/*
 *  An informal interface for delegates to Something
 */
interface SomethingDelegate {

  optional bool shouldDoStuff(Somethin) = true;
  optional void willDoStuff(Something);
  optional void didDoStuff(Something);

}

/*
 *  The Something class.
 *  That will ask a delegate for permission, and in
 *  general keep it informed when doing stuff.
 *  If one is set that is.
 */
class Something {

  SomethingDelegate delegate;
  
  void doStuff() {
    if (delegate.shouldDoStuff(this)) {
      delegate.willDoStuff(this);
      writefln("Doing stuff");
      delegate.didDoStuff(this);
    }
  }
  
  char[] toString() { return "Foo"; }
}

/*
 *  An actual delegate that keeps track of Somethings doing.
 *  But ignores telling it what to do.
 */
class MyDelegate : SomethingDelegate {
  void willDoStuff(Something s) {
    writefln(s.toString ~ " will do stuff");
  }
  void didDoStuff(Something s) {
    writefln(s.toString ~ " did do stuff");
  }
}

/*
 *  This will output:
 *    Foo will do stuff
 *    Doing stuff
 *    Foo did do stuff
 */
void main() {
  Something something = new Something();
  something.delegate = new MyDelegate();
  
  something.doStuff();
}