@@can.onKeyValue( key, handler(newValue) )

The @@can.onKeyValue symbol points to a function that registers handler to be called back with the new value of key when key changes.

var obj = {
   handlers: {},
   setKeyValue: function(key, value){
       this[key] = value;
       var self = this;
       obj.handlers[key].forEach(function(handler){
           handler.call(self, value);
       });
   }
};

obj[canSymbol.for("can.onKeyValue")] = function(key, handler){
   if(!obj.handlers[key]) {
       obj.handlers[key] = [];
   }
   obj.handlers[key].push(handler);
}