[TYPO3-english] How to prevent saving if evaluation failed?

[Chaiyawut Keereeto]

Hi,

I'm trying to find the way to prevent saving record in the backend if 
evaluation is failed. I've built custom eval class and add it to TCA. 
What I want is prevent the saving if the evaluation failed.

I know that I have to do it in evaluateFieldValue function of the class. 
But I have no idea how to do that since I've tried various ways I can 
think of.

I would be so nice if someone can give me an advice.

Thanks,
C.Keereeto