Re: Generic UI question

[George Gatling (Contractor)]

The way I manage my queues is maybe a little eccentric.... i create a 
reentrant subvi for each queue I want to use in my application.  I name 
this queue the same name as the subvi (using a property node for the VI), 
and the subvi has an input to specify whether to enque or deque (it also 
has data in, data out and timeout terminals).  I put one instance of this 
vi in the deque loop with timeout set to -1.  Then, wherever I want to 
enque data I drop instances of this subvi with the operation terminal set 
to enque.  This has some memory overhead because an instance of the 
reentrant VI is created everywhere I use the queue, but that's ok for me 
because I prefer the readability and convenience.  It is easy to see where 
data is getting into the queue be looking for instances of the queue 
subvi.  With the queue timeout set to -1 the deque operation will suspend 
until data is placed in the queue... this is what makes this approach event 
driven.  (And is what also creates the need for and exit command for the 
queue... there is no other way to stop the loop while it is suspended in 
the deque).

I will send an example by private mail.
George
At 03:37 PM 6/21/2004, you wrote:
Hi Georges,
I think I understand your approach, but it seems to me that the separate 
loop that is needed in the MainLevel.vi will have the same "problem" as 
the polling strategy I was mentioning, that is, unecessary activity (you 
can't use a queue without volontarily polling it, ie you will again need 
to relieve the CPU of unwanted load by having a Wait 100 ms in your side 
loop). Plus, how do you enqueue something in a subVi, when the queue is 
read in the TopLevel.vi ? Somehow you need a Queue RefNum global, if I am 
not mistaken.
What do you think ?
X.

George Gatling (Contractor) wrote:
One possible solution would involve using queues.  Create a queue that 
uses a cluster of an enum and a variant as the type.  Put a loop in your 
main VI that waits for elements to arrive in the queue.  When they show 
up, it uses the enum to determine which control to update and then casts 
the variant to the appropriate type and does the update.  I uses this 
method all the time very successfully for things like moving the cursor 
on a graph around, enabling/disabling menu items, updating control 
values, hiding/disabling controls or indicators and on and on.  Finally, 
one of the items of my enum is "exit"... and so when the app is exiting 
it sends the exit message to the UI Update loop to stop it so the 
application can exit gracefully.
This way you can put a -1 for the timeout on the deque and make your ui 
updates "event driven".

If this sounds like something you'd like to try but it is still not 
entirely clear what I am talking about, let me know and I will send you 
an example.

George
George Gatling
Applied Technology Division, SFA Inc.
Space Physics Simulation Chamber
US Naval Research Laboratory
202-404-5405 (phone)
202-767-3553 (fax)
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