Interface helpers or delegating interface parent

Question

If I have an existing IInterface descendant implemented by a third party, and I want to add helper routines, does Delphi provide any easy way to do so without redirecting every interface method manually? That is, given an interface like so:

IFoo = interface
  procedure Foo1;
  procedure Foo2;
  ...
  procedure FooN;
end;

Is anything similar to the following supported?

IFooHelper = interface helper for IFoo
  procedure Bar;
end;

or

IFooBar = interface(IFoo)
  procedure Bar;
end;

TFooBar = class(TInterfacedObject, IFoo, IFooBar)
private
  FFoo: IFoo;
public
  procedure Bar;
  property Foo: IFoo read FFoo implements IFoo;
end;

I'm specifically wondering about ways to that allow me to always call Foo1, Foo2, and Bar with a single variable reference (IFoo, IFooBar, or TFooBar), without switching between them, and without adding all of IFoo's methods to TFooBar.

Answer 1

Change your class to read:

TFooBar = Class(TInterfacedObject, IFoo, IFooBar)
private
  FFoo: IFoo;
public
  procedure Bar;
  property Foo: IFoo read FFoo implements IFoo ;
end;

You will also need a constructor or some method to create the instance of IFoo and place it in FFoo.

Answer 2

You cannot access the methods of IFoo through a reference to TFooBar because TFooBar doesn't implement IFoo - it delegates IFoo. But you shouldn't be using a TFooBar reference to access an interfaced object anyway, that's the whole point of using interfaces in the first place!

NOTE: To deliberately prevent this I adopt a convention of implementing interface methods on my classes as "protected", specifically to deny access to those implementation details except as surfaced via the interface itself.

Where-ever you are currently obtaining a reference to TFooBar, change this to instead obtain the IFooBar interface implemented by TFooBar and you will be sorted.

Answer 3

Hi,

I'm not sure I understood all of your concerns, but here's my suggestion anyway :

IFooBar = interface(IFoo)
  procedure Bar;
end;

TFooDelegate = class(TInterfacedObject, IFoo )
private
  FFoo: IFoo;
public
  property Foo: IFoo read FFoo implements IFoo;
end;

TFooBar = class( TFooDelegate, IFooBar )
  procedure Bar;
end;