Exposed class method returns a class instance

[Molochnik]

TB = class 
end;

TA = class 
published
     function GetB:TB;
end;

var
     A: TA;
     B: TB;
...
function TA.GetB:TB;
begin
     Result:=B;
end;

TA and TB are exposed classes.
A is an exposed TA instance.
In the script

var
     B: TB;
begin
     B := A.GetB;//compile error function returns Variant instead of TB
end;

So A.GetB returns a value of type Variant instead of TB. How to convert it to TB?

[EricGrange]

I'm not sure to understand, would you have a working r example that would reproduce the issue ? Thanks

[CzajnikXD]

I'm not sure to understand, would you have a working r example that would reproduce the issue ? Thanks

Hi, i know what he is trying to say because im having the exact same issue. The problem is that when we for example expose classA that has a property returning another class (classB for example), that class is considered a Variant and not a instance of classB. Let me show you on a example.

Let's define those two example classes:

TClassB = class
    private
      FStoredName : string;
    public
      function GetName : string;
      procedure SetName(const AName : string);
    published
      property Name : String read GetName write SetName;
      constructor Create;
    end;
TClassA = class
    private
      FClassB: TClassB;
    protected
      function GetClassB : TClassB;
    public
      constructor Create;
      destructor Destroy; override;
    published
      property DataClassB : TClassB read GetClassB;
    end;

we then expose both classes to dwsunit (im using ExposeRTTI) and inside of script we run something like this:

var NewClassA : TClassA;
NewClassA := TClassA.Create;
println(NewClassA.DataClassB.Name);

we get: "Syntax Error: There is no accessible member with name "Name" for type Variant [line: 3, column: 27]".
And the problem is that script sees NewClassA as object of type TClassA, but it doesn't see NewClassA.DataClassB as a TClassB, instead seeing it as a Variant type.

In the dwsRTTIExposer.pas unit, inside the TdwsRTTIExposer.RTTITypeToScriptType function, there is a case statement that explicitly maps tkClass to SYS_VARIANT:

// Lines from 355-370 in the dwsRTTIExposer unit
case aType.TypeKind of
         tkSet, tkProcedure, tkPointer, {tkDynArray, }tkInterface, tkArray,
            tkEnumeration, tkClassRef, tkClass, tkMethod : begin
             Result := SYS_VARIANT; // todo, someday maybe...
         end;

This seems to be the reason why a property returning a class instance is treated as a Variant instead of its actual, specific class type. A potential solution would be to change this logic. Instead of returning SYS_VARIANT, the function should probably try to resolve the tkClass type to its corresponding script type. This might involve ensuring that the nested class (like TClassB in my example) is also exposed/registered with the script engine, and then returning the correct script type handle.

[EricGrange]

Ah, I'm not using the RTTI exposer, so I didn't think of that one.

I guess the TdwsRTTIHelper could used to disambiguate the class name and find the script-side class name (if it has already been exposed), for classes self-referencing each other, some form of class forwarding would be required.