XForms Essentials

Model Item Properties

An individual property that can be applied to a node is called a model item property. Some of the properties are XPath expressions (called computed expressions in the specification), which the XForms Processor tracks and reevaluates as necessary:

• readonly

• required

• relevant

• calculate

• constraint

The remaining properties are static, and don’t get reevaluated:

• type

• p3ptype

Some of the model item properties have an effect on child nodes as well. The rules for this behavior can be summarized like this:

• Setting a node to readonly sets all child nodes to readonly, unless specifically overridden.

• Setting a node to non-relevant sets all child nodes to non-relevant, unless specifically overridden.

• For all other model item properties, setting that property on a node has no effect on child nodes.

  Warning

  A common mistake is to write code like this:

  required="true"

  This almost certainly doesn’t have the intended effect of making the node always required. Computed model item properties are XPath expressions, and under XPath rules, "true" is interpreted as a node-set matching all element nodes named true. Unless there is such an element, the expression will evaluate to an empty node-set, which is interpreted as an XPath boolean false! Always use the functions true( ) and false( ), which return the expected XPath boolean values.

  required="true(  )"