On 7/5/11 12:12 PM, Boris Zbarsky wrote:
On 7/5/11 3:00 PM, David Flanagan wrote:
Boris, you have hinted that making the DOM readonly would cause all
kinds of problems, such as: a mutation listener that attempted to set
certain global variables would throw an exception. I'm coming at this
from the perspective of DOM Core and haven't gotten into the details of
HTML yet, so I don't understand what these problems are. Could you
provide more detail?
The problem is that web developers don't have a firm grasp on what's
part of the DOM and what's part of ECMAScript. And libraries that
hide underlying things don't help it any.
So in practice, a web developer looking at code that involves a set
call has no way to tell whether it will work in a given web browser in
a mutation listener if listeners disable DOM mutations. A web
developer looking at a function call has no way to tell either.
I've assumed that mutation events are an advanced feature that will
mostly be used by sophisticated developers and library authors. But I
see your point. I was worried you were saying that there quirks to the
DOM itself that made a read-only mode impractical.
The only way to make it possible to tell without learning every inch
of the specs is to not give any access to DOM objects at all from
mutation listeners. Then any code you write there will work.
But that option is completely impossible, right? Not an avenue to
investigate.
Note that this isn't a capability system: we don't have to prevent the
listener from having any access to the DOM. That wouldn't be possible
and the listeners need read access to the document and its nodes. We
just need to make any modification attempts fail.
And I'm saying that this will lead to hard-to-diagnose bustage due to
unintended DOM modifications that the caller doesn't even realize are
DOM modifications.
At least the bustage would fail fast with an exception. We could define
a new RECURSIVE_MUTATION_ERR exception code that would make it easy to
diagnose the problem.
I have to agree with jjb: developers won't like a readonly DOM in
mutation listeners, but the problems that it will cause are not as bad
as those that arise when recursive mutations are allowed.
David
-Boris
