On Dec 6, 2005, at 8:20 PM, Adam Wolff wrote:
This is actually just a quirk in the compiler -- it emits warnings for
el.notthere
but not for
el[ "notthere" ]
This quirk is there specifically so that there's a warning-free way
to test for the presence of an attribute.
In standard JavaScript, you can do this with the 'in' relation:
if ('notthere' in el) ...
but OpenLaszlo JavaScript don't implement 'in'.
Why doesn't OpenLaszlo JavaScript implement 'in'? I'm not sure,
actually. JavaScript doesn't allow 'in' inside an initializer in a
'for' expression, and goes through all kinds of hoops to prevent it
(http://www.mozilla.org/js/language/grammar14.html), presumably
because of some kind of ambiguity having to do with:
for (var index in sequence) ...
for (var index = expr; ...; ...) ...
if expr contains an 'in'. JavaCC is an RDP and can enforce this with
inherited attributes (implemented as RDP parameters); it's not
incredibly tricky like it is with a shift/reduce parser such as bison/
yacc. When I was implementing the compiler I just didn't have time
even to implement it with inherited attributes, but now I don't even
see what the ambiguity is.
People used this as a shortcut for sending events in code, but I
think the
best form here is declare the event as null (until we get tucker's
<event>
tag) This has no major runtime impact since literal class attribute
values
with no setter are stored on the class prototype.
<node>
<attribute name="time" setter="setTime(time)"/>
<attribute name="ontime" value="null"/>
That way you can write
if (ontime) this.ontime.sendEvent();
and it won't cause warnings. You're also guaranteed that the ontime
slot
will not get set to null after the event has been created, so it's
safe.
A
On Nov 9, Jim Grandy wrote:
A particular question about how to implement setters. Here's an
edited version
of the example from the DevGuide (Chapter 25, Section 5.5):
<node>
<attribute name="time" setter="setTime(time)"/>
<method name="setTime" args="t">
if (!this.isinited) { this.time = t; return; }
// do some custom stuff here
this.time = t;
if (ontime) this.ontime.sendEvent();
</method>
</node>
In certain cases, I'm seeing a runtime error with code similar to
this, saying
that 'ontime' is not a defined property of the given object.
Grepping through the LPS sources, I've seen a different way of
calling the
event handler:
if (this['ontime'])) this.ontime.sendEvent();
When I use this idiom, I don't get the error. This makes sense to
me -- if no
one has declared an event for this attribute, it shouldn't be
defined. But
maybe I'm not understanding things and there's another reason the
documented
idiom isn't working for me.
Anyone care to comment?
jim
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