On 2006-06-13, at 21:03 EDT, Sarah Allen wrote:
>
> ok, I just code reviewed a fix in scrollrichedittext that I don't
> really
> understand. Perhaps one of the Javascript gurus on the list can
> enlighten us.
More than just Javascript. There is lots of LZX black magic here.
One wonders why you are having to call applyConstraint in the first
place rather than using a declarative constraint?
> The following code generated a waring that inp was undefined:
> this.applyConstraint("stepsize",
> function() { this.setAttribute("stepsize",
> parent.inp.lineheight); },
> [p.inp, "lineheight"]);
I'm surprised there was not a warning that `parent` was undefined.
But maybe there is a global named `parent`.
> The bug was fixed by adding an explicit 'this' before parent.inp:
> this.applyConstraint("stepsize",
> function() { this.setAttribute("stepsize",
> this.parent.inp.lineheight); },
> [p.inp, "lineheight"]);
This looks like it still has a bug, but maybe you are not telling me
that p == parent in this context.
> My first thought was... I thought you never needed an explicit this
> for
> parent since it is defined at construct time.
You would not have if you used a declarative constraint, because
declarative constraints are implicitly evaluated in a `with (this)`
context. [Seemed like a good idea at the time, but we really regret
that we ever did that. It has caused no end of pain.] Since you are
creating your constraint by hand, you need to explicitly say 'this'
where you mean it. (Or you could have put the function body inside
`with (this)` which is what the tag-compiler does.)
> My second thought was... um, what the heck is 'this' in the middle
> of an
> anonymous function?
More magic. A constraint function is evaluated as:
<constraint>.call(this);
So that 'this' in the constraint is the object that has the
constrained attribute as a member.
> I would have thought it was no longer in the scope
> of the object. Why does this work?
If you said:
<attribute name='stepsize' value='${parent.inp.lineheight}' />
and compiled that using `lzc --script` you will see all the magic.
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