How to pass a member function/delegate into a mixin template?

[岩倉 澪]

Hi all,

I've been reading D Cookbook, in which the author recommends the 
use of mixin templates to essentially hold boilerplate code for 
classes (page 28). Referencing TDPL reaffirms this strategy. With 
this design choice in mind, I would like to be able to use a 
mixin template that creates a slot for me (as in signals & slots) 
and provides a constructor that connects it to the signal.

The closest I have come is in the simplified example given below:

EventHandler event_handler;

class EventHandler{
    ...

    mixin std.signals.Signal!Event;

    ...
}

mixin template EventListener(void delegate(Event) slot){
    private this()
    in{ assert(event_handler); }
    body{ event_handler.connect(&eventListener); }
    private void eventListener(Event e){ slot(e); }
}

class Foo{
    mixin EventListener!((e){ ... });
}

Sadly, this code does not compile.
My understanding (and correct me if I'm wrong) is that this does 
not compile because I cannot create such a delegate, as the scope 
that it would be in is not available at compile time (it would be 
objects instantiated from class Foo in this example).

One strategy that works is to not pass anything to the mixin 
template, and have the mixin template use a function that is 
presumed to exist. It is then the duty of the class writer to 
make sure a function of the correct name and signature exists if 
they use this mixin. However, I worry that this is poor/brittle 
design.

What is the best approach to achieve this?

Thanks, Mio