Describes asynchronous services provided by the client-server framework.
The majority of Symbian platform asynchronous services are provided by
servers written in accordance with the Symbian platform client-server framework.
A server is a thread running in user mode, which services requests from client
threads also running in user mode. An example of asynchronous service provided
by a server is the EventReady(TRequestStatus* aStat) function
provided by the RWsSession class — the client API to
the window server. This function requests an event from the window server,
which the user program can extract and analyse using other RWsSession functions.
As with services provided by the kernel, server-provided services are presented
to clients through a client API, typically an R class such
as RWsSession, derived from RSessionBase.
When the RSessionBase is opened, a server-side object is
created and the client’s thread id is noted. Request functions are converted
into an inter-thread message which encapsulates all the parameters. When request
functions complete, the server posts the request status and signals the client
thread’s request semaphore.
As with services provided by the kernel, the process of requesting asynchronous service from a server, and handling its completion, involves messaging and inter-thread communication. Symbian platform provides efficient services to support these.