ScriptUI Checkbox onClick fires differently in CS5 vs CS3?

Question

I'm developing an application consisting of a ScriptUI interface and a custom InDesign template. The ScriptUI interface contains a few checkboxes, with onClick event handlers assigned to read their value and store them to a preferences variable whenever they change. This all works perfectly in InDesign CS3 which was my primary development environment.
Now I need to get this application compatible with InDesign CS5, but the checkboxes seem to consistently be returning the wrong value. No matter whether my event handler reads this.value, event.target.value or a direct reference to the checkbox, the returned value is the opposite of the expected. If I use the direct reference to read the value from elsewhere though, the returned value is correct! It almost seems like in CS5 the onClick event handler is fired before the value is changed, while in CS3 it is fired after the value is changed. Just tried CS4 and it seems to behave the same way as CS5.
Did they actually change the checkboxes to behave this way (which makes no sense to me) or am I doing something wrong? Is there any way to get the old behavior back, or is the only solution to manually invert the value if the script detects CS4 or later as the runtime?

Answer 1

Mayhem SWE wrote:
 I wonder how the execution order is in a web browser. The only valid reason I can come up with to justify the CS4/5 change would be if this is how a web browser would behave (perhaps in order to give you an opportunity to cancel the event before the value is changed?), but it is still quite confusing and illogical IMHO...
I totally agree with you. The reason behind this —I think— is that the ScriptUI event model has been modified in CS4 in order to enhance the compliance with the DOM Level 3 Events Specification — http://www.w3.org/TR/DOM-Level-3-Events — which states that the click event must be cancelable. Thus you probably can use myClickEvent.preventDefault() in CS4/CS5 while this probably has no effect in CS3 (not tested). I suppose that the checkbox value should be actually changed if and only if the event is completed and not canceled. That makes sense, but the counterpart is that there is no more equivalence between using the onClick handler and a 'pure' click event listener: myWidget.addEventListener(...)
On this subject, the specification says that using the shortcut myWidget.onEvent = myFunction to register an 'event listener' depends on the host language: "Because the details of this are often language-specific, this type of event listener registration is not defined in this specification, but in general, any event type may be used as an attribute in this way by adding the prefix on- to the event type name, and events so dispatched should behave consistently with the event registration and propagation defined in this specification, with the same interfaces, properties, and methods." —DOM Level3 Events Specification, 4.3 "Interface EventTarget", note 2.
Of course, the easiest approach is to use myCheckbox.onClick = ... since the handler is called once the value has been updated, whatever the version of ScriptUI. But I don't believe it's a good practice if you're working on a complex project —especially a library!— because the onClick handler can be easily rewritten by the client code, and because you may need to deal with event propagation/cancellation at a deeper level. For my part I always use addEventListener(...) and removeEventListener(...) in my library code, although it implies that I fit my implementation to the ScriptUI version. (In CS3, reversing the checkbox value, etc.) That way the client code cannot pollute the basic behavior of my widgets.
Hope that helps,
@+
Marc