Business rules

XForms makes it very easy to create dependencies between different parts of a web application. Of major significance is that these dependencies are established at the level of the data, before being reflected in the user interface. For example, say a form contains a data value for 'number of years at the current address' and another for 'previous address'; a rule could be created that says the latter need not be completed if the form-filler has been at their current address for less than three years.

In XForms this rule is defined completely independently of the actual user interface itself, but the XForms processor will ensure that any form control that is representing the 'previous address' fields will only be made relevant (i.e., available for the user to fill in), once the correct conditions are met.

It's also possible to define a rule that says the 'previous address' field is required if the form-filler has only lived at their current address for a certain amount of time. An XForms processor can prevent the form from being submitted to the server, if this condition is not met.

Since these 'business rules' are defined using a standard mark-up language and XPath statements, they become very easy to develop, maintain, and test. This is because, unlike traditional e-forms systems where such rules are tightly coupled with the user interface, in XForms there is a separate data layer within which the logic sits. (XForms was designed to make use of the Model-View-Controller pattern.)

In the following screenshot from the mortgage application sample (see the showcase area) a user has indicated that the purpose of the mortgage they are applying for is to buy a bungalow:

However, when the selection is changed to indicate the purchase of a flat, three further fields automatically become available to the user, to allow the capture of more information: