| Sharing Variables within a Widget [message #8453] |
Mon, 17 March 1997 00:00  |
john Slavich
Messages: 5
Registered: March 1997
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Junior Member |
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To anyone who can help me,
I'm in the process of building my first widget program and now I'm at
an impass. This is how my widget is presently srtuctured:
-> Function 1
-> Function 2
-> PRO, Event
-> PRO (main)
Function 1 and 2 have no widgets, they are called upon to make
calculations on data set. Function 1 requires additional information
calculated in Function 2 (i.e. the data Func. 1 requires is not part of
the Return from Function 2). Since Functions can only return 1 "thing,"
I would like to store information, calculated in Function 1 and 2, into
the top event, so that I may call upon the information in other
functions or procedures which are not widgets. For example, in the PRO
event, I can call up information stored in a structure from the PRO
main, by typing:
Widget_control, event.top, Get_UValue=info
All the info I need can be accessed in the structured called "info,"
but when I do the same thing in Function 1 and 2, it doesn't work.
I hope this is making sense, I'm still the learning process.
Thanks to anyone who's willing to help.
John
jcslavic@ouray.cudenver.edu
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| Re: Sharing Variables within a Widget [message #8534 is a reply to message #8453] |
Tue, 18 March 1997 00:00  |
J.D. Smith
Messages: 214
Registered: August 1996
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Senior Member |
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David,
Thanks for correcting my oversight... Quite right. Local variables
remain local until exported back to the user value (or handle) from
which they were obtained.
Sorry for any confusion.
JD
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| Re: Sharing Variables within a Widget [message #8538 is a reply to message #8453] |
Tue, 18 March 1997 00:00 |
peter
Messages: 80
Registered: February 1994
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Member |
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john Slavich (jcslavic@ouray.cudenver.edu) wrote:
: To anyone who can help me,
: I'm in the process of building my first widget program and now I'm at
: an impass. This is how my widget is presently srtuctured:
: -> Function 1
: -> Function 2
: -> PRO, Event
: -> PRO (main)
: Function 1 and 2 have no widgets, they are called upon to make
: calculations on data set. Function 1 requires additional information
: calculated in Function 2 (i.e. the data Func. 1 requires is not part of
: the Return from Function 2). Since Functions can only return 1 "thing,"
: I would like to store information, calculated in Function 1 and 2, into
: the top event, so that I may call upon the information in other
: functions or procedures which are not widgets. For example, in the PRO
: event, I can call up information stored in a structure from the PRO
: main, by typing:
: Widget_control, event.top, Get_UValue=info
: All the info I need can be accessed in the structured called "info,"
: but when I do the same thing in Function 1 and 2, it doesn't work.
: I hope this is making sense, I'm still the learning process.
Presumably, function1 and function2 are called from the event handler.
So, retrieve the uvalue in the event handler (as you are doing) then
pass that info to f1 and f2. After f1 and f2 are done with the
structure, you'll return to the event handler, which can then put the
modified structure back into the uvalue of the widget.
This makes an appropriate clean separation between routines that build
and manage the widget, and routines that process data.
A more general comment: functions can return only one thing, but
procedures (and functions) can change as many of their arguments as you
want, since IDL used call-by-reference argument passing (most of the
time!). So it is possible to "return" as many things as you want.
Hope this helps,
Peter
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| Re: Sharing Variables within a Widget [message #8549 is a reply to message #8453] |
Mon, 17 March 1997 00:00 |
davidf
Messages: 2866
Registered: September 1996
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Senior Member |
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JD Smith writes an extremely lucid and helpful reply to John Slavich's
widget programming question. But I would like to further clarify one
of his points, which I *know* will be misinterpreted by someone.
JD writes:
> I think your difficulty is with understanding event procedures. Your
PRO,Event is the
> *only* procedure which is automatically passed an event (i.e., is the
only procedure
> which can say 'Widget_control, event.top, Get_UValue=info'). To let a
function have
> access to your info structure, simply pass it as an argument. You can do
this from
> one of two places: in your widget setup routine (where you define the
structure info),
> or in your PRO, Event routine (where your Widget_control call retrieves
info). Here's
> an example:
>
> pro my_event_handler, event
> Widget_control, event.top, Get_UValue=info
>
> answer1=func1(arg1,arg2,...,info) ; info passed on to answer1
> end
>
> and an example function ;
>
> function func1, arg1,arg2,...,info
> info.item1=some_crazy_calculation
> end
>
>
> This works because structures are passed by reference, and so setting a
field of info
> inside func1,or func2 (or both) changes the 'info' in the calling routine.
As a matter of fact, this example of JD's *won't* work exactly as he claims.
It is true that calling the function in the event handler and passing the
info structure to the function so that it can be changed in the function
will work. At least it will work IN THE EVENT HANDLER. In other words,
if you print the new info structure in the event handler after you return
from the function call, you will see the field has changed.
But if you depend on that new field value in subsequent program operations,
you will be sorely disappointed. The reason is that you have made the
change to a LOCAL VARIABLE! That is, the info structure in the event
handler is a LOCAL variable, copied into the event handler from a global
storage location (the user value of the top-level base widget). In order
to make the change "permanent", you must copy the LOCAL info structure
back to the global storage location before you leave the event handler.
This is what is missing from JD's example code, but not, I feel confident,
from his real code.
The correct sequence of events will look like this:
pro my_event_handler, event
Widget_control, event.top, Get_UValue=info
answer1=func1(arg1,arg2,...,info) ; info passed on to answer1
WIDGET_CONTROL, event.top, SET_UVALUE=info
end
and an example function ;
function func1, arg1,arg2,...,info
info.item1=some_crazy_calculation
end
And (just to nitpick for the cognosenti) I would do the getting and setting of
the user value with the NO_COPY keyword. No sense making multiple
copies of the same data. :-)
Cheers!
David
-----------------------------------------------------------
David Fanning, Ph.D.
Fanning Software Consulting
2642 Bradbury Court, Fort Collins, CO 80521
Phone: 970-221-0438 Fax: 970-221-4762
E-Mail: davidf@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com
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