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<concept xml:lang="en" id="GUID-8DB1C618-597C-560C-95A2-C0AB2CEBB027"><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><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-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. </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-E50F6710-937C-32EC-A101-01E2EABD0160"><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_d0e212439_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_d0e212460_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_d0e212469_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-DE01101D-52D5-37DC-A397-94C23CBB36CE"><apiname>CCoeControl::DrawNow(TRect&amp;)</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. Sometimes this causes some subtle changes in behavior, as explained below. </p> </section> <section><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-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><title>Tips for migrating old code</title> <p>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. The main points to note are provided below under separate subheadings. </p> <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>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><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>