--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Symbian3/SDK/Source/GUID-C73318E7-44E2-5F42-BDB0-AAE9BD72BF5C.dita	Thu Jan 21 18:18:20 2010 +0000
@@ -0,0 +1,63 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
+<!-- This component and the accompanying materials are made available under the terms of the License 
+"Eclipse Public License v1.0" which accompanies this distribution, 
+and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
+<!-- Initial Contributors:
+    Nokia Corporation - initial contribution.
+Contributors: 
+-->
+<!DOCTYPE concept
+  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
+<concept id="GUID-C73318E7-44E2-5F42-BDB0-AAE9BD72BF5C" xml:lang="en"><title>Using
+TCallBack</title><shortdesc>This document describes the TCallBack class.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
+<p>The class encapsulates a pointer to a function which takes an argument
+of type <codeph>TAny*</codeph> and returns a <codeph>TInt</codeph>. The class
+is generally useful but, in particular, simplifies the programming interface
+of the <codeph>CIdle</codeph> and <codeph>CPeriodic</codeph> classes.</p>
+<p>Given a suitable function and a pointer to an object, a callback is constructed
+simply. The function must be a non-member function or a static member of a
+class. For example:</p>
+<codeblock id="GUID-B70261EB-D95A-52EE-954E-D7062BF8A7EF" xml:space="preserve">TInt Foo(TAny *);  // a non-member function
+X* pX=new X;       // a class X object</codeblock>
+<p>or, as a static member of class <codeph>X</codeph>:</p>
+<codeblock id="GUID-DA631174-BFA6-5348-9807-5417DCA17523" xml:space="preserve">TInt X::Foo(TAny *); // a static function of class X
+X* pX=new X;         // a class X object</codeblock>
+<p>A callback function returns a true value to indicate whether it should
+be called again. This is important when used with the <codeph>CIdle</codeph> and <codeph>CPeriodic</codeph> classes.
+The following code fragment shows the programming paradigm:</p>
+<codeblock id="GUID-58BF3CFB-D8E3-549B-B30F-568F2C566295" xml:space="preserve">TCallBack cb(Foo,pX);  // construction of the callback</codeblock>
+<codeblock id="GUID-3B88A62E-0350-53DF-9479-D5749D96E2FD" xml:space="preserve">for (;;)
+ {
+ if (!cb.CallBack()) // invoke callback until it returns
+  {    // a false value
+  break;
+  }
+ }</codeblock>
+<p>Calling <codeph>cb.CallBack()</codeph> results in a call to the callback
+function <codeph>Foo()</codeph> passing it the pointer <codeph>pX</codeph>.</p>
+<p>A common requirement is for the callback function to be a non-static member
+of a class. This can be implemented by passing, to the callback function,
+a pointer to an instance of the class of which it is a static member. For
+example:</p>
+<codeblock id="GUID-85DDB62D-FDDC-5B04-9F9C-D5C20C8735DB" xml:space="preserve">class X
+ {
+ static X* NewL();
+ static TInt Foo(TAny* pX);
+private:
+ TInt DoFoo();
+ ...
+ }</codeblock>
+<p>where the static function <codeph>Foo()</codeph> is implemented as:</p>
+<codeblock id="GUID-D3FD2FA5-960D-540C-9BCA-0206004CC92C" xml:space="preserve">static TInt X::Foo(TAny* pX)
+ {
+ return ((X*)pX)-&gt;DoFoo();
+ }</codeblock>
+<p>Typically, create an instance of class <codeph>X</codeph> and, at some
+later stage, create the callback:</p>
+<codeblock id="GUID-4E685C8E-9185-50FA-9402-A2B4351C3AEA" xml:space="preserve">...
+X* pX = X::NewL();
+...
+TCallBack cb(Foo,pX);
+...</codeblock>
+</conbody></concept>
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