1 Terminology
The key words MUST, MUST NOT, REQUIRED,
SHALL,
SHALL
NOT, SHOULD, SHOULD
NOT, RECOMMENDED, MAY, and OPTIONAL
in this specification are to be interpreted as described in [RFC 2119].
The terms base URI and relative URI are used in this
specification as they are defined in [RFC
2396].
2 The XPath Data
Model
The value "xpath" for the 'datamodel' attribute results in an
XML data model with XPath used as the expression language.
Implementations that support this data model MUST support
[XPath 1.0].
2.1 Data
Model
The underlying structure of the XPath data model follows the
document syntax closely. In particular, the SCXML Processor MUST maintain
the XPath data model as an XML document with a root element called
. For each element in the document,
the SCXML Processor MUST create a child of
called with an 'id' attribute whose value is the same
as that of the 'id' attribute of the document element.
The Processor MUST insert the value of the document
element as the child(ren) of the datamodel
element. The Processor MUST also bind
an XPath variable of the same name to that datamodel
element.
If either the 'src' attribute or in-line content is provided in
the document element, then if the content (whether
fetched or provided in-line) is an XML document or node set, the
SCXML Processor MUST insert the XML as the child of the datamodel
element. Otherwise the Processor MUST treat the
content as a space-normalized string literal and insert it as the
child of the datamodel element. If no value is
assigned, the SCXML Processor MUST leave the datamodel element
empty, as follows:
2.1.1 Scoping
The Processor MUST place all variables in a single global
XPath scope. Specifically, the SCXML Processor MUST allow any
data element to be accessed from any state. Ordering dependencies
between elements are not permitted. In the case of
early binding, the SCXML Processor MUST evaluate
all elements at initialization time but MAY do so in
any order it chooses. When late binding is selected, the SCXML
Processor MUST create data model elements at
initialization time but MAY do so in any order it chooses. Similarly,
the processor MUST assign the specified initial values to
data elements only when the state containing them is first entered,
but MAY do
so in any order it chooses.
Example company
example.com
John Doe
2.2 Conditional Expressions
The SCXML Processor MUST accept any XPath 1.0 expression as a
conditional expression and MUST convert it into its effective boolean
value as described in section 2.4.3 of the [XPath
2.0] specification. (Note that the definition of effective
boolean value is taken from the XPath 2.0 specification, even
though the XPath datamodel requires support only for expressions as
defined in XPath 1.0, and not for the broader range of expressions
defined in XPath 2.0.) The following example illustrates this
usage.
The SCXML processor MUST define an XPath function 'In()' that
takes a stateID as its argument and returns 'true' if and only if
that state is in the current state configuration, as described in
Conditional Expressions. For examples of the use of this
predicate (but in an ECMAScript context), see microwave
example.
Function signature: In($stateID as xs:string?) as
xs:boolean
Returns an xs:boolean indicating whether or not the
state with ID $stateID is one of the currently active states.
2.3 Location Expressions
The SCXML Processor MUST accept any XPath expression as a location
expression. The following example illustrates this usage:
2.4 Value Expressions
The SCXML Processor MUST allow any XPath expression to be used as
a value expression. If the result of the value expression is a
node-set, the Processor MUST make a deep copy of the subtree rooted at
each node. The Processor MAY convert the expression result to a string
In contexts where a string is expected.
2.5
When is a child of , the
Processor MUST interpret its value as defined in 2.7.1 _event.data. When
is a child of , the interpretation of
its value depends on the Event I/O Processor. When
is a child of , the interpretation of its value is
platform-specific.
2.6
Implementations supporting the XPath data model MUST support
the following additional attributes for the
element.
| Name |
Required |
Attribute Constraints |
Type |
Default Value |
Valid Values |
Description |
| type |
false |
|
enum |
replacechildren |
replacechildren, firstchild, lastchild, previoussibling,
nextsibling, replace, delete, addattribute |
Defines the nature of the insertion to be performed. See below
for details. |
| attr |
false |
This attribute must be present if and only if 'type' is
'addattribute'. |
NMTOKEN |
none |
|
The attribute name to add at the specified location. |
The SCXML Processor MUST modify the data model as directed by the
'type' attribute as described below. If it is unable to do so, it
MUST place
the error error.execution on the internal event queue:
- replacechildren. Replace all the children at 'location' with
the value specified by 'expr'.
- firstchild. Insert the value specified by 'expr' before all of
the children at 'location'.
- lastchild. Insert the value specified by 'expr' after all of
the children at 'location'.
- previoussibling. Insert the value specified by 'expr' before
the node specified by 'location', keeping the same parent.
- nextsibling. Insert the value specified by 'expr' after the
node specified by 'location', keeping the same parent.
- replace. Replace the node specified by 'location' by the value
specified by 'expr'.
- delete. Delete the node specified by 'location'. ('expr' is
ignored.)
- addattribute. Add an attribute with the name specified by
'attr' and value specified by 'expr' to the node specified by
'location'.
If the specified location contains a node set, the Processor MUST perform
the assignment to each node in the set. It MAY do so in
any order it chooses, and MUST evaluate 'expr' only once.
Note that in the case of an XML data model, it is not required
to assign to the root of a tree (i.e., the "name" value in a
tag), since the path expression can reach down into
the tree to assign a new value to an internal node. The following
examples show various aspects of assignment in the XPath data
model. Suppose we have a data model of the following form:
The Zen Mind
Freakonomics
Here is an example of assignment of a string to an element
node.
results in
My favorite book
Freakonomics
...
Now suppose we assign an XML structure to an element node. The
following assignment statement would have the effect of replacing
the children of the element "$cart/myCart/books/book[1]" by the XML
tree rooted in .
results in
12334455
some author
Freakonomics
...
Here are examples of legal and illegal assignment to an
attribute:
Now suppose we assign a string to a nodeset. The following
assignment statement would have the effect of replacing the
children of each node in the nodeset $cart/myCart/books/book with
the string "The Zen Mind":
results in
The Zen Mind
The Zen Mind
...
Finally suppose we assign a structure to a nodeset. The
following statement would iterate over the elements in the nodeset
of elements and replace their children with the
structure:
results in
If the evaluation of any of the expressions in an
element causes an error to be raised, evaluation of the element
terminates immediately and the has no effect. For
example, the following assignment statement would raise an error
because the sample data model we are using does not have an
node as a child of :
2.7 System Variables
For each system variable defined in System
Variables, the SCXML Processor MUST create a
data model element with an 'id' attribute whose value
is the name of the system variable. The Processor also MUST bind an
XPath variable whose name is the name of the system variable to
this element. For the _event system variable (i.e., the child of ), for each of the fields
defined in
The Internal Structure of Events, the Processor also MUST create an
child element whose name is the name of the field. For the
_ioprocessors system variable the Processor MUST create an
child element for each Event I/O processor that
it supports, with a 'name' attribute whose value is the name of the
I/O processor. For the SCXML and BasicHTTP Event I/O processors,
the Processor MUST create a child under the
child element, assigning the access URI as its text child. For
example, in systems that support the BasicHTTP Event I/O Processor,
the access URI of the BasicHTTP Event I/O processor can be a
accessed as
$_ioprocessors/processor[@name="/http://www.w3.org/TR/scxml/#BasicHTTPEventProcessor/"]/location/text().
2.7.1
_event.data
The _event variable's child is populated with
content provided by an external event or by or
. In some circumstances, the entity that generated
the event may have indicated how the data is to be interpreted. In
such cases, the Processor SHOULD try to format the data according to
those indications. Otherwise, if the SCXML Processor can interpret
the content as key-value pairs, then for each unique key, the
Processor MUST create a element as a child
of _event's child with an 'id' attribute whose value
is the name of the key. The Processor MUST insert
the value of the key-value pair as the content of this element. In
the case of duplicate keys, the behavior is platform-specific.
(Note that content specified by or delivered as POST
parameters via the BasicHTTP Event I/O Processor consists
unambiguously of key-value pairs.) Otherwise (i.e., if the content
does not consist of key-value pairs), if the Processor can
interpret the content is a valid XML document, the it MUST insert
the XML structure as the content of _event's child.
Otherwise, the Processor MUST treat the content as a space-normalized
string literal and insert it as the content of _event's
child.
Suppose as part of executing a state machine named "myName" with
a platform-assigned sessionid "12345", we are processing an event
with the name "foo.bar" and the following XML payload:
Then the underlying XML data model would have the following form
(not all the fields of the event are shown):
myName
12345
foo.bar
42
http://example.com/scxml-http/12345
As an example, here is a sample transition that accesses the
$_sessionid variable in that data model.
2.8
Serialization
In certain circumstances, e.g. when including data in events
sent via the BasicHTTP Event I/O Processor, the SCXML Processor is
required to serialize data from the XPath data model for
transmission to a remote entity. In such cases, the Processor MUST serialize
the data as a string.
2.9 Scripting
There is no scripting language.
2.10
In the XPath data model, the legal iterable collections are
Node-Sets. The legal values for the 'item' attribute are legal
XPath variable names. The iteration order is the order of the
underlying Node-set, and goes from an index of 1 by increments of
one to an index of count(node-set).
2.11 Unsupported Elements
The script
element is not supported in the XPath Data Model.