I’ve finally put up the IDispatch wrapper code I mentioned in my previous post — see here for some information and the CodeCentral link. Compared to the compiled demo I posted earlier, I’ve implemented a few more things, e.g. read-only dynamic array support and the automatic exposure of most enumerators, the latter enabling For Each in VBScript where you would use for/in in Delphi.
Nonetheless,whether it’s practically useful or not I don’t really know — writing it just scratched a very old itch really, having played around with the script control many years ago. Moreover, as a demo of the new RTTI, my code is not exactly the best, since most of it concerns implementing IDispatch rather than using Rtti.pas — though of course, that’s as much a tribute to the latter as it is a criticism of my code qua demo of it.
Having said that, writing the proxy classes did make explicit certain limitations of the new RTTI — basically, while its coverage is very good, it isn’t perfect:
- Where TRttiMethod.Invoke accepts a TValue, object or class as its first parameter, TRttiProperty.GetValue only accepts a pointer. This probably just refects underlying limitations, but nonetheless, the interface should match Invoke IMO.
- Indexed properties are not surfaced at all.
- You can’t tell whether such-and-so property is a default property.
- Class vars are not surfaced.
- Method pointers (= events) seem tricky
(even impossible)to work with when put into a TValue . ( Note they aren’t put in as TMethod records, but anonymous method interfaces, or at least, seem to be.Ignore that — I must have been testing incorrectly. They are in fact stored as TMethod records, as you would expect. See my post here for how to invoke an event handler using RTTI.)
- Sets aren’t exposed as nicely as arrays are.
- An interface type requires the explicit addition of $M+ (or be derived from IInvokable) for its methods to be surfaced. Even then however, interface properties are ignored. (I’m guessing this is yet another couple of quirks due to the Delphi interface type’s origins as a COM support feature.)
Overall though, the new RTTI is still a very impressive feature and one I think people should use in confidence.
[Update: Barry Kelly, the author of the new RTTI, explains the limitations just listed in the comments.]