| Re: Event handler as an object method ?? [message #41698 is a reply to message #41601] |
Tue, 09 November 2004 04:30  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Antonio Santiago writes:
> Hi,is there a reason why an object method can't be use as an event handler?
No reason at all. In fact, it is only short step
now before you realize widgets should be objects. :-)
>
> I just created an object that "contains" a WIDGET_DRAW. I'd like that
> the event ocurred on WIDGET_DRAW was handled by a method of my class,
> but EVENT_FUNCTION and EVENT_PRO keywords not accpet a method.
>
> The solution i have adopted is to catch the event of WIDGET_DRAW outside
> of the object (on a WIDGET_BASE where i put the object (realy de
> WIDGET_DRAW)) and redirect it to the method i want
> (MyObject->EventHandler), but this is an ugly solution.
In the absence of help from RSI, most solutions are more
or less ugly. The solution I usually adopt is to have
all events go to a generic event handler. In the event
handler, a "command" structure is extracted from the user
value of the widget that caused the event. The "command"
is an anonymous structure containing an "object" and
a "method" field. All the event handler does is extract
the command structure and use CALL_METHOD to call the
method field on the object field, passing the event structure
as a parameter to the "event handler method".
To make this work, every widget that is going to generate
an event has a "command" structure stored in its user value:
drawID = Widget_Draw(tlb, XSize=400, YSize=400, $
UValue={object:myobject, method:'MyEventHandlerMethod'})
The generic widget event handler does this:
Widget_Control, event.id, Get_UValue=cmd
Call_Method, cmd.method, cmd.object, event
In my Catalyst Library, all widgets are objects and the
"fields" of the "event structure" all point to objects
as well, so this slight of hand feels a little more
natural. Instead of using Event_Pro or Event_Func
to direct events, you can use an Event_Method keyword
to define the appropriate method to handle the event
If an event method is not specified, we use a EventHandler
method that is always associated with an widget object.
Cheers,
David
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http:/www.dfanning.com/
Phone: 970-221-0438, IDL Book Orders: 1-888-461-0155
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