In the previous article, I showed you how to manage user input in the XForms model. I discussed the process of submitting an XForm and – more importantly - validating user input prior to submission using built-in XML Schema support. In this concluding article, find out how to use the <xforms:bind> element to perform calculations on form input values, integrate XPath expressions into your XForms model and get a crash course in the XForms event model.
Price of most expensive book: <xforms:output ref="/inventory/books/mostExpensive" /> <br />
Price of least expensive book: <xforms:output ref="/inventory/books/leastExpensive" /> <br />
Average price: <xforms:output ref="/inventory/books/averagePrice" />
As usual, all the action is centered around the definition of the XForms model. This time, the instance data has been populated with the inventory of the neighbourhood bookstore, and XPath functions like sum(), min(), max() and avg() have been used to perform calculations on that data, and assign the results of those calculations to specific summary elements in the instance data. This data is then displayed in the form using the <xforms:output> element.
You'll also notice a new element in the form above - the <xforms:repeat> element. This element gives XForms authors the ability to loop over a collection of nodes without having to resort to complex "while" or "for" loops. In this example, the <xforms:repeat> element causes the XForms processor to iterate over the <books> collection from the instance data, and display the values of the title, author, price and quantity. The "nodeset" attribute specifies the node over which iteration should take place, while the loop counter is handled internally by the processor. Looping will stop once no further match is found for the "nodeset" criteria.
Why stop just at numeric functions, though? This next example uses the days-from-date() function to calculate the number of days between an entered date and January 01 1970.
The XForms model will look something like this:
<!-- form model --> <xforms:model id="dateCalculator"> <xforms:instance> <calc> <dt /> <numDays /> </calc> </xforms:instance> <xforms:bind nodeset="/calc/numDays" calculate="days-from-date(/calc/dt)" /> </xforms:model>