PSA: nsISimpleEnumerator now works as a JS iterator

Mike Conley mconley at mozilla.com
Thu Aug 23 22:48:56 UTC 2018


Wow, this is a great improvement! Thank you!

On Thu, Aug 23, 2018, 6:26 PM Kris Maglione <kmaglione at mozilla.com> wrote:

> Oh, and from C++, there's also now a range iterator wrapper with
> similar semantics:
>
>   for (auto& docShell : SimpleEnumerator<nsIDocShell>(docShellEnum)) {
>     ...
>   }
>
> On Thu, Aug 23, 2018 at 03:19:55PM -0700, Kris Maglione wrote:
> >As of bug 1484496, any C++-implemented nsISimpleEnumertor instance can
> >be used as a JS iterator. And, when used this way, the iterators now
> >have intrinsic type information, and therefore do not require QIing
> >their elements.
> >
> >Which is to say, now you can simply do:
> >
> > for (let window of Services.wm.getEnumerator("navigator:browser")) {
> >   ...;
> > }
> >
> > for (let docShell of docShellEnum) {
> >   ...;
> > }
> >
> >rather than:
> >
> > let winEnum = Services.wm.getEnumerator("navigator:browser");
> > while (winEnum.hasMoreElements()) {
> >   let window = winEnum.getNext();
> >   ...
> > }
> >
> > while (docShellEnum.hasMoreElements()) {
> >   let docShell = winEnum.getNext().QueryInterface(Ci.nsIDocShell);
> >   ...
> > }
> >
> >If you happen to be using an nsIArray from directly from JavaScript,
> >you unfortunately still need to specify the expected types, since
> >nsIArray has no idea what types it can contain:
> >
> > for (let thing of array.enumerate(Ci.nsIThing)) {
> >   ...
> > }
> >
> >Aside from being easier to maintain, these forms should also be
> >somewhat faster than the old protocol, since they only require one
> >XPConnect call per iteration rather than 3(+).
> >
> >-Kris
>
> --
> Kris Maglione
> Senior Firefox Add-ons Engineer
> Mozilla Corporation
>
> There is no greater mistake than the hasty conclusion that opinions
> are worthless because they are badly argued.
>         --Thomas Huxley
>
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>
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