Hey there,
I've got a prototype having a method I can add callbacks with:
/*
* Add a callback function that is invoked on every element submitted and must return a data object.
* May be used as well for transmitting static data.
*
* The callback function is supposed to expect a jQuery element as single parameter
* and must return a data object (for additional data to be sent along with the one already given upon initialization).
* Adding multiple callback functions results in those functions being invoked in the same order as they were added.
*
* 1) Therefore subsequent assignments to the same key in the data array override those that were performed by a previous callback function.
* 2) If data_arg = true is given, the data returned by the callback function that was previously called is given to the new_callback as 2nd argument, so it can be manipulated.
* However, even if it isn't, the unchanged data must be returned anyway to have any effect.
*/
this.add_data_callback = function(new_callback, data_arg) {
if(this.data_callback) {
old_callback = this.data_callback;
if(!data_arg) {
//alert('add it');
//alert(old_callback);
//alert(new_callback);
this.data_callback = function(element) {
//alert('do it');
//alert(old_callback);
//alert(new_callback);
return jQuery.extend(old_callback(element), new_callback(element));
};
}
else {
this.data_callback = function(element, data) {
return new_callback(element, old_callback(element));
};
}
}
else {
//alert('add the first');
//alert(new_callback);
this.data_callback = new_callback;
}
};
(Please ignore the else part for data_arg = true as it's not relevant.)
In my concrete case I'm adding three callback functions. If this.data_callback() is finally invoked for an element, however, it results in the whole thing looping infinitely. In my attempt to track the bug down, the alerts above (yeah, I know that there're debugging tools for that but it was way more comfortable like that) delivered the following insight into the issue:
- The anonymous function / closure (the one that contains jQuery.extend()) is called. new_callback contains the 3rd callback function. old_callback contains another...
- anonymous function which is called. new_callback contains the 2nd callback function. old_callback should contain the 1st callback function, but actually it's yet another...
- anonymous callback which is called. new_callback contains the 2nd callback function again. old_callback contains the anonymous function
- ...
Now, what do I miss? Is it some strange closure magic or just some obvious bug I'm apparently unable to see?
Thanks in advance!