Symbian3/SDK/Source/GUID-7C75642D-C0C9-5392-9354-711F42DC2C61.dita
author Dominic Pinkman <dominic.pinkman@nokia.com>
Fri, 11 Jun 2010 12:39:03 +0100
changeset 8 ae94777fff8f
parent 7 51a74ef9ed63
child 13 48780e181b38
permissions -rw-r--r--
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<?xml version="1.0" encoding="utf-8"?>
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<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
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<!-- This component and the accompanying materials are made available under the terms of the License 
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"Eclipse Public License v1.0" which accompanies this distribution, 
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and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
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<!-- Initial Contributors:
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    Nokia Corporation - initial contribution.
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Contributors: 
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-->
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<!DOCTYPE concept
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  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
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<concept id="GUID-7C75642D-C0C9-5392-9354-711F42DC2C61" xml:lang="en"><title>Server-Initiated
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vs. Application-Initiated Redrawing</title><shortdesc>This topic introduces the reasons that the screen display may need
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to be updated and the difference between redrawing that is initiated by an
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application and that initiated by the Window Server. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
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<p> <b>Variant</b>: Both (ScreenPlay and non-ScreenPlay). <b>Target
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audience</b>: Application developers. </p>
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<section id="GUID-D5FE0214-ECEB-4C0F-84C9-2BB44C19E477"><title>Introduction</title> <p>The Window Server maintains the UI
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display taking into account the region, ordinal position and visibility requirements
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of all displayable windows. The UI may be updated for several reasons: </p> <ul>
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<li id="GUID-017DF20B-5881-5B8A-A8A3-0019614535B7"><p>at direct application
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request by an application drawing to a window through a <xref href="GUID-0AEE5955-C530-35F1-A904-69183331B294.dita"><apiname>CWindowGc</apiname></xref>  </p> </li>
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<li id="GUID-8B9CFD40-0ECB-5F0D-92BA-F8EB327B31CA"><p>due to regions of the
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display becoming invalid </p> </li>
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<li id="GUID-6F5C9D10-BCE4-5F60-9F85-BE69C4C58F2C"><p>due to actions internal
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to the Window Server, such as pointer cursor movements. </p> </li>
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</ul> <p>When a window is moved, resized, or otherwise manipulated by an application,
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it may cause parts of other windows to become invalid so that they must be
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redrawn. The Window Server maintains a list of all invalid regions and causes
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them to be redrawn in one of several ways: </p> <ul>
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<li id="GUID-2FFC86C2-0971-539B-BD21-E95E9FEB4202"><p>for a region with no
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window, the Window Server redraws the invalid region in the default background
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color </p> </li>
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<li id="GUID-B0289B7D-97E2-5FBD-9983-130A5109FDFF"><p>for a blank window,
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the Window Server redraws the invalid region in the window's color </p> </li>
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<li id="GUID-931BDE7B-4C56-5350-A26B-F89CCBA72DE6"><p>for a backed-up window,
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the invalid region is redrawn from a backup bitmap maintained by the Window
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Server </p> </li>
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<li id="GUID-24155452-0F5D-508E-B04D-96E06FD0913F"><p>if the invalid region
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is owned by an animation DLL, the Window Server invokes the animation DLL
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directly to redraw the area: the region is then valid again </p> </li>
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<li id="GUID-2A5080AF-1415-5CBF-B775-D547D9C3F222"><p>for a normal <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita"><apiname>RWindow</apiname></xref>,
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whenever possible, the Window Server replays the sequence of drawing commands
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stored in the redraw stores. However, sometimes this is not possible, because,
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for example, the redraw stores have been lost due to low memory conditions.
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In these circumstances, the Window Server generates a redraw event and sends
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it to the application. This is described in more detail below. </p> </li>
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</ul> </section>
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<section id="GUID-50054B7B-FF18-40B2-A582-B90FE66D269F"><title>Window Server-initiated redrawing</title> <p>When the Window
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Server needs to repaint a standard <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita"><apiname>RWindow</apiname></xref> window and it
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is not possible to do this by replaying the drawing commands stored in the
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redraw stores, it sends a <xref href="GUID-B5F16BF3-569D-3985-AAB7-439E3410468D.dita"><apiname>TWsRedrawEvent</apiname></xref> message to the
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client. </p> <p>It is part of the client's contract with the Window Server
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to listen for these events and respond to them by performing a redraw. The
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normal way for a client application to do this is through an active object,
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which is sometimes called a <b>redrawer</b>. This is normally provided by
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the application framework and should have a lower priority than the active
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objects for standard events. The client issues an asynchronous request for
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the next redraw event, calling the <xref href="GUID-643DDA78-C7A7-386D-AB3F-8710141DDDA9.dita#GUID-643DDA78-C7A7-386D-AB3F-8710141DDDA9/GUID-DADAFFD6-4B63-35EE-8F50-F397C119CC27"><apiname>RWsSession::RedrawReady()</apiname></xref> function. </p> <p>The
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redrawer's <xref href="GUID-12C281FF-546C-318D-8783-F26B0F619E11.dita"><apiname>RunL()</apiname></xref> function (which handles the completed
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request) calls <xref href="GUID-643DDA78-C7A7-386D-AB3F-8710141DDDA9.dita#GUID-643DDA78-C7A7-386D-AB3F-8710141DDDA9/GUID-6008C20D-AA95-3803-8B57-54948FA61616"><apiname>RWsSession::GetRedraw()</apiname></xref>. This gets the <xref href="GUID-B5F16BF3-569D-3985-AAB7-439E3410468D.dita"><apiname>TWsRedrawEvent</apiname></xref>,
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which contains the handle of the window and the rectangle within it that needs
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to be redrawn and then does the redraw. This involves: </p> <ol id="GUID-078DF0F5-E856-5FFF-ADEB-1A09B4BBDFB0">
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<li id="GUID-75A15778-D570-5B5D-A07E-5660FFCAB00F"><p>Calling <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> on
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the relevant window and passing in the rectangle. </p> </li>
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<li id="GUID-C63A002A-B433-5054-BD73-0368E2FB6C1B"><p>Issuing the <xref href="GUID-0AEE5955-C530-35F1-A904-69183331B294.dita"><apiname>CWindowGc</apiname></xref> drawing
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commands that define how to draw the area of the window specified by the rectangle
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passed in the <xref href="GUID-B5F16BF3-569D-3985-AAB7-439E3410468D.dita"><apiname>TWsRedrawEvent</apiname></xref>. These drawing commands must
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exactly match what was drawn originally, because the Window Server clips the
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drawing to the passed rectangle. </p> </li>
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<li id="GUID-AA03D7DD-0B8A-5241-87B1-D45BC8AAA369"><p>Calling <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> </li>
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</ol> <p>At the end of the redrawer’s <xref href="GUID-12C281FF-546C-318D-8783-F26B0F619E11.dita"><apiname>RunL()</apiname></xref>, a call is
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made to <xref href="GUID-643DDA78-C7A7-386D-AB3F-8710141DDDA9.dita#GUID-643DDA78-C7A7-386D-AB3F-8710141DDDA9/GUID-DADAFFD6-4B63-35EE-8F50-F397C119CC27"><apiname>RWsSession::RedrawReady()</apiname></xref> to enable the client
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to listen for further requests from the Window Server. </p> <p>The following
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diagram demonstrates that typically the <xref href="GUID-12C281FF-546C-318D-8783-F26B0F619E11.dita"><apiname>RunL()</apiname></xref> starts with
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a <xref href="GUID-C57CFECB-72CD-39EB-A83E-82A9E985F56B.dita"><apiname>GetRedraw()</apiname></xref> and ends with a call to <xref href="GUID-E3F0CB70-58E4-32FD-9828-71DF2F9976D3.dita"><apiname>RedrawReady()</apiname></xref>. </p> <fig id="GUID-B092D136-8C18-5237-A095-F8C3BBFFDDB4">
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<title>              Window Server-initiated redraw            </title>
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<image href="GUID-0528A294-AC1B-5CDD-B59C-A105F39D748F_d0e190203_href.png" placement="inline"/>
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</fig> <p>A redrawer's <codeph>RunL()</codeph> function must <b>not</b> perform
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any non-redraw drawing (that is, any drawing operations that are not bracketed
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within the calls to <codeph>BeginRedraw()</codeph> and <codeph>EndRedraw()</codeph>).
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If it does, this causes a panic with code 79 (<codeph>EWservPanicWindowBeginRedrawNotCalled</codeph>)
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in the ScreenPlay variant. This is to avoid the infinite loop that would otherwise
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be caused by an endless cycle of the non-redraw drawing within the <codeph>RunL()</codeph> triggering
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a Window Server-initiated redraw. </p> </section>
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<section id="GUID-0238F737-3D2B-4811-8E46-561922210143"><title>Application-initiated redrawing</title> <p>An application
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initiates redrawing when it wants to update the UI to reflect a change of
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state. There are two scenarios, which we describe under separate headings
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below. </p> <p><b>The "draw now" scenario </b> </p> <p>The "draw now" redraw scenario is
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when an application wants to push a state change to a window immediately.
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Typically the application does this by calling <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>.
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However, it is possible to achieve the same results using <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita"><apiname>RWindow</apiname></xref> and <xref href="GUID-0AEE5955-C530-35F1-A904-69183331B294.dita"><apiname>CWindowGc</apiname></xref> calls
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directly. To illustrate how to do this, consider a digital clock application
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that updates the time displayed in the window from 12:02 to 12:03. </p> <p>The
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application does the following: </p> <ol id="GUID-D9D43E2B-B29D-5C6F-A1AB-ABA192D3D562">
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<li id="GUID-C59E802D-0D4F-595B-9F1C-BED1090EE7D7"><p>Calls <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita#GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79/GUID-28202F81-52FE-30F5-8B8C-ABED0915822E"><apiname>RWindow::Invalidate()</apiname></xref>,
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optionally passing in the bounding rectangle. When called without passing
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in a rectangle, this function invalidates the entire window. </p> </li>
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<li id="GUID-491CB745-6B47-5656-874E-A87DE1BE6254"><p>Calls <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>.
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If the application passed a bounding rectangle to <codeph>Invalidate()</codeph>,
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it must pass the same bounding rectangle to <codeph>BeginRedraw()</codeph>.
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If the application did not pass a bounding rectangle to <codeph>Invalidate()</codeph>,
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it must not pass one to BeginRedraw(). </p> </li>
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<li id="GUID-8CA69A7D-0387-5604-9C10-983FB50514B5"><p>Issues the <xref href="GUID-0AEE5955-C530-35F1-A904-69183331B294.dita"><apiname>CWindowGc</apiname></xref> draw
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commands. </p> </li>
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<li id="GUID-B2474EE6-CC03-55A8-9972-071B76512B26"><p>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> </li>
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</ol> <fig id="GUID-6E2637DC-F92A-56DE-BB07-F109F5CA681B">
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<title>              Application-initiated redrawing, "draw now" scenario
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           </title>
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<image href="GUID-4BC95F70-2F3F-5CFE-B057-0C5A97430573_d0e190311_href.png" placement="inline"/>
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</fig> <p><b>The "draw deferred" scenario </b> </p> <p>The "draw deferred" redraw scenario
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is when an application simply marks a window or a rectangle within a window
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as invalid. This eventually triggers a Window Server-initiated redraw. Typically
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the application does this 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>.
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However, it is also possible to do this by calling <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita#GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79/GUID-28202F81-52FE-30F5-8B8C-ABED0915822E"><apiname>RWindow::Invalidate()</apiname></xref>. </p> <fig id="GUID-8B78CE09-EE92-50B3-8000-731BD9C20890">
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<title>              Application-initiated redrawing, "draw deferred" scenario
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           </title>
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<image href="GUID-21EF9E6F-A0BF-5EB2-9E3F-CA6A011D4C89_d0e190335_href.png" placement="inline"/>
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</fig> <p>This approach is useful when the data to be displayed might be updated
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frequently but it is not necessary to update the display instantaneously. </p> <p>To
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illustrate how it works, consider a window that has a rectangle that is used
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to display icons (a star, a cross and a triangle) that represent three different
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events that can occur in the application. Suppose the events can occur in
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quick succession. If you use the "draw now" approach when each event occurs,
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the Window Server updates the display separately for each one, as shown in
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the following diagram. </p> <fig id="GUID-04BCFA21-8D44-5FEA-BE29-57B40AC9CF99">
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<title>              Icons displayed using the "draw now" approach       
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    </title>
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<image href="GUID-89185EBD-223F-5F52-B1CA-9F9692F7D21F_d0e190349_href.png" placement="inline"/>
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</fig> <p>However, if you use the "draw deferred" approach when each event
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occurs, the Window Server may not update the display for each one. For example,
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the Window Server does not update the display when it is superceded by a later
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call. </p> <fig id="GUID-1E0E2A3C-1E63-58F6-940F-0D439FA6C721">
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<title>              Icons displayed using the "draw deferred" approach  
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         </title>
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<image href="GUID-EDC405DC-F222-59CD-836E-180F8E1D2D11_d0e190360_href.png" placement="inline"/>
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</fig> <p>If it is important that the display is updated immediately, you
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would generally use the "draw now" approach. However, if a short delay in
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the display being updated is unlikely to inconvenience the user, the "draw
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deferred" approach might be appropriate and has a performance advantage. </p> </section>
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</conbody><related-links>
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<link href="GUID-484B51EC-2209-5492-8E9C-9D792AB0DF35.dita"><linktext>Graphics
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and Drawing </linktext></link>
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</related-links></concept>