@@can.onKeysAdded( handler(newValue) )

The @@can.onKeysAdded symbol points to a function that registers handler to be called back when the keys on the object change via new properties added.

var obj = {
    handlers: {},
    setKeyValue: function(key, value){
        var newKey = this[canSymbol.for("hasOwnKey")](key);
        this[key] = value;
        var self = this;
        if(newKey) {
            obj.handlers.__keysAdded.forEach(function(handler){
                handler.call(self, [key]);
            });
        }
    }
};

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