is less efficient than the Window Server replaying

the draw operations for the affected area. </p> <p>Symbian recommends

that all drawing is now performed as redraw drawing. Typically this

involves dividing the window up into rectangular areas that represent

different aspects of the user interface—for example, as shown in the

following diagram. If anything spoils the screen, the Window Server

then only needs to redraw the corresponding portions of the user interface. </p> <fig id="GUID-BAD0C762-0EBD-51E2-BC64-DF38749CA7D8">

<image href="GUID-F2E8CB4B-C1BB-577D-B018-7FF231B70D8C_d0e236815_href.png" placement="inline"/>

</fig> <p>In earlier versions of Symbian, before the introduction

of the <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-250ED45A-76F3-3623-BA4B-22D31613A2EF"><apiname>CCoeControl::DrawNow(const TRect &amp;)const</apiname></xref> overload, developers sometimes used non-redraw drawing to update

a small part of a control. This technique has often been used for

virtual keyboards and calendar controls. This type of use case is

now particularly unsuited to non-redraw drawing because in ScreenPlay

each non-redraw drawing command triggers a server-initiated client

redraw of the entire window. This can be very slow if the window requires

many drawing operations to redraw the scene. </p> <p>Symbian recommends

that all non-redraw drawing is changed to redraw drawing. Migrating

non-redraw drawing to redraw drawing means that you must enclose all

drawing operations between <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita#GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79/GUID-9337538E-7A53-3153-A330-968B5E4F2FF2"><apiname>RWindow::BeginRedraw()</apiname></xref> and <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita#GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79/GUID-3DE16607-AD3B-3946-BEB3-88512EAAB9CE"><apiname>RWindow::EndRedraw()</apiname></xref> calls or migrate it

to use the <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-D5458F8C-E199-37DD-B821-050B749122C6"><apiname>CCoeControl::DrawNow()</apiname></xref> and <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> methods. Sometimes this causes

some subtle changes in behavior, as explained in the Notes below. </p></section>

<section id="GUID-E88E139C-F852-4CAB-B6EB-7E5861D607D6"><title>Restrictions</title> <ol id="GUID-3BE36F7E-CFEC-5BFD-A311-E36D67D9097D">

<li id="GUID-C6236CD8-C83E-5252-BCC6-336FE8677DCF"><p>You must supply <b>only</b> drawing operations between the <codeph>BeginRedraw()</codeph> and <codeph>EndRedraw()</codeph> calls. For example, you must not

change the window extent between a <codeph>BeginRedraw()</codeph> and <codeph>EndRedraw()</codeph> call. </p> </li>

<li id="GUID-616C81BF-6296-4D10-BA9F-5A037995D773"><p>For opaque windows,

you must define pixel content for the <b>entire area</b> being redrawn.

For example, when calling <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-250ED45A-76F3-3623-BA4B-22D31613A2EF"><apiname>CCoeControl::DrawNow(const TRect

&amp;)const</apiname></xref>, you must supply drawing operations for the

entire rectangle passed to the function. Similarly when calling the <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-8DE580DF-459C-3FCF-8ABA-B26741C294CC"><apiname>CCoeControl::DrawNow()const</apiname></xref> overload, you must supply drawing

operations for the entire window. However, this is not necessary for

transparent windows.</p></li>

<li id="GUID-652DF093-046F-58A3-961F-2A51DEB2FD74"><p>You must <b>not</b> call any functions that can leave between the <codeph>BeginRedraw()</codeph> and <codeph>EndRedraw()</codeph> calls. This is because drawing

operations must never leave. </p> </li>

</ol> <p>Note: A redrawer's <codeph>RunL()</codeph> function must <b>never</b> perform any non-redraw drawing. See <xref href="GUID-7C75642D-C0C9-5392-9354-711F42DC2C61.dita">Server-Initiated

vs. Application-Initiated Redrawing</xref> for more information. </p> </section>

<section id="GUID-620268C0-2F67-4D87-BF99-E1843DBE9BCA"><title>Notes</title> <p><b>Performance </b> </p> <p>You can improve performance by passing

the smallest possible bounding rectangle to the <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita#GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79/GUID-9337538E-7A53-3153-A330-968B5E4F2FF2"><apiname>RWindow::BeginRedraw()</apiname></xref> and <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-D5458F8C-E199-37DD-B821-050B749122C6"><apiname>CCoeControl::DrawNow()</apiname></xref> methods. If this still

results in poor performance, because, for example, there is a large

amount of incremental screen updating, the recommended solution in

ScreenPlay is to render to a surface. </p><p><b>High frequency rendering</b> </p><p>It is possible for a client to provide batches of redraw

drawing (that is, drawing bracketed in <codeph>BeginRedraw()</codeph> and <codeph>EndRedraw()</codeph> calls) for the same region of a

window faster than they can be displayed on the screen.

When this happens, the Window Server may discard the drawing commands

in one or more of the earlier batches of redraw drawing and simply

draw the pixels defined in a later redraw batch to the screen. This

can result in missing frames, but is only likely to be noticeable

in an application like a game that performs high-frequency rendering.</p><p>The solution is to call <xref href="GUID-643DDA78-C7A7-386D-AB3F-8710141DDDA9.dita#GUID-643DDA78-C7A7-386D-AB3F-8710141DDDA9/GUID-B431DC60-D11F-3239-8F52-4257B9B0E0C9"><apiname>RWsSession::Finish()</apiname></xref> after each <codeph>EndRedraw()</codeph> call. This slows the client

so that it does not generate pixel content faster than the Window

Server can display it on the screen. </p><p>To summarize: When you

want to ensure that every single frame is shown on the screen, call <codeph>Finish()</codeph> after each <codeph>EndRedraw()</codeph> call.</p> <p><b>Delay of execution </b> </p> <p>After you migrate non-redraw

drawing to redraw drawing, the execution of the drawing operations

is deferred compared to previously. This means that if the arguments

of the drawing operations result in a panic, the panic also appears

later than before. </p> <p><b>CRemoteGc usage </b> </p> <p>Device

creators can use <xref href="GUID-9965F9F1-ACEE-398C-BDAB-B3D5D61D4EBE.dita"><apiname>CRemoteGc</apiname></xref> to create a data buffer

containing a series of drawing operations. These can then be executed

to display the drawing on the screen by using <xref href="GUID-B25363C2-8757-36D4-93D2-769CDA1BDF86.dita#GUID-B25363C2-8757-36D4-93D2-769CDA1BDF86/GUID-35C68602-7684-3A1C-ADA2-9C8822E18DE0"><apiname>CCommandBuffer::Play()</apiname></xref>. </p> <p>For example, consider a weather program that has one sub-system

responsible for deciding the drawing operations that comprise a weather

symbol and another sub-system that places weather symbols at different

locations on the screen. Here we potentially have a Cloud symbol buffer

created once and played many times. </p> <p>The points to note are: </p> <ul>

<li id="GUID-A8DCB4C4-9D80-5F0F-A894-512A432A9E11"><p>Any <codeph>BeginRedraw()</codeph> and <codeph>EndRedraw()</codeph> commands

that are placed into the <codeph>CRemoteGc</codeph> buffer are <b>not</b> transferred to the target window when the buffer is played </p> </li>

<li id="GUID-4D5FB2B7-44F0-50F1-AB60-3A12B0AF287B"><p>You <b>must</b> bracket any call to <xref href="GUID-B25363C2-8757-36D4-93D2-769CDA1BDF86.dita#GUID-B25363C2-8757-36D4-93D2-769CDA1BDF86/GUID-35C68602-7684-3A1C-ADA2-9C8822E18DE0"><apiname>CCommandBuffer::Play()</apiname></xref> within <codeph>BeginRedraw()</codeph> and <codeph>EndRedraw()</codeph> calls. </p> </li>

</ul> <p>It may be puzzling to understand why any <codeph>BeginRedraw()</codeph> and <codeph>EndRedraw()</codeph> commands would be placed in the <codeph>CRemoteGc</codeph> buffer at all. This facility is to allow the commands

already in the <codeph>CRemoteGc</codeph> buffer to be replaced with

new drawing operations. In the weather example, it might be done to

allow a "7<sup>o</sup> Celsius" label to partially occlude a cloud

graphic placed earlier into the <codeph>CRemoteGc</codeph> buffer. </p> </section>

<section id="GUID-D263317D-ED6F-45D2-8F89-95891F6B7540"><title>Configuring

the emulator to panic clients that ignore the           convention</title> <p>In both ScreenPlay and the non-ScreenPlay variant, you can configure

the TechView emulator to panic clients that ignore the convention

(described above) that all drawing operations are performed as redraw

drawing. </p> <p>To enable this feature, add the following line to

the <filepath>epoc32/data/epoc.ini</filepath> file: </p> <codeblock id="GUID-50D79AD9-BB89-5A49-91A3-DDDAF9543551" xml:space="preserve">debug_wserv_exe_EnforceRedrawCallingConvention 1</codeblock> <p>This feature is disabled if you do not specify this parameter

or if you set it to zero, like this: </p> <codeblock id="GUID-ACCA692B-42C6-593E-BA02-43D2EF52E9C8" xml:space="preserve">debug_wserv_exe_EnforceRedrawCallingConvention 0</codeblock> <p>The feature takes effect in debug emulator (WINSCW) builds only—it

never affects ARM builds. </p> <p>When a Window Server panic code

79 (<codeph>EWservPanicWindowBeginRedrawNotCalled</codeph>) then occurs,

it means that a non-redraw drawing operation has occurred. </p> <p>However, the Window Server buffers client requests. This means that

the drawing that is at fault may have been issued by the client some

time earlier. It is therefore helpful to enable <i>autoflushing</i>, because this removes the buffering between the client issuing the

drawing operations and the Window Server processing them. This makes

it easier to identify the drawing operation that is at fault. </p> <p>There are three ways to enable autoflushing: </p> <ol id="GUID-22E4AACD-5718-5774-B9B1-BF28D287ED76">

<li id="GUID-EF604CC8-5138-5747-AD4F-CA8BE8DC335A"><p>To enable autoflushing

globally across all clients, define <codeph>__AUTO_FLUSH</codeph> in <filepath>client/client.h</filepath> and re-compile the Window Server code. </p> </li>

<li id="GUID-ACC8C257-1530-5808-9838-B905D2483455"><p>To enable autoflushing

in your client-side code only, call <xref href="GUID-643DDA78-C7A7-386D-AB3F-8710141DDDA9.dita#GUID-643DDA78-C7A7-386D-AB3F-8710141DDDA9/GUID-85232F8B-9FB8-3E19-9ECD-E64B63748A11"><apiname>RWsSession::SetAutoFlush(ETrue)</apiname></xref>. </p> </li>

<li id="GUID-3CCA44EB-5375-59AA-B96B-E3C3E241D88B"><p>To enable autoflushing

on an ad hoc basis in the TechView emulator, press <userinput>Ctrl-Alt-Shift-F</userinput>. </p> </li>

</ol> </section>

</conbody><related-links>

<link href="GUID-484B51EC-2209-5492-8E9C-9D792AB0DF35.dita"><linktext>Graphics

and Drawing </linktext></link>

<link href="GUID-6C16417B-5B37-5310-B59A-750D971AA6D4.dita"><linktext>The

UI Control Framework (CONE)</linktext></link>

</related-links></concept>