[Rails-spinoffs] Re: Event.observe problems

Thu, 07 Sep 2006 09:19:04 -0700

Interesting. I assumed new_i = i would be making a value copy to the new reference, which should be scoped each time through the loop... hmm.

So I wonder if this is what I needed to do... (yes agreed it's academic, but fun to carry on)

for (var i=1; i<=5;i++){
   var new_i = new Number(i);
   Event.observe($('star'+new_i), 'click',
function(e){objRating.setRating(new_i)});
}

...or even...

for (var i=1; i<=5;i++){
   var new_i = i.toString();
   Event.observe($('star'+new_i), 'click',
function(e){objRating.setRating(new_i)});
}

Now I would really assume either of those should work to force a value copy to the new variable with each iteration. If not, I give up :-)

On 9/7/06, Chris Lear <[EMAIL PROTECTED]> wrote:

* Ryan Gahl wrote (07/09/06 15:42):
> What Eric was saying was not that they are all using the same callback,
> but that they are all referencing the same i variable at the same
> address in memory, which at the end of your loop holds the value 6. You
> would have been fine if you had created a locally scoped variable new_i
> within the loop and used that in the callback (of course Eric's
> alternative also works fine, but I'm just trying to clear this issue up
> a little in your mind so you know next time)
>
> This would have worked:
>
> for (var i=1; i<=5;i++){
>    var new_i = i;
>    Event.observe($('star'+new_i), 'click',
> function(e){ objRating.setRating(new_i)});
> }

I don't think even this works. new_i is in the same scope. When I tried
it, I got all 5s (rather than 6s), which would correspond to all the
closures using one value for new_i - the last one assigned.
Variable scoping in closures is hard (for me, anyway) to get to grips
with, so I don't think I'll try an explanation - probably Ryan could doo
better than me anyway.

These should both work, and are viable alternatives to Eric's solution:


// Solution One (simplest case of returning a closure from a function)

for (var i=1; i<=5;i++){
     Event.observe($('star'+(x)), 'click', myfunc(i));
}
// Return the closure from a new function that uses a variable in local
// scope
function myfunc(x) {
     return function(e){objRating.setRating(x)}
}


// Two
// Use the prototype bind function to bind the "this" variable
for (var i=1; i<=5;i++){
     Event.observe(
        $('star'+(x)),
        'click',
        function(e){objRating.setRating(this)}.bind(i)
     );
}

Eric's solution does the same thing, but hides the detail behind more
prototype cleverness.

Apologies if I've transcribed these wrong. They worked in a test page I
made, but I just used alerts rather than objRating.setRating etc.

This is all a bit academic considering that the OP says he's happy now,
but I find closure code fascinating. I also find the bind() and
bindAsEventListener() methods to be some of the best things about prototype.

Chris

Athena Group, Inc.
Inquire: 1-888-919-8700 x2903
Blog: http://www.someElement.com
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