| Re: Help with pick menu widget code [message #12879 is a reply to message #12877] |
Wed, 16 September 1998 00:00   |
davidf
Messages: 2866
Registered: September 1996
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Senior Member |
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Doug Larson (djl@REMOVECAPSloki.srl.caltech.edu) writes:
> Well, I did what David told me, and of course it works :)
Well, you're lucky. I figured your chances were better
than average at most when I fired the answer off at
11:00 last night. :-)
> The second part with the "Widget_Info" still confuses me as to why it
> works.
Here is the code:
IF (Widget_Info(event.top, /Valid_ID) THEN $
WIDGET_CONTROL, event.top, SET_UVALUE= pickPtr, /NO_COPY
You are probably confused because I did the usual advanced IDL
thing of combining two commands in one. Widget_Info is a function
that returns (in this case) either a zero or one based on whether
the event.top widget is still a valid widget or not. I'm using the
return value as the test in the IF statement. If the widget ID is valid,
I put something in it's user value. If the widget ID is not valid, I
just skip this statement.
> With the destruction of the top-level base preceding the SET_UVALUE, how
> does PickFinish ever know what was done to the values in the pickPtr? I
> guess I still have not grasped this whole event driven style of code yet.
To tell you the truth, PickFinish doesn't even care. :-)
Each time you go through *any* of your event handlers and do
something to the data pointed to by pickPtr, you are changing
the thing pointed to, not the pointer itself.
And the thing pointed to exists someplace else. (It exists on
the HEAP, which is where all pointers and objects live.) What
you have in this program is a *reference* to that space. When
you destroy that reference, you do not destroy the things that
the reference points to. This is both good news and bad
news, although I would say it is mostly bad news for
your program at the moment.
I say this because it looks like to me that when you
destroy that top-level base you are destroying the only
reference you have to that stuff stored in the pointer.
This results in a condition we call "leaking memory".
In other words, each time you run this program some of
your memory gets allocated and then leaks away as you
destroy the only way you have to get to it. Over time,
you will find that you have no more memory to do anything
useful. (And Rose talks about cryptic error messages. You
should see this one!).
To clean this pointer up, I recommend you use a CLEANUP
routine. The purpose of the cleanup routine is to destroy
pointer and object references, delete pixmaps, and generally
do the sorts of things that need to be done when a program
exits. This routine can be assigned to the top-level
base at the time you call XMANAGER with the CLEANUP
keyword:
XMANAGER, 'picked', pickTLB, GROUP_LEADER = pickTLB, $
CLEANUP='PICKMENU_CLEANUP'
The routine (and please place *all* your event handler and
other routines IN FRONT OF your PickMenu routine. That way
your program will work correctly when it is called and
you won't have to individually compile it before you use
it.) is written like this:
PRO PICKMENU_CLEANUP, pickTLB
Widget_Control, pickTLB, Get_UValue=pickPtr
Ptr_Free, pickPtr
END
When the TLB dies, this routine is called. It's sort of
like calling a priest to issue last rites. The TLB is "mostly
dead" to use one of my favorite phrases from the movie "The
Prince's Bride". All the TLB can do is whisper the name of
the thing in its user value. :-) You use this name to your
own advantage, in this case freeing both the pointer and the
data the pointer points to, thereby avoiding memory leakage.
> Would it be better to have the destruction "percolate up" to
> a handler for the pickTLB? If so, how do I do this?
Better, I think, to have a separate event handler for the
Done button. I would assign it when I created the button,
like this:
done = WIDGET_BUTTON(finishbase, VALUE = 'DONE', $
Event_Pro='Out_of_Here')
It would be written like this (provided I had written the
Cleanup routine as I recommended above):
PRO Out_Of_Here, event
Widget_Control, event.top, /Destroy
END
Cheers,
David
P.S. I would *really* give that event handler the name
"PICKMENU_OUT_OF_HERE" if I was trying to write code that
works all of the time. :-)
----------------------------------------------------------
David Fanning, Ph.D.
Fanning Software Consulting
E-Mail: davidf@dfanning.com
Phone: 970-221-0438, Toll-Free Book Orders: 1-888-461-0155
Coyote's Guide to IDL Programming: http://www.dfanning.com/
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