| Re: passing parameters from base to base [message #33109 is a reply to message #33004] |
Tue, 03 December 2002 09:14  |
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Stein Vidar Hagfors H[2]
Messages: 28
Registered: October 2002
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Junior Member |
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"Pavel A. Romashkin" <pavel_romashkin@hotmail.com> writes:
> JD Smith wrote:
>>
>> In an ideal system, ObjMsg objects would find each other and setup
>> their own intercommunication themselves. You get into lots of
>> chicken-and-egg dependency issues in this case though.
>
> This is exactly what I was talking about :-)
I may not be getting exactly what you're talking about, but this is
exactly where I think a singleton might be useful: You can always
"find" it (it'll be created & initialized if it doesn't exist!) as
long as you know it's name, e.g. (pardon any mis-programming here, I
haven't been keeping up with objects in daily life!)
;; Locate the message center
dummy = obj_new("message_center",object=MsgCtr)
MsgCtr->Register,self,"mainprog",uniqkey ;; Key allows multiple instances
;; of mainprog
;; The LocateObjMsg could generate a proxy object if ObjMsg
;; hasn't registered yet! The key makes sure we don't pick a
;; wrong instance of ObjMsg (that has already been connected
;; with another self-class object)
objmsg = MsgCtr->LocateObjMsg("ObjMsg",uniqkey)
if objmsg->isproxy() then begin
objmsg->queue_state,/on ;; Keep all messages I send
objmsg->inform_me_when_registered,self,"through_this_method "
end
objmsg->send_message,self,"Send me data anytime","through_this_method"
Etc, etc.. Now, when "ObjMsg" registers with the message center, it
(MsgCtr) will generate a call to the above object's
"through_this_method" method, with information that the proxy "objmsg"
is now replaced with a true ObjMsg instance, with so-and-so unique
key. It will then dump all the queued messages from the proxy onto
the registering ObjMsg (maybe triggered by ObjMsg calling a
MsgCtr->deliver_pending_messages method, gives you more control over
whether or not the object is fully initialized).
The number of variations on this scheme is endless, and details will differ
according to what kind of messaging system is already implemented. However,
it sketches out a method to deal with chicken-and-egg problems.
One key problem might be how to use the unique keys (e.g. system time
at object creation) to enable creation of multiple independent interlinked
systems (e.g., two display windows with separate sets of linked controls).
Only a half-thought-through solution, but I don't see that you have problems
that cannot be solved by this general approach
--
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Stein Vidar Hagfors Haugan
ESA SOHO SOC/European Space Agency Science Operations Coordinator for SOHO
NASA Goddard Space Flight Center, Tel.: 1-301-286-9028
Mail Code 682.3, Bld. 26, Room G-1, Cell: 1-240-354-6066
Greenbelt, Maryland 20771, USA. Fax: 1-301-286-0264
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