Rails - Feb 2006 - JavaScript Prototype Help?

I have a form with a text area.  I want to have an observer watch the  
text area and update a div with the running count of characters in  
the text area.  I tried "onchange" but that doesn''t update
until you
leave the text area.  I was able to get it working using AJAX, but  
it''s silly to make a needless round trip to the server, when the page  
already knows everything it needs to know.  What I wanted was an  
element observer which executes JavaScript immediately.  So I decided  
to write my own helper using the observe_field code as a model.

In my view I have:

<%= observe_field_length(''text_area'', {
		:frequency => 5,
		:update => ''count_display'',
		:with => "''Current characters = '' +
value.length"}) %>

And obviously I also have the form text area called "text_area" and a
div called "count_display".

My helper looks like this:

def observe_field_length(field_id, options = {})
	options[:content] = options[:with] || ''value''
	callback = update_element_function("#{options[:update]}", options)
	javascript  = "new Form.Element.Observer(''#{field_id}'',
"
	javascript << "#{options[:frequency]}, "
	javascript << "function(element, value) {"
	javascript << "#{callback}}"
	javascript << ")"
	javascript_tag(javascript)
end

This does half the job: it updates the div while the user types, but  
instead of evaluating the JavaScript in options[:content] it''s  
outputting the string I want evaluated.  I need a way to evaluate the  
JavaScript in options[:content].  I''m 99% sure my answer lies in the  
JavaScript Prototype Library but I can''t seem to find it.  The AJAX  
helper observe_field manages to evaluate the JavaScript sent to it in  
options[:with].  How can I do the same thing?

Kevin Skoglund

On Friday, February 03, 2006, at 4:17 PM, Kevin Skoglund
wrote:
>I have a form with a text area.  I want to have an observer watch
>the text area and update a div with the running count of characters
>in the text area.
You probably don''t need to bother with Rails helpers here at all. See  
this example:
http://javascript.internet.com/forms/limit-textarea.html

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On 2/3/06, Kevin Skoglund <kevin@pixelandpress.com>
wrote:
> My helper looks like this:
>
> def observe_field_length(field_id, options = {})
>         options[:content] = options[:with] || ''value''
>         callback = update_element_function("#{options[:update]}",
options)
>         javascript  = "new
Form.Element.Observer(''#{field_id}'', "
>         javascript << "#{options[:frequency]}, "
>         javascript << "function(element, value) {"
>         javascript << "#{callback}}"
>         javascript << ")"
>         javascript_tag(javascript)
> end
Andrew''s solution is what I would do myself.. but I thought I would
mention that another way to write the above is:
javascript = <<-CODE
new Form.Element.Observer(''#{field_id}'',
#{options[:frequency]},
function(element, value) {
#{callback}} )
CODE

"CODE" can be any string you make up, like JAVASCRIPT, or
JACK_DANIELS.