[Laszlo-dev] Proposal to tweak the behavior of attributes with a setter and an initial value

[P T Withington]

Currently, if you create an attribute with both a setter _and_ an  
initial value, the compiler will initially set the attribute to  
`null`, and then call the setter with the initial value.  This is not  
actually right, but it lets setters get away with code like:

<attribute name="foo" value="null" />

<setter name="foo" args="newfoo">
   if (this.foo !== newfoo) {
     ...
   }
</setter>

People sometimes write this as an optimization, to avoid a complex  
setter if the value is not actually changing.

I propose that the 'initial initial' value of an attribute like this  
be `undefined`, rather than `null`, because it really has not been set  
yet.  Furthermore, in an optimization like the above, the setter code  
will not be called on the `null` initial value of the attribute  
(because the compiler has already set it to `null`).  This can lead to  
subtle bugs.

Pro:

The setter code for cases like the above will not be erroneously  
skipped if the initial value is `null`.

Con:

The setter code must take care checking the value of the attribute.   
Since it will initially be `undefined`, referencing it as `this.foo`  
will generate a warning in the swf8 runtime.  To avoid the warning,  
the code must be re-written as `this['foo']`.

Comments?  Questions?