Re: [naviserver-devel] running callbacks

[Vlad S. <vl...@cr...>]

Is it possible to retrieve callback operation from within callback 
without providing required first argument all the time.

something line ns_callback operation, ns_callback script, ns_callback ....

and in the callback and ns_callback command use special tls which will 
be set with callback struct. this way we keep compatibility and have a 
lot of ways to extend this

Zoran Vasiljevic wrote:
> Hi !
> 
> There is a call in tclcallbacks.c:
> 
> Ns_TclCallbackArgProc(Tcl_DString *dsPtr, void *arg)
> {
>      int             ii;
>      Ns_TclCallback *cbPtr = arg;
> 
>      Tcl_DStringAppendElement(dsPtr, cbPtr->script);
>      for (ii = 0; ii < cbPtr->argc; ii++) {
>          Tcl_DStringAppendElement(dsPtr, cbPtr->argv[ii]);
>      }
> }
> 
> 
> This way of constructing Tcl script works only if the
> script is a single word, like a procedure name. If the
> script consists of two or more "words" then it won't.
> Common examples are in OO sysytems where you would specify
> the class and the object to be the callback.
> Good example can be made using XOTcl like this:
> 
> Class Logger
> Logger log args {
>     puts stderr $args
> }
> 
>    ns_logctl register "Logger log" arg1 arg2
> 
> This above would actually build the Tcl line as:
> 
>    {Logger log} arg1 arg2
> 
> and this will of course not work as Tcl will not
> find the {Logger log} command.
> 
> It should have rather be:
> 
>     Logger log arg1 arg2
> 
> So the preferred way would actually look like:
> 
>      Tcl_DStringAppend(dsPtr, cbPtr->script);
>      for (ii = 0; ii < cbPtr->argc; ii++) {
>          Tcl_DStringAppendElement(dsPtr, cbPtr->argv[ii]);
>      }
> 
> Any objections to chaging this behaviour in the current code?
> I do not know if this will break anything, therefore I ask.
> 
> More...
> 
> During the general callback registration and un-registration
> it would be extremely helpful if the callbakck would "know"
> that is being registered or called regularily or un-registered.
> This way it could perform some tasks like internal caching
> of resources (on registration) or destroying the caches (on
> un-registration).
> So, I plan to do that with the log callbacks. For that I will
> "invent" the:
> 
> typedef enum {
>      unknown,
>      register,
>      execute,
>      unregister
> } Ns_CallbackOperation;
> 
> and would put this into the Ns_TclCallback structure as one
> additional element. Also I would add two wrapper functions
> 
> Ns_TclCallbackSetRegister
> Ns_TclCallbackSetUnregister
> Ns_TclCallbackSetExecute
> 
> which will just flip the bit. This is of course not a change
> we would like to propagate to existing callbacks as this will
> mean breaking the compatiblity with existing scripts which is
> always a pain in the neck. But we can use this in any new
> callbacks that we define the [ns_logctl register] being the
> obvious first candidate.
> 
> In this way the Tcl script will always be called with at least
> one argument: callback operation, followed by any number of
> other required arguments.
> 
> I hope I managed to bring you the whole picture.
> Are there any objections or better ideas how to do that?
> 
> Cheers
> Zoran
> 
> 
> 
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-- 
Vlad Seryakov
571 262-8608 office
vl...@cr...
http://www.crystalballinc.com/vlad/