Question: I want to do some validation whenever the value of a textfield changes. I don't see an on change event mentioned in the documentation though.

Answer: 
Depending on what exactly you mean, there are a couple of ways to go about this.

The simplest case is the one where you've added variables or methods to an already-existing class, as in this example:

```
class String
  def rot13
    return self.tr('a-z', 'n-za-m')
  end
end

```

Here we've added the rot13 method to class String. As soon as this code is run, every String everywhere in your program will be able to #rot13. Thus, if you have some code that needs rot13-capable strings, you just ensure that the code above is run before the code in question, e.g. by putting the rot13 code in a file someplace and require()ing it. Very easy!

But maybe you've added a class variable to a class, and you want to preserve not just its existence but its value, as in:

```
class String
  @@number_of_tr_calls_made = 0
  # Fix up #tr so that it increments @@number_of_tr_calls_made
end

```

Now if you want to save the value of @@number\_of\_tr\_calls\_made, you can do so in the same way you would with any other serializable Ruby value: via the Marshal library. Also easy!

But something in the way you phrased your question makes me suspect that you're doing something like this:

```
greeting = "Hello"
class <<greeting
  def rot13
    return self.tr('a-z', 'n-za-m')
  end
end
encrypted_greeting = greeting.rot13

```

This is very different from what we did in the first example. That piece of code gave every String in your program the power to rot13 itself. This code grants that power to only the object referred to by the name 'greeting'. Internally, Ruby does this by creating an anonymous Singleton subclass of String, adding the rot13 method to it, and changing greeting's class to that anonymous subclass.

The problem here is that Singletons can't be Marshal'd (to see why, try to figure out how to maintain the Singleton invariant when any call to Marshal.load can generate copies of extant Singleton objects). Now greeting has a Singleton in its inheritance hierarchy, so if you want to save and load it you are hosed. Make a subclass instead:

```
class HighlySecurableString < String
  def rot13
    return self.tr('a-z', 'n-za-m')
  end
end
greeting = HighlySecurableString.new("hello")

```
You're editing the class on the fly, and you want to save that? You could try using the Marshal module, it'll allow you to save objects to a file, and read them back in memory dynamically.