How to implement a client interface with subsessions

A client side subsession is represented by an instance of a class derived from RSubSessionBase which provides the behaviour for:

• creating a subsession in the server

• sending messages to the subsession.

In the following code fragment, the class RCounter, derived from RSubSessionBase, represents the client side subsession with a server, called the count server in this example. The assumption is made that the client has already established a session with the server as represented by the RCountServ class.

class RCounter : public RSubSessionBase
 {
public:
 TInt Open(RCountServ &aServer);
 void SetFromString(TDesC& aString);
 void Close();
 void Increase();
 void Decrease();
 void IncreaseBy(TInt anInt);
 void DecreaseBy(TInt anInt);
 void Reset();
 TInt CounterValue();
 };

class RCountServ : public RSessionBase
 {
public:
 RCountServ();
 TInt Connect();
 ...
 TVersion Version() const;
 void Close();
 };

The important points in this example are:

• Open() creates a subsession within the client-side session. A reference to the client-side session is specified as the function’s single argument. The function calls RSubSessionBase::CreateSubSession() which, in turn, causes a server side subsession object to be created and its handle number to be returned.

• Client subsession interface functions, such as CounterValue(), send a specific message to the server.

TInt RCounter::CounterValue()
 {
 TInt res=0;
 TPckgBuf<TInt> pckg;
 if (SubSessionHandle())
  {
  TAny *p[KMaxMessageArguments];
  p[0]=(TAny *) &pckg;
  SendReceive(ECountServValue,&p[0]);
  res = pckg();
  }
 return res;

}

• Close() closes the subsession.

• The operation code passed to RSubSessionBase::CreateSubSession() must be interpreted by the server as a request to create a subsession.

• When sending a message to the server with a call to RSubSessionBase::SendReceive(), only three elements in the message argument array can be used. This function always uses the fourth argument to hold the client subsession handle number which is used to identify the corresponding server side subsession object. The message argument array is subsequently passed to a call to RSessionBase::SendReceive().

• The operation code passed to RSubSessionBase::CloseSubSession() must be interpreted by the server as a request to close the subsession.