XPath Expressions in XForms

7 XPath Expressions in XForms

XForms uses XPath to address instance data nodes in binding expressions, to express constraints, and to specify calculations. XPath expressions other that are not syntactically valid, including attempted calls to undefined functions, result in an exception (4.5.4 The xforms-compute-exception Event), except for binding expressions, which produce a different exception (4.5.1 The xforms-binding-exception Event).

7.1 XPath Datatypes

XPath data types are used only in binding expressions and computed expressions. XForms uses XPath datatypes boolean, string, number, and node-set. A future version of XForms is expected to use XPath 2.0, which includes support for XML Schema datatypes.

7.2 Instance Data

For each model element, the XForms Processor maintains the state in an internal structure called instance data that conforms to the XPath Data Model [XPath 1.0]. XForms Processors that implement DOM must provide DOM access to this instance data via the interface defined below.

Note:

Instance data always has a single root element, and thus corresponds to a DOM Document.

interface XFormsModelElement : org.w3c.dom.Element

The method getInstanceDocument returns a DOM Document that corresponds to the instance data associated with this XForms Model.

This parameter specifies the ID of an instance element.

Return value: org.w3c.dom.Document

raises (DOMException); if there is no model with the specified instance-id.

This method signals the XForms processor to rebuild any internal data structures used to track computational dependencies within this XForms Model.

This method takes no parameters.

Return value: None

This method raises no exceptions.

This method signals the XForms processor to perform a full recalculation of this XForms Model.

This method takes no parameters.

Return value: None

This method raises no exceptions.

This method signals the XForms processor to perform a full revalidation of this XForms Model.

This method takes no parameters.

Return value: None

This method raises no exceptions.

This method signals the XForms processor to perform a full refresh of form controls bound to instance nodes within this XForms Model.

This method takes no parameters.

Return value: None

This method raises no exceptions.

Note:

Script invocation of recalculate() is not necessarily equivalent to performing the recalculate action handler. Though the script is assumed to have modified instance data prior to invoking recalculate(), the DOM mutations are not cached. Thus, a full recalculation is necessary to ensure the proper changes are effected throughout the XForms model.

7.3 Evaluation Context

Within XForms, XPath expressions reference abstract instance data (using the "path" portion of XPath), instead of a concrete XML document. This reference is called a binding expression in this specification. Every XPath expression requires an evaluation context. The following rules are used in determining evaluation context when evaluating elements containing binding expressions in XForms:

The context node for outermost binding elements is the top level element node, or the single node returned by /*. A binding element is any element that is explicitly allowed to have a binding expression attribute. A binding element is "outermost" when the node-set returned by the XPath expression ancestor::* includes no binding element nodes.
The context node for non-outermost binding elements is the first node of the binding expression of the immediately enclosing element. An element is "immediately enclosing" when it is the first binding element node in the node-set returned by the XPath expression ancestor::*. This is also referred to as "scoped resolution".
The context node for computed expressions (occurring on element bind) is the first node of the node-set returned from the binding expression in the sibling ref attribute.
The context size and position are both exactly 1.
No variable bindings are in place.
The available function library is defined below, plus any functions supplied by the implementation. Extension functions required for operation of the form should be declared, as described at 7.10 Extension Functions.
Any namespace declarations in scope for the attribute that defines the expression are applied to the expression.

Example: Binding Expression Context Nodes

<group ref="level2/level3">
  <select1 ref="@attr" ... />
</group>

In this example, the group has a binding expression of level2/level3. According to the rules above, this outermost element would have a context node of /level1, which is the root element of the instance data. The select1 form control then inherits a context node from the parent group. Matching instance data follows:

Example: Sample Instance

<level1>
  <level2>
    <level3 attr="xyz"/>
  </level2>
</level1>

7.4 Binding Expressions

A binding expression is an XPath LocationPath expression used in binding a model item property to one or more instance nodes, or to bind a form control to the instance, or to specify the a node or node set for operation by an action.

Not every possible XPath expression is acceptable as a binding expression. In particular, there restrictions on binding expressions that create dynamic dependencies, which are defined as follows:

Any XPath predicate expression (in square brackets) is a dynamic dependency unless both sides of the comparison are "fixed", where fixed means either a constant, or a value that will not change between operations explicitly defined as rebuilding computational dependencies.

Note:

A dynamic dependency does not result if either side of the predicate is one of the following: position(), last(), count(), or index(); the specification mandates a dependency rebuild after any event that could change the values returned by these functions. Note also that property() is also permitted, since it always returns the same values for the duration of form processing.

Another dynamic dependency is any use of the id() function, unless both the parameter to the function and the matching attribute of type xsd:ID are fixed. In the same way, the instance() function is dynamic unless the parameter to the function is fixed.

XPath variables that change in value from one recalculate to the next would also create dynamic dependencies (though XForms 1.0 defines an empty variable context for all XPath expressions).

Authors that define extension functions are encouraged to follow these rules.

7.4.1 Model Binding Expressions

A model binding expression is a kind of binding expression used in bind that can be used to declare model item properties.

No dynamic dependencies are allowed in model binding expressions. Upon detecting a dynamic dependency in a model binding expression, form processing terminates with an exception (4.5.1 The xforms-binding-exception Event).

7.4.2 UI Binding Expressions

Binding references can be used to bind form controls to the underlying data instance as described here. Different attribute names, ref and nodeset distinguish between a single node and a node-set respectively. See 3.2.3 Single-Node Binding Attributes and 3.2.4 Node-Set Binding Attributes.

Dynamic dependences are allowed in UI binding expressions based on the conformance profile.

7.4.3 UI Binding in other XML vocabularies

The XForms binding mechanism allows other XML vocabularies to bind user interface controls to an XForms Model using any of the techniques shown here. As an example, XForms binding attribute bind might be used within legacy HTML user interface controls as shown below. See 3.2.3 Single-Node Binding Attributes and 3.2.4 Node-Set Binding Attributes.

Example: XForms Binding In Legacy HTML User Interface Controls

<html:input type="text" name="..." xforms:bind="fn"/>

7.4.4 Binding Examples

Consider a document with the following XForms model:

<xforms:model id="orders">
  <xforms:instance xmlns="">
    <orderForm>
      <shipTo>
        <firstName>John</firstName>
      </shipTo>
    </orderForm>
  </xforms:instance>
  <xforms:bind nodeset="/orderForm/shipTo/firstName" id="fn" />
</xforms:model>

The following examples show three ways of binding user interface control xforms:input to instance element firstName declared in the model shown above.

Example: UI Binding Using Attribute ref

<xforms:input ref="/orderForm/shipTo/firstName">...

Example: UI Binding Using Attribute bind

<xforms:input bind="fn">...

Example: Specifies Model Containing The Instance Explicitly

<xforms:input model="orders" ref="/orderForm/shipTo/firstName">...

7.5 XForms Core Function Library

The XForms Core Function Library includes the entire [XPath 1.0] Core Function Library, including operations on node-sets, strings, numbers, and booleans.

This following sections define additional required functions for use within XForms.

7.6 Boolean Methods

7.6.1 The boolean-from-string() Function

boolean boolean-from-string(string)

Function boolean-from-string returns true if the required parameter string is "true" or "1", or false if parameter string is "false", or "0". This is useful when referencing a Schema xsd:boolean datatype in an XPath expression. If the parameter string matches none of the above strings, according to a case-insensitive comparison, processing stops with an exception (4.5.4 The xforms-compute-exception Event).

7.6.2 The if() Function

string if(boolean, string, string)

Function if evaluates the first parameter as boolean, returning the second parameter when true, otherwise the third parameter.

7.7 Number Methods

7.7.1 The avg() Function

number avg(node-set)

Function avg returns the arithmetic average of the result of converting the string-values of each node in the argument node-set to a number. The sum is computed with sum(), and divided with div by the value computed with count().

7.7.2 The min() Function

number min(node-set)

Function min returns the minimum value of the result of converting the string-values of each node in argument node-set to a number. "Minimum" is determined with the < operator. If the parameter is an empty node-set, the return value is NaN.

7.7.3 The max() Function

number max(node-set)

Function max returns the maximum value of the result of converting the string-values of each node in argument node-set to a number. "Maximum" is determined with the < operator. If the parameter is an empty node-set, the return value is NaN.

7.7.4 The count-non-empty() Function

number count-non-empty(node-set)

Function count-non-empty returns the number of non-empty nodes in argument node-set. A node is considered non-empty if it is convertible into a string with a greater-than zero length.

7.7.5 The index() Function

number index(string)

Function index takes a string argument that is the idref of a repeat and returns the current position of the repeat index for the identified repeat—see 9.3.1 The repeat Element for details on repeat and its associated repeat index. If the specified argument does not identify a repeat, processing stops with an exception (4.5.4 The xforms-compute-exception Event).

Example: index

<xforms:button>
  <xforms:label>Add to Shopping Cart</xforms:label>
  <xforms:insert ev:event="ev:activate" position="after"
      nodeset="items/item" at="index('cartUI')"/>
</xforms:button>

7.8 String Methods

7.8.1 `The property() Function`

string property(string)

Function property returns the XForms property named by the string parameter.

The following properties are available for reading (but not modification).

version: version is defined as the string "1.0" for XForms 1.0
conformance-level: conformance-level strings are defined in 12 Conformance.

Example: property

<xforms:instance>
  ...
  <xforms:bind ref="info/xforms-version"
      calculate="property('version')"/> ...
</xforms:instance>

7.9 Date and Time Functions

Note:

The following XML Schema datatypes do not have specific functions for manipulation within XForms expressions: xsd:time, xsd:gYearMonth, xsd:gYear, xsd:gMonthDay, xsd:gDay, xsd:gMonth. Extension functions (7.10 Extension Functions) may be used to perform needed operations on these datatypes.

7.9.1 The now() Function

string now()

The now function returns the current system date and time as a string value in the canonical XML Schema xsd:dateTime format. If time zone information is available, it is included (normalized to UTC). If no time zone information is available, an implementation default is used.

Note:

Attaching a calculation of "now()" to an instance data node would not result in a stream of continuous recalculations of the XForms Model.

7.9.2 The days-from-date() Function

number days-from-date(string)

This function returns a whole number of days, according to the following rules:

If the string parameter represents a legal lexical xsd:date or xsd:dateTime, the return value is equal to the number of days difference between the specified date and 1970-01-01. Hour, minute, and second components are ignored. Any other input parameter causes a return value of NaN.

Examples:

days-from-date("2002-01-01") returns 11688
days-from-date("1969-12-31") returns -1

7.9.3 The seconds-from-dateTime() Function

number seconds-from-dateTime(string)

This function returns a possibly fractional number of seconds, according to the following rules:

If the string parameter represents a legal lexical xsd:dateTime, the return value is equal to the number of seconds difference between the specified dateTime and 1970-01-01T00:00:00Z. If no time zone is specified, an implementation default is used. Any other input string parameter causes a return value of NaN.

7.9.4 The seconds() Function

number seconds(string)

This function returns a possibly fractional number of seconds, according to the following rules:

If the string parameter represents a legal lexical xsd:duration, the return value is equal to the number specified in the seconds component plus 60 * the number specified in the minutes component, plus 60 * 60 * the number specified in the hours component, plus 60 * 60 * 24 * the number specified in the days component. The sign of the result will match the sign of the duration. If no time zone is specified, an implementation default is used. Year and month components, if present, are ignored. Any other input parameter causes a return value of NaN.

Examples:

seconds("P1Y2M") returns 0
seconds("P3DT10H30M1.5S") returns 297001.5
seconds("3") returns NaN

Note:

Even though this function is defined based on a lexical xsd:duration, it is intended for use only with derived-from-xsd:duration datatypes, specifically xforms:dayTimeDuration.

7.9.5 The months() Function

number months(string)

This function returns a whole number of months, according to the following rules:

If the string parameter represents a legal lexical xsd:duration, the return value is equal to the number specified in the months component plus 12 * the number specified in the years component. The sign of the result will match the sign of the duration. Day, hour, minute, and second components, if present, are ignored. Any other input parameter causes a return value of NaN.

Examples:

months("P1Y2M") returns 14
months("-P19M") returns -19

Note:

Even though this function is defined based on a lexical xsd:duration, it is intended for use only with derived-from-xsd:duration datatypes, specifically xforms:yearMonthDuration.

7.10 Extension Functions

XForms documents may use additional XPath extension functions beyond those described here. The names of any such extension functions must be declared in attribute functions on element model. Such declarations are used by the XForms Processor to check against available extension functions. XForms Processors perform this check at the time the document is loaded, and stop processing by signaling an exception (4.5.4 The xforms-compute-exception Event) if the XForms document declares an extension function for which the processor does not have an implementation.

Note:

Explicitly declaring extension functions enables XForms Processors to detect the use of unimplemented extension functions at document load-time, rather than throwing a fatal error during user interaction. Failure by authors to declare extension functions will result in an XForms Processor potentially halting processing during user interaction with a fatal error.


Table of Contents	Prev	Next	Bottom

7 XPath Expressions in XForms

7.1 XPath Datatypes

7.2 Instance Data

7.3 Evaluation Context

7.4 Binding Expressions

7.4.1 Model Binding Expressions

7.4.2 UI Binding Expressions

7.4.3 UI Binding in other XML vocabularies

7.4.4 Binding Examples

7.5 XForms Core Function Library

7.6 Boolean Methods

7.6.1 The boolean-from-string() Function

7.6.2 The if() Function

7.7 Number Methods

7.7.1 The avg() Function

7.7.2 The min() Function

7.7.3 The max() Function

7.7.4 The count-non-empty() Function

7.7.5 The index() Function

7.8 String Methods

7.8.1 The property() Function

7.9 Date and Time Functions

7.9.1 The now() Function

7.9.2 The days-from-date() Function

7.9.3 The seconds-from-dateTime() Function

7.9.4 The seconds() Function

7.9.5 The months() Function

7.10 Extension Functions

7.8.1 `The property() Function`