This is one of the best explanations of anything that I have seen on this
list do date! Nice job Derrell!
Jim
On Fri, Mar 12, 2010 at 10:43 AM, Derrell Lipman <
[email protected]> wrote:
> On Fri, Mar 12, 2010 at 13:13, skar <[email protected]> wrote:
>
>>
>> > this article should answer your question:
>> >
>> http://qooxdoo.org/documentation/1.0/antipatterns#reference_types_in_member_section
>> >
>>
>> Derrell. Is this a qooxdoo specific problem or a problem
>> in JS in general?
>>
>
> Hmmm... How to answer that...
>
> What we're talking about here is a feature of the JavaScript language that
> is very useful. Objects (including Arrays) can become very large. Instead of
> passing copies of objects and arrays around, and taking the possibly large
> amount of time required to make those copies, only a single copy of each
> object is created, and when assigned to another variable or passed to a
> function or method, a reference to that object is passed. For function
> calls, this is typically referred to as "call be reference" instead of "call
> by value." (Scaler types are passed by value.)
>
> In the case of your question, you created a class like this (with a few
> changes by me):
>
>
> qx.Class.define("testproject.ArrayClass",
> {
> extend: qx.ui.basic.Atom,
>
> construct : function()
> {
> this.base(arguments);
>
> this.arr2 = new Array();
> },
> members :
> {
> elementsRemaining : 10,
> arr : new Array()
> }
> });
>
> What we have here is a JavaScript object (aka a "map") being passed to a
> static function called qx.Class.define. The define() function creates a
> JavaScript object prototype, which creates a new class, and uses the
> provided map to initialize that class. The way that define() is implemented
> puts member initialization into the prototype, so each instance of the class
> gets initialized with the initial values from the members portion of the
> map. The initial values are evaluated at define() time and placed into the
> prototype.
>
> Looking at the above example, you'll see this is very desirable to allow a
> new instance of testproject.ArrayClass to have its elementsRemaining member
> initialized to 10. This will occur automatically for each new instance of
> testproject.ArrayClass since the prototype of this class had an
> elementsRemaining member with a value of 10.
>
> But what happened with the arr member? Its value, too, got evaluated at the
> time that qx.Class.define() was called, so the prototype contains a *
> reference* to that Array that was created. Each instantiation of
> testproject.ArrayClass gets the reference to that same array in its arr
> member, because the prototype had a reference to that array in the class'
> arr member.
>
> So is it a bug or a feature? It is certanly a JavaScript feature. I suppose
> that one might have designed qx.Class.define() a bit differently, and placed
> into the members portion of the map only a list of member variables
> (implicitly asking that they each be added to the prototype with a value of
> null), and requiring that all member variables be initialized in the
> constructor. This would have prevented the "issue" (I like that word better
> than "problem" in this context) that we're seeing here because there would
> be no opportunity to have a reference type added to the prototype. On the
> other hand, it would complicate the code (to a fairly large extent, with
> many classes with lots of scaler initialization) by forcing initialization
> of every member variable in the constructor, rather than in the much cleaner
> representation of the members: section of the map.
>
> So there are the pros and cons. That should provide some food for thought.
>
> Cheers,
>
> Derrell
>
>
>
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