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
-----------------------------------------------------------
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
