Re: [naviserver-devel] running callbacks

[Stephen D. <sd...@gm...>]

On 7/13/06, Zoran Vasiljevic <zv...@ar...> 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.
>


Ns_TclCallbackArgProc is only for reporting purposes, so it doesn't
much matter. There should be lots of tests for this in the test suite
for all the existing systems which use Tcl level callbacks.  Maybe
have a look if this particular cases is tested for...



> 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).


Why can't you initialise resources when you register the callback?  I
don't think I understand the problem here...



> 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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