Signals far from Slots

[Jim Hewes via Digitalmars-d-learn]

Is there a good way to connect signals and slots when the objects are 
far apart? All tutorials for signals and slots show the objects being 
readily accessible by the main() function. But what if they're not? Is 
there an elegant design?

For example, here's a typical minimal demo:

---------------------------------------------------------
class Foo
{
     void Listen()
     {
         writeln("Foo: I hear you!");
     }
}

class Bar
{
     mixin Signal!() barSignal;
     void SaySomething()
     {    emit();   }
}

int main(string[] argv)
{
     Foo f = new Foo();
     Bar b = new Bar();

     b.barSignal.connect(&f.Listen);
     b.SaySomething();
     return 0;
}
---------------------------------------------------------------

The main() function is a kind of controller that connects up the 
components. But let's say main() doesn't have access to Bar because it's 
behind a layer:

------------------------------------------------------------------

class BarContainer
{
     this() { b = new Bar(); }

     void SetBarSignalHandler(b.barSignal.slot_t dg)
     {    b.barSignal.connect(dg);  }

     private Bar b;
};

int main(string[] argv)
{
     Foo f = new Foo();
     BarContainer bc = new BarContainer();

     bc.SetBarSignalHandler(&f.Listen);
     return 0;
}
------------------------------------------------------------------
This can get worse if there is also a FooContainer and objects get 
further away.

My first thought is to pass the delegates through the layers, as in the 
SetBarSignalHandler() function in BarContainer. But this seems kind of 
ugly as all the layers need to know about the connection between the 
signaling object and the observer object. It would be nice if they 
didn't have to.

Do you know any any cleaner way?

Jim