Re: [cgmo-webcgm] more questions about onload

[Lofton Henderson <lofton@rockynet.com>]

At 04:39 PM 12/14/2005 -0500, Benoit Bezaire wrote:
>Hi all,
>
>   So we've agreed that we want the 'onload' event to be dispatched
>   every time 'src' changes. Ok.
>
>   Here's another question that has been bothering me lately... Assume
>   we have the following js code:
>
>   var metafile = document.getWebCGMMetafile();
>   metafile.src = 'myCGM.cgm';
>   metafile.addEventListener("onload", myLoadFunction);
>   metafile.src = 'myCGM2.cgm';

Note, per my last email, that the name of the event in 5.7.10 is 'load', 
not 'onload'.


>   The current spec wording says that event listeners are added to the
>   WebCGMMetafile object, but 'src' is loading a new metafile;
>   shouldn't the event listener 'myLoadFunction' be discarded when
>   'src' is modified?

Hmmm... interesting question.

The getWebCGMDocument() method on the WebCGM metafile interface says:  "If 
no WebCGM document is open in the viewer, the WebCGMMetafile object will 
still be returned."  I.e., we said once upon a time that "the metafile 
object always exists, even if no WebCGM has yet been defined."  Which 
enables the case that there is no 'data' attribute on <object>, then 
execution of the script:

>   var metafile = document.getWebCGMMetafile();
>   metafile.src = 'myCGM.cgm';

An 'onload' <param> definition in the <object> element certainly persists 
across the "metafile.src = ..." statement, right?  Why wouldn't the 'load' 
(onload) event listener definition that is made via 
"addEventListener("load", myLoadFunction)" persist in similar fashion?

Conclusion.  It seems to me that we want the definition of the 'load' 
(onload) event listener to persist across definition or redefinition of the 
metafile ('src' attribute), don't we?

If that is all correct, what we seem to have come around to is this:

1.) for the case that the WebCGM metafile instance is defined on the 'data' 
attribute of the <object>, then you must use the 'onload' <param> if you 
want to catch the onload event.

2.) If you omit the 'data' attribute of the <object>, and define the 
metafile via the .src attribute in script, then you may use the 
addEventListener("load", ...) before setting the .src attribute.

>[...]
>   Do we want to have the event listeners attached to the user agent or
>   to the metafile? If it's to the user agent, then the
>   'addEventListener' spec description needs to change.

I'm not sure how to answer that question.  When we say "metafile", do we 
mean a particular WebCGM instance, or a metafile object (which apparently 
can exist without having a particular WebCGM instance attached.)  The above 
line of reasoning implies that you need to be able to attach the event 
listener to the metafile object, even when a particular WebCGM instance has 
not yet been associated with it.  (Am I making any sense, or is this all 
nonsense?)

-Lofton.