Write an OnChange event handler to respond to any changes in the field's data. OnChange allows a response once the data has been successfully written to the record buffer. 

When writing the value of a field to the current record buffer, the following steps occur:

1. The OnValidate event handler is called to validate the data.
2. If the OnValidate event handler accepts the current Value, the data is written to the record buffer.
3. If writing the data does not raise an exception, the OnChange event handler is called to allow a response to the change.

OnValidate 

AfterPost 

Value