When a user attempts to leave a data field, a system validation function is called, if one is necessary for that field type. The system validation function automatically checks the validity of the entry in the field. On errors, the validation function beeps, displays an error message, and returns the user to the field. The system validation functions are all named with the prefix "vf_" and are listed in Table 9.1. If you would like details on what system checks are being performed during field validation for a particular field type, see the description of the appropriate validation function in the Function Reference.
You can specify a range for most data field types. All standard data entry field types, except Boolean fields and check boxes, support range checking. If a range is specified, the value entered by the user is checked by the system validation function during field validation to ensure that it falls within the desired range. How to specify a valid range for a field is described later in this chapter.
System validation functions are provided for most data field types. See Table 9.1 for a listing. If you are using a multi-toggle or a spin button, the system uses the validation function for the underlying data field type.
In addition to the system validation function, you can install your own validation function for any data field. An installed validation function is called after the system validation function. See Chapter 32, "Writing User Validation Functions," for more information.
