iOS - validate if a signature field is signed.

Hello,

I'm trying to check if a required signature field is signed, but when I iterate over the doc fields and get the signature value is always null. It happens too in Complete Reader Sample. Below a sample code I'm using.

        if let iterator = self.pdfCtrl.getDoc().getFieldIterator()
        {
            while iterator.hasNext()
            {
                if let field = iterator.current()
                {
                    if field.getType() == e_ptsignature
                    {
                        if field.getValue() != nil
                        {
                            print("Valid")
                        }
                    }
                }
                
                iterator.next()
            }
        }

What is null? The field.GetValue result is always null, even for a signed digital signature?

If so, how do you know it is signed?
Does this happen with any document, or only specific ones?