380597 - Methods returning an array suck for JS implementations/callers

Status: RESOLVED DUPLICATE of bug 523817

(Core :: XPConnect, enhancement)

Description

[Justin Dolske [:Dolske]]

While rewriting password manager as a JS component (bug 374723), I wanted a couple of functions to return arrays. For example, nsIPasswordManager.idl has this:

void getAllDisabledLogins(out unsigned long count,
              [retval, array, size_is(count)] out wstring hostnames);

The header generated for C++ is sensible:

NS_IMETHOD GetAllDisabledLogins(PRUint32 *count, PRUnichar ***hostnames) = 0;

But JS callers are stuck with having to provide a useless argument for the |count| outparm:

var dummy = { value : null; }
var hostnames = pwmgr.getAllDisabledLogins(dummy);
if (hostnames.length != dummy.value) { /* omgwtfbbq? */ }

That can be reduced to .getAllDisabledLogins({}), but it doesn't serve any obvious purpose and is confusing for callers.

Fodder for Mozilla2, perhaps.

Comment 1

[Justin Dolske [:Dolske]]

The flip side of this problem is that JS implementations needlessly have to set the array length as an outparam, XPCOM ought to do that automatically (when the caller needs it).

JS code irritatingly has to do the following:

function returnsAnArray(count) {
    var foo = [1, 2, 3];
    count.value = foo.length;
    return foo;
}

instead of just

function returnsAnArray() {
    var foo = [1, 2, 3];
    return foo;
}

Comment 2

[Alex Vincent [:WeirdAl]]

See bug 380839; I think that should make this easier.