Right now, the received wisdom as I understand it is that Promises are
the hot, hip new way to do asynchronous programming. So, over here in
$WE_WANT_TO_REMOVE_CRUFT, I proposed adding a new API that returns a
Promise for all the JS consumers. This new API, naturally, is to be
implemented in JS. However, we also have a large body of code that may
want to use this API who are presently in C++ and can't really move to
JS (doing so would require moving tens or hundreds of thousands of line
of code more or less simultaneously).
Now, creating a Promise (via mozilla::dom::Promise) from C++ and passing
it to JS is possible but kind of difficult, since I need to materialize
an nsIGlobalObject which requires too much code. It's not really
possible to cast to this method from a JS implemented Promise (since the
notion of "promise" in JS is "I have a callable property named then").
It's also the only way you can react to a promise from C++ without
stabbing yourself. Since I need C++ to be able to react to
JS-implemented stuff, this kind of means I have to use XPIDL instead of
WebIDL.
Can promises be specified in XPIDL in such a way that they are usable
from C++? Not quite: while [function] automatically converts a function
to an nsISupports-derived interface, it doesn't allow the same
nsISupports-derived object to be called as a function pointer. e.g., if
I defined an interface:
[function]
interface AnyAnyCallback {
jsval doCallback(in jsval arg);
};
interface nsIPromise {
nsIPromise then(in AnyAnyCallback resolve, in AnyAnyCallback reject);
};
Then an implementation of a JS .then method couldn't call its resolve
argument with resolve(val) but would have to do resolve.doCallback(val).
I've thought about ways to fix this, but they all require modifying
xpconnect to some degree, so I wondered if anyone had better solutions
to this problem space.
--
Joshua Cranmer
Thunderbird and DXR developer
Source code archæologist
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