// Copyright (c) 2006-2009 Nokia Corporation and/or its subsidiary(-ies).

// All rights reserved.

// This component and the accompanying materials are made available

// which accompanies this distribution, and is available

//

// Initial Contributors:

// Nokia Corporation - initial contribution.

//

// Contributors:

//

// Description:

// An agent plugin may have to service an asynchronous request, for example when

// <code>ContentAccess::CAgentManager::NotifyStatusChange()</code> is called.

// If the agent plugin must make an asynchronous SendReceive() call to service the request

// then it must be careful to ensure that any memory that is passed as an argument

// in the call is still valid when the agent server that receives the call processes

// and uses the memory.

// There are two ways that this can be achieved:

// <hr>

// If the agent plugin cannot guarantee that a variable to be passed in the asynchronous

// SendReceive() call will still be in scope when the agent server comes to access and use

// it then the agent plugin should store a local heap copy of the data and pass this

// in the call instead. It is the responsibility of the agent plugin to maintain this heap 

// memory and delete it when appropriate. Depending on how and when the agent server uses 

// the memory, it may be safe to delete the memory after the asynchronous call has 

// been accepted, or not until the asynchronous request has completed.

// For example, an agent plugin could implement the API <code>ContentAccess::CAgentManager::NotifyStatusChange()</code> 

// as illustrated below. Note that for this API the agent plugin can make no assumption about the 

// scope of the descriptor passed to aURI. 

// void CTestAgentManager::NotifyStatusChange(const TDesC& aURI, TEventMask aMask, TRequestStatus& aStatus)

// HBufC* uri = aURI.Alloc();

// if(uri)

// // store the heap variable in a local array

// iAsyncDataArray.Append(uri); // takes ownership of uri

// SendReceive(EManagerNotifyStatusChange, TIpcArgs(uri,aMask),aStatus);		

// <hr>

// Alternatively, the agent plugin can use the variables that are in scope at the time of the 

// asynchronous SendReceive() call if it makes a synchronous SendReceive() call afterwards,

// within the same function scope, as illustrated below. The synchronous message can 

// be a 'no operation' in the agent server.

// void CTestAgentManager::NotifyStatusChange(const TDesC& aURI, TEventMask aMask, TRequestStatus& aStatus)

// SendReceive(EManagerNotifyStatusChange, TIpcArgs(&aURI,aMask),aStatus);

// // this call doesn't have to be immediately after the asynchronous call, but within this function

// SendReceive(ENoOp,TIpcArgs(NULL));

// The synchronous call causes the message queue to be flushed into the agent server before the thread returns from the 

// function and unwinds the call stack. The intention is that the agent server will only complete the second (synchronous) 

// message after receiving and doing initial processing of the first (asynchronous) message, which may include, for 

// example, reading the uri descriptor. 

// However, an obvious disadvantage of this pattern is that it incurs a second IPC call, and so may degrade performance.<br>

// Moreover, there are several caveats which must hold true in order for the pattern to work:

// 1. The kernel delivers messages in the order that they are sent (this is currently true).<br>

// 2. The agent server is guaranteed to finish processing the first message before completing the second message.<br>

// This requires understanding of the agent server implementation.<br>

// 3. After initial processing of the first (asynchronous) message the agent server does not need to 

// access the memory supplied in the message again. This requires understanding of the agent server implementation.<br>

// 4. The synchronous call is a request that has no effect on the state of the agent server - a 'no operation'

// may or may not be possible.

// <b> 

// For these reasons, this pattern should only be used as a last resort - for example, if the agent plugin cannot store member data in its class for compatibility reasons.

// <b>

// 

//

/**

 @page CAFAsyncSendReceive Making an asynchronous SendReceive() call in an agent plugin

 - @ref StoreLocalCopy

 - @ref SyncSendReceive

 @section StoreLocalCopy Storing a local heap copy of transient data

 @code

 @endcode

 @section SyncSendReceive Following the asynchronous call with a synchronous call

 @code

 @endcode

*/