Symbian3/SDK/Source/GUID-2BF409EA-82BF-407C-B048-DA0973B7F61D.dita
author Dominic Pinkman <Dominic.Pinkman@Nokia.com>
Thu, 21 Jan 2010 18:18:20 +0000
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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-2BF409EA-82BF-407C-B048-DA0973B7F61D" xml:lang="en"><title>Optimizing
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feedback latency</title><prolog><metadata><keywords/></metadata></prolog><conbody>
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<p>The general latency requirement for touch feedback is 30 milliseconds.
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For direct feedback it is not always possible to achieve this, as in high
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CPU load cases it may take more than 30 ms already before the pointer event
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reaches the application. </p>
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<p>However, you can also affect the latency in your application: the maximum
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additional latency for direct feedback is as long as it takes for the longest
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running <codeph>RunL</codeph> of any active object running in application’s
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thread to execute. Hence keeping all <codeph>RunL</codeph> functions short
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improves the feedback latency (and of course also improves the overall responsiveness
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of the application). </p>
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<p>Another way of improving the latency is optimizing redrawing in such a
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way that only the necessary area of the screen is drawn. </p>
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<p>For example, if there are 20 buttons in the application and one of them
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is pressed down by a pointer event, then only that button should be redrawn
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and not the whole screen. This optimization improves both direct and area
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registry based feedback, because the window server cannot receive pointer
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events from the touch driver in case it is performing a draw operation, and
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unnecessarily massive draw operations can thus have significant effect on
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the latency.</p>
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<p>Finally, it is recommended that you trigger direct feedback in <codeph>HandlePointerEventL</codeph> as
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the first thing before doing any other processing, such as redrawing.</p>
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</conbody></concept>