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XForms Processors are free (and encouraged) to skip or optimize any steps in this algorithm, as long as the end result is the same. The XForms recalculation algorithm considers model items and model item properties to be vertices in a directed graph. Edges between the vertices represent computational dependencies between vertices.
Following is the default handling for a recalculate
action. Action recalculate
is defined in 10.1.4 The recalculate
Element.
A master dependency directed graph is created as detailed in D.1 Details on Creating the Master Dependency Directed Graph.
To provide consistent behavior, implementations must reduce the number of vertices to be processed by computing a pertinent dependency subgraph consisting only of vertices and edges that are reachable from nodes that require recomputation. This is detailed in D.2 Details on Creating the Pertinent Dependency Subgraph. Note that on a first recomputation (such as on form load), the pertinent dependency subgraph will be the same as the master dependency directed graph.
A topological sort is performed on the vertices of the pertinent dependency subgraph, resulting in an order of evaluation in which each vertex is evaluated only after those vertices on which it depends and before all vertices which depend on it. The topological sort algorithm is discussed at [Algorithms].
The recalculate
process completes.
The master dependency directed graph can be considered an array with one record for each vertex, each having the following fields:
InstanceNode: a reference to the associated instance data node
type: indicates the aspect of the instance node represented by the vertex (the text content or a model item property such as readOnly or required)
depList: a list of vertices that refer to this vertex
in-degree: the number of vertices on which this vertex depends
visited: a flag used to ensure vertices are not added to a subgraph multiple times
index: an association between vertices in the master dependency directed graph and a subgraph
The depList
for each vertex is assigned to be the
referenced XML nodes of a given instance node, which are obtained
by parsing the computed expression in the node (e.g., the
calculate, relevant, readonly, or required property). Any
expression violating any Binding Expression Constraint causes an
exception (4.5.4 The
xforms-compute-exception Event), terminating the
recalculate
process.
The depList
for a vertex v
is assigned
to be the vertices other than v
whose computational
expressions reference v
(described below). Vertex
v
is excluded from its own depList
to
allow self-references to occur without causing a circular reference
exception.
A computational expression appearing in a calculate
attribute controls the text content (value) of one or more instance
nodes. A vertex exists for each instance node to represent the
expression in the context of the node. Likewise, computational
expressions for model item properties such as readOnly
and required
are applied to one or more instance
nodes, and vertices are created to represent such expressions in
the context of each applicable node. The computational expression
of each vertex must be examined to determine the XML nodes to which
it refers. Any expression violating any Binding Expression
Constraint causes an exception (4.5.4 The
xforms-compute-exception Event), terminating the
recalculate
process. A computation expression refers
to a vertex v
if a subexpression indicates the
InstanceNode for v
and v
represents the
instance node text content (its value). In this version of XForms,
model item properties such as readOnly
and
required
cannot be referenced in an expression.
If all calculations must be performed, which is the case on form load, then the pertinent dependency subgraph is simply a duplicate of the master dependency directed graph. If the recalculation algorithm is invoked with a list of changed instance data nodes since the last recalculation, then the pertinent dependency subgraph is obtained by exploring the paths of edges and vertices in the computational dependency directed graph that are reachable from each vertex in the change list. The method of path exploration can be depth first search, a suitable version of which appears in the pseudo-code below.
This algorithm creates a pertinent dependency subgraph
S
from a list of changed instance data nodes
L<sub>c</sub>
. Variables such as
v
and w
represent vertices in the master
dependency directed graph. The same variables ending with
S
indicate vertices in the pertinent dependency
subgraph S
.
// Use depth-first search to explore master digraph subtrees rooted at // each changed vertex. A 'visited' flag is used to stop exploration // at the boundaries of previously explored subtrees (because subtrees // can overlap in directed graphs). for each vertex r in Lc if r is not visited { Push the pair (NIL, r) onto a stack while the stack is not empty { (v, w) = pop dependency pair from stack if w is not visited { Set the visited flag of w to true Create a vertex wS in S to represent w Set the index of w equal to the array location of wS Set the index of wS equal to the array location of w Set the InstanceNode of wS equal to the InstanceNode of w Set the type of wS equal to the type of w For each dependency node x of w Push the pair (w, x) onto the stack } else Obtain wS from index of w if v is not NIL { Obtain vS from index of v Add dependency node for wS to vS Increment inDegree of wS } } } // Now clear the visited flags set in the loop above for each vertex vS in S { Obtain v from index of vS Assign false to the visited flag of v }
Note that the number of vertices and dependency nodes in the
pertinent dependency subgraph is not known beforehand, but a method
such as array doubling (see [DDJ-ArrayDoubling]) can be used to
ensure that building the subgraph is performed in time linear in
the size of S
.
The following steps process vertices, resulting in a recalculated form:
A vertex with inDegree of 0 is selected for evaluation and removed from the pertinent dependency subgraph. In the case where more than one vertex has inDegree zero, no particular ordering is specified. If the pertinent dependency subgraph contains vertices, but none have an inDegree of 0, then the calculation structure of the form has a loop, and an exception (4.5.4 The xforms-compute-exception Event) must be thrown, terminating processing.
If the vertex corresponds to a computed item, computed expressions are evaluated as follows:
calculate
: If the value of the model item changes,
the corresponding instance data is updated and the dirty flag is
set.
relevant
, readOnly
,
required
, constraint
: If any or all of
these computed properties change, the new settings are immediately
placed into effect for associated form controls.
For each vertex in the depList
of the removed
vertex, decrement the inDegree by 1.
If no vertices remain in the pertinent dependency subgraph, then the calculation has successfully completed. Otherwise, repeat this sequence from step 1.
For example, consider six vertices a
,
b
, v
, w
, x
, and
y
. Let a
and b
represent the
text content of instance nodes that will be set by a binding from
user input controls. Let v
and w
be
vertices representing the calculated value and the validity
property of a third instance node c
. These vertices
would result from a bind
element B
with
calculate
and constraint
attributes and a
nodeset
attribute that indicates c
.
Suppose that the value of c
is the product of
a
and b
and that the value is only valid
if it does not exceed 100. Likewise, suppose x
and
y
are vertices representing the calculated value and
the validity property of a fourth instance node d
. Let
the value of d
be the sum of a
and
b
, and let d
be valid if the value does
not exceed 20. The figure below depicts the dependency digraph for
this example.
Vertices a
and b
have edges leading to
v
and x
because these vertices represent
the calculate expressions of c
and d
,
which reference a
and b
to compute their
product and sum, respectively. Similarly, v
and
x
have directed edges to w
and
y
, respectively, because w
and
y
represent the constraint
expressions of
c
and d
, which reference the values of
c
and d
to compare them with boundary
values.
If a
and b
are initially equal to 10,
and the user changes a
to 11, then it is necessary to
first recalculate v
(the value of c
) then
recalculate w
(the validity property of the value of
c
). Likewise, x
(the value of
d
) must be recalculated before recalculating
y
(the validity property of the value of
d
). In both cases, the validity of the value does not
change to false
until after the new product and sum
are computed based on the change to a
. However, there
are no interdependencies between v
and x
,
so the product and sum could be computed in either order.
The pertinent subgraph excludes b
and only vertex
a
has in-degree of zero. The vertex a
is
processed first. It is not a computed vertex, so no recalculation
occurs on a
, but its removal causes v
and
x
to have in-degree zero. Vertex v
is
processed second. Its value changes to 121, and its removal drops
the in-degree of vertex w
to zero. Vertex
x
is processed next, changing value to 21. When
x
is removed, its neighbor y
drops to
in-degree zero. The fourth and fifth iterations of this process
recalculate the validity of w
and y
, both
of which change to false.
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