This is where something like an external cancellation source could be more
effective:
```js
function getUser1(cancelToken) {
return doXHR("/user.json", cancelToken);
}
function getUser2(cancelToken) {
// immediate result, no need for cancellation
return { "name": "domenic" };
}
function getUser3(cancelToken) {
return doXHR("/user.json", cancelToken).then(function (user) {
user.metadata = "<meta> tags are old school";
});
}
var cts = new CancellationTokenSource();
var usersF = Future.every(getUser1(cts.token), getUser2(cts.token),
getUser3(cts.token));
usersF.done(function(users) {
});
cts.cancelAfter(5000); // timeout after 5 seconds.
```
Rather than subclassing Future, you use an external source provided by the
caller to manage cancellation. Now it doesn't matter whether either of the 3
methods have a return value that supports cancellation, and if the caller
doesn't need to cancel, it can provide null or undefined.
Ron
> -----Original Message-----
> From: [email protected] [mailto:es-discuss-
> [email protected]] On Behalf Of Domenic Denicola
> Sent: Wednesday, May 1, 2013 11:23 AM
> To: Tab Atkins Jr.; Bill Frantz
> Cc: [email protected]; Brendan Eich; es-discuss
> Subject: RE: Future cancellation
>
> From: Tab Atkins Jr. [[email protected]]
>
> > I think you're making this far too complicated. It's much simpler than
> > this:
>
> I disagree. An abstraction boundary gets broken. Consider:
>
> ```js
> function getUser1() {
> return doXHR("/user.json");
> }
>
> function getUser2() {
> return { "name": "domenic" };
> }
>
> function getUser3() {
> return doXHR("/user.json").then(function (user) {
> user.metadata = "<meta> tags are old school";
> });
> }
> ```
>
> Here, `getUser1()` returns a cancellable promise, but `getUser2()` does not,
> even though they should have the same semantics. Worse, `getUser3()` isn't
> cancellable, even though it was originally derived from an XHR. Trying to fix
> the `getUser3()` case is what takes you down the slope of complex additional
> semantics.
>
> Much better would be if a hypothetical future XHR utility returned a `{
> promise, abort }` object, with no logical connection between the `abort` and
> the `promise` (besides maybe rejecting the promise with a well-known
> error). This seems much better than trying to make a general cancellation
> concept for promises and overloading them with additional semantics about
> the operation being performed.
> _______________________________________________
> es-discuss mailing list
> [email protected]
> https://mail.mozilla.org/listinfo/es-discuss
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