CODEWITHSUNDEEP
delphirttidelphi-10.2-tokyo
Stephane
Stephane

Reputation: 3402

Using function reference returned by RTTI method invocation

In a Delphi program, have the following pattern:

TDelegate=reference to procedure(const Arg: TMyType); 

TRouter = class
  ...
public
  procedure RegisterHandler(const route: string: handler: TDelegate);
end;

THandlerContainer = class
public
  function getDelegate: TDelegate;
  procedure register(const Router: TRouter);
end; // class

...
procedure THandlerContainer.register(const router: TRouter)
begin
  router.RegisterHandler('route', getDelegate);
end;

Basically, I'm registering function references to be used to handle some message handling (based on the "route" string).

I would like to simplify the pattern for my coworkers so that they do not have to call router.RegisterHandler themselves for each implementation but simply have to add an attribute to their class and then pass an instance to a method of TRouter that will use RTTI to find all method decorated by that attribute and register them.

I have therefore created a simple attribute RegisterMessageHandlerAttribute for that decoration (with a custom constructor for receiving the routing string) and wrote a method of TRouter that uses the RTTI to find all method decorated with that atribute:

function TRouter.RegisterHandlers(const HandlerContainerClass:
    TObject);
var
  RTTIContext: TRttiContext;
  RttiType : TRttiType;
  prop: TRttiMethod;
  Attr: TCustomAttribute;
begin
  RTTIContext := TRttiContext.Create;
  try
    RttiType := RTTIContext.GetType(HandlerContainerClass);
    if assigned(RttiType) then
    begin
      for prop in RttiType.GetMethods do
      begin
         for Attr in Prop.GetAttributes do
         begin
           if (Attr is RegisterMessageHandlerAttribute) then
           begin
               Self.RegisterHandler(
                (Attr as RegisterMessageHandlerAttribute).Route,
                TDelegate(Prop.Invoke(HandlerContainerClass, []).AsPointer); // <--- this fails
               );
           end;
         end;
      end;
    end;
  finally
    RTTIContext.Free;
  end;
  result := Handlers.ToArray;
end;

Unfortunately, the compilers complains on the line where I retrieve the lambda by calling the method:

TDelegate(Prop.Invoke(HandlerContainerClass, []).AsPointer);
...
[dcc32 Error] GIT.MessageQueue.Router.pas(169): E2089 Invalid typecast

My problem is that I have no idea how to take the TValue type returned by Prop.Invoke and use it as a function reference of type TDelegate.

Upvotes: 2

Views: 430

Answers (1)

Stefan Glienke
Stefan Glienke

Reputation: 21713

Just use .AsType<TDelegate>() - this returns the content of the TValue as TDelegate. That function is also making sure that you don't turn something that is inside the TValue into something that is not explicitly assignment compatible (not like Variants do). But since that is the exact return type of your function it will just work.

P.S. You need to type the parentheses explicitly because otherwise you might get an E2010 error from the compiler.

Upvotes: 5

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