Waiting for a text change in WatiN - ajax

Question

I am trying to test a web page that has ajax calls to update a price.
An ajax call is fired on page load to update an initially empty div.
This is the extension method I'm using to wait for a change in the inner text of the div.
public static void WaitForTextChange(this IE ie, string id)
{
string old = ie.Element(id).Text;
ie.Element(id).WaitUntil(!Find.ByText(old));
}
However it's not pausing even though when I write out the old value and ie.Element(id).Text after the wait, they are both null. I can't debug as this acts as a pause.
Can the Find.ByText not handle nulls or have I got something wrong.
Has anyone got some code working similar to this?

Answer 1

I found my own solution in the end after delving into WatiN's constraints.
Here's the solution:
public class TextConstraint : Constraint
{
private readonly string _text;
private readonly bool _negate;
public TextConstraint(string text)
{
_text = text;
_negate = false;
}
public TextConstraint(string text, bool negate)
{
_text = text;
_negate = negate;
}
public override void WriteDescriptionTo(TextWriter writer)
{
writer.Write("Find text to{0} match {1}.", _negate ? " not" : "", _text);
}
protected override bool MatchesImpl(IAttributeBag attributeBag, ConstraintContext context)
{
return (attributeBag.GetAdapter<Element>().Text == _text) ^ _negate;
}
}
And the updated extension method:
public static void WaitForTextChange(this IE ie, Element element)
{
string old = element.Text;
element.WaitUntil(new TextConstraint(old, true));
}
It assumes that the old value will be read before the change so there's a slight chance of a race condition if you use it too long after setting off the update but it works for me.