It's also possible to specify that a particular element of the instance data be read-only via the "readonly" property. Consider the following example:


<!-- form model -->
<xforms:model id="shop">
 <xforms:instance>
  <item>
   <code />
   <price />
   <discount>2.00</discount>
   <quantity />
   <grossTotal />
   <netTotal />
  </item>
 </xforms:instance>
<xforms:bind nodeset="/item/discount" readonly="true()" />
</xforms:model>



In this case, since you don't want users messing with the discount rate, you can use the "readonly" property to make it non-modifiable. As before, you can combine this with comparison tests to make the property's relevance conditional.

One extremely useful property in this context - and one you'll probably be using a lot - is the "calculate" property, which lets you calculate some of the form data on the fly. Consider the following extension of the example above, which illustrates:


<!-- form model -->
<xforms:model id="shop">
 <xforms:instance>
  <item>
   <code />
   <price />
   <discount>2.00</discount>
   <quantity />
   <grossTotal />
   <netTotal />
  </item>
 </xforms:instance>
<xforms:bind nodeset="/item/discount" readonly="true()" />
<xforms:bind nodeset="/item/grossTotal" 
calculate="/item/price */item/quantity" />
<xforms:bind nodeset="/item/netTotal" calculate="/item/grossTotal -
((item/grossTotal * /item/discount)/100)" />
</xforms:model>



In this case, the "calculate" property has been used to dynamically calculate the value of the <grossTotal> element from the values of the <price> and <quantity> elements. The <netTotal> is then again calculated from the <grossTotal> by accounting for the <discount>.

In case you were wondering why I haven't used a "readonly" property for the two dynamically-calculated values; I don't need to, since these values are automatically made read-only when a "calculate" property is used on them.