Rails spinoffs - Jan 2007 - "this."refers to a different object in an ajax callback handler

Hello,

I am experiencing some difficulties using prototype 1.4 and i hope
someone here can help me. I''ve tried a quick google, but to no avail
(hard to search for "this." ;)

My situation: i am writing a form which will have a number of widgets
of various types. I am working on a given widget (itself composed of
several html inputs of different kinds). I have defined a js object
(using prototype''s myobj = Class.create followed by myobj.prototype
blah blah). The various event handlers for the different widget
component parts (i.e. the html fields that make up this widget) are
defined as methods of myobj and assigned to the correct form fields in
myobj''s constructor using the bindAsEventListener function. This way i
can reuse the widget several times on the same page, i just create a
new instance of the object and that instance will bind to the correct
fields based on the constructor argument passed. So far, so good, all
working fine.

Now for the problem: i need to get some values from the server and
alter/populate one of the widget components. I define a method which
creates an Ajax.Request, and another method to act as the callback
handler. I hook them up together, and the callback handler gets called
by the Ajax.Request.

Here''s the problem: the value of "this."in the callback
handler is not
the same as the value of "this."in the other methods of my object - it
looks to me like the value has been "hijacked" and now refers to the
ajax request?

Here''s a short example of pseudo-code in case i''m not being
clear:

---start ---
var myobj = Class.create();

myobj.prototype = {
  /*
  * ctor
  */
  initialize: function(fieldName) {
    //find all the widget components (they''re
    //all called "something" + fieldName, and
    //bind event listeners, etc
    this.widgetcomponent = $(''something_'' + fieldName);
  },

  /*
  *  function to get data via ajax
  */
  getDataViaAjax: function(){
    var params = {
      //my params
    };
    var myAjax = new Ajax.Request(
      //my endpoint
      {
        method:''post'',
        parameters:$H(params).toQueryString(),
        onComplete:this.ajaxCallBackHandler,
    });
  },

  /*
  *  callback functions for the ajax
  */
  ajaxCallBackHandler: function(requestObj,respData){
    this.widgetcomponent.value = respData; //ERROR HERE
    //basically, "this." seems to refer to the Ajax.Request,
    //NOT the instance of the object....
  }
}
--- end ---


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On 1/12/07, tanguyr-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
<tanguyr-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> Here''s the problem: the value of "this."in the callback
handler is not
> the same as the value of "this."in the other methods of my object
- it
> looks to me like the value has been "hijacked" and now refers to
the
> ajax request?
>
Hi,

Please try to replace this line :
> onComplete:this.ajaxCallBackHandler,
by this one :
> onComplete:this.ajaxCallBackHandler.bind(this),
HTH,
Nicolas Terray

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Brilliant!

thanks vey much. I had worked out a hack using a closure, but this (no
pun intended) is *much* more elegant.

Thanks again Nicolas.

Regards,
/t

On 1/12/07, Nicolas Terray
<nicolas.terray-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
> On 1/12/07, tanguyr-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
<tanguyr-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > Here''s the problem: the value of "this."in the
callback handler is not
> > the same as the value of "this."in the other methods of my
object - it
> > looks to me like the value has been "hijacked" and now
refers to the
> > ajax request?
> >
>
> Hi,
>
> Please try to replace this line :
> > onComplete:this.ajaxCallBackHandler,
> by this one :
> > onComplete:this.ajaxCallBackHandler.bind(this),
>
> HTH,
> Nicolas Terray
>
> >
>
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* tanguyr wrote (12/01/07 14:35):
> Brilliant!
> 
> thanks vey much. I had worked out a hack using a closure, but this (no
> pun intended) is *much* more elegant.
This much more elegant thing is still a closure. The reason your closure
looks like a hack in comparison to this elegant solution is that the
prototype author(s) just do it a bit better :-) I find this is often the
case when comparing my code to Prototype code.
> 
> Thanks again Nicolas.
> 
> Regards,
> /t
> 
> On 1/12/07, Nicolas Terray
<nicolas.terray-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>>
>> On 1/12/07, tanguyr-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
<tanguyr-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> > Here''s the problem: the value of "this."in the
callback handler is not
>> > the same as the value of "this."in the other methods of
my object - it
>> > looks to me like the value has been "hijacked" and now
refers to the
>> > ajax request?
>> >
>>
>> Hi,
>>
>> Please try to replace this line :
>> > onComplete:this.ajaxCallBackHandler,
>> by this one :
>> > onComplete:this.ajaxCallBackHandler.bind(this),
>>
>> HTH,
>> Nicolas Terray
>>
>> >
>>
> 
> > 

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well, when it comes to frameworks, i''m a fan of swimming with the
current... if this is the "prototype way" to do it then so be it.

Now (*ahem*) i also noticed that "this." gets hijacked inside
iterators (each, find, etc). Oh mighty prototype masters, enlighten
this poor seeker after knowledge...

On 1/12/07, Chris Lear
<chris.lear-kZMsvDh4tCZWk0Htik3J/w@public.gmane.org>
wrote:
>
> * tanguyr wrote (12/01/07 14:35):
> > Brilliant!
> >
> > thanks vey much. I had worked out a hack using a closure, but this (no
> > pun intended) is *much* more elegant.
>
> This much more elegant thing is still a closure. The reason your closure
> looks like a hack in comparison to this elegant solution is that the
> prototype author(s) just do it a bit better :-) I find this is often the
> case when comparing my code to Prototype code.
>
> >
> > Thanks again Nicolas.
> >
> > Regards,
> > /t
> >
> > On 1/12/07, Nicolas Terray
<nicolas.terray-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >>
> >> On 1/12/07, tanguyr-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
<tanguyr-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >> > Here''s the problem: the value of "this."in
the callback handler is not
> >> > the same as the value of "this."in the other
methods of my object - it
> >> > looks to me like the value has been "hijacked" and
now refers to the
> >> > ajax request?
> >> >
> >>
> >> Hi,
> >>
> >> Please try to replace this line :
> >> > onComplete:this.ajaxCallBackHandler,
> >> by this one :
> >> > onComplete:this.ajaxCallBackHandler.bind(this),
> >>
> >> HTH,
> >> Nicolas Terray
> >>
> >> >
> >>
> >
> > >
>
>
> >
>
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On 1/12/07, tanguyr <tanguyr-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> Now (*ahem*) i also noticed that "this." gets hijacked inside
> iterators (each, find, etc).
... and also in Event.observe, where you have to bindAsEventListener() :)

Some documentation:
http://blog.livollmers.net/?p=17
http://particletree.com/notebook/prototype-and-the-this-keyword/

Yours,
Nicolas Terray

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.bind is available to every function via Function.prototype. Any
situation in which you want ''this'' to be specifically set to
your
object, bind() it to the function and have a nice day... :D

-E

On 1/12/07, Nicolas Terray
<nicolas.terray-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
> On 1/12/07, tanguyr <tanguyr-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> > Now (*ahem*) i also noticed that "this." gets hijacked
inside
> > iterators (each, find, etc).
>
> ... and also in Event.observe, where you have to bindAsEventListener() :)
>
> Some documentation:
> http://blog.livollmers.net/?p=17
> http://particletree.com/notebook/prototype-and-the-this-keyword/
>
> Yours,
> Nicolas Terray
>
> >
>

-- 
Eric Ryan Harrison

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Nicolas,

Thank you very much for those links. It''s all becoming clearer, bit by
bit...

Regs,
/t

On 1/12/07, Nicolas Terray
<nicolas.terray-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
> On 1/12/07, tanguyr <tanguyr-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> > Now (*ahem*) i also noticed that "this." gets hijacked
inside
> > iterators (each, find, etc).
>
> ... and also in Event.observe, where you have to bindAsEventListener() :)
>
> Some documentation:
> http://blog.livollmers.net/?p=17
> http://particletree.com/notebook/prototype-and-the-this-keyword/
>
> Yours,
> Nicolas Terray
>
> >
>
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