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Binds

Introduction
The XForms xf:bind element is used to point to data in the data model. It allows to associate with data:
an id
a name (see Model Bind Variables)
properties, called Model Item Properties (MIPs)
This serves the following functions:
in the model
determine which part of the data is valid, readonly, and relevant
apply calculations to the data (formulas)
export variables used in calculations (see Model Bind Variables)
in the model and in the view
as an indirection to the data model, with the bind attribute or the bind() function
Extensions
Nested MIP elements
[SINCE Orbeon Forms 4.3 for xf:constraint, and 4.9 for other elements]
Instead of the type, readonly, required, relevant, calculate, constraint, and xxf:default attributes, you can use nested elements:
xf:type
xf:readonly
xf:required
xf:relevant
xf:calculate
xf:constraint
xxf:default (see below)
All elements except xf:type (whose value is not an XPath expression but a type literal) have a value attribute. The following binds are equivalent:
<xf:bind
    ref="control-1"
    type="xf:decimal"
    xxf:default="42"
    constraint=". ge 10"
    required="true()"/>
and:
<xf:bind ref="control-1">
    <xf:type>xs:decimal</xf:type>
    <xxf:default value="42"/>
    <xf:constraint value=". ge 10"/>
    <xf:required value="true()"/>
    <xf:relevant value="$qty gt 0"/>
</xf:bind>
This enables two features:
the ability to assign a specific id attribute to a MIP and refer to it from an `​
the ability to specify multiple readonly, required, relevant, and constraint rules (which are combined using either a boolean "or" or a boolean "and")
See XForms Validation for details about the validation-related elements (xf:type, xf:required, and xf:constraint).
Multiple binds pointing to the same node
NOTE: This is scheduled to be standardized in XForms 2.
Model Item Properties (MIPs) have a default value:
required: false
valid: true (depends on a series of conditions, including required)
relevant: true
readonly: false
The resulting value of a MIP on a given node when multiple binds touch that node is the result of a boolean combination:
required: boolean "or"
valid: boolean "and"
relevant: boolean "and"
readonly: boolean "or"
The values are also combined this way when multiple nested elements are specified on a same bind.
Consider the following example:
<xf:model>
    <xf:instance>
        <instance>
            <some-node/>
        </instance>
    </xf:instance>
    <xf:bind ref="instance()//*" required="true()"/>
    <xf:bind ref="instance()/some-node" required="false()"/>
</xf:model>
some-node's required property is here set to true, and the order of the two binds doesn't matter.
NOTE: This is the behavior with Orbeon Forms 3.9 onwards. With Orbeon Forms 3.8, the combination was based on bind order.
Extension XPath functions
The xxf:bind() function returns the node-set of a given bind.
NOTE: This is also available as a plain bind() function since it is proposed for XForms 2.0.
The xxf:evaluate-bind-property() function evaluates a property of a given bind.
The xxf:type() function returns the type of the instance data node passed as parameter.
For details, see XPath Function Library.
Custom MIPs
You can place user-defined Model Item Properties (MIPs) on the <xf:bind> element.
When xxf:custom-mips is missing (see below), any attribute not in a standard Orbeon namespace is interpreted as a custom MIP:
<xf:bind ref="*" foo:bar="if (starts-with(., 'g')) then 'is-g' else 'is-not-g'"/>
The value of the attribute must be a valid XPath expression. The expression result is converted to a string to set the MIP value.
Custom MIPs have the side effect of placing CSS classes on controls bound to affected nodes. Class names are computed as the concatenation of:
MIP attribute prefix
"-"
MIP attribute local name
"-"
MIP value
With the example above, the following class names can be set: foo-bar-is-g or foo-bar-is-not-g.
[SINCE Orbeon Forms 4.0.1]
The model supports the xxf:custom-mips attribute, which lists the names of the custom MIPs in use. When this attribute is present, only the attributes with names listed are interpreted as MIPs. Other non-built-in attributes are ignored.
When this attribute is missing, the backward-compatible behavior is enabled and all non-built-in attributes are considered custom MIPs.
It is recommended to use the xxf:custom-mips attribute to specify which attributes are custom MIPs.
<xf:model xxf:custom-mips="foo:bar foo:baz">
    <xf:bind ref="name" foo:bar="normalize-space(.) = ''" foo:baz="42"/>
</xf:model>
Dynamic initial values
The xxf:default MIP
In XForms, default or initial values can be set by pre-populating an instance document's elements and attributes with initial data, for example:
<xf:instance>
    <form>
        <username>jdoe</username>
        ...
    </form>
</xf:instance>
For dynamic values, for example coming from request parameters or session values, there is no declarative notation and you must use xforms-submit-done, xforms-model-construct-done, or xforms-submit-ready, which is sometimes cumbersome:
<xf:setvalue
    event="xforms-model-construct-done"
    ref="username"
    value="xxf:get-request-header('MY_USER')"/>
For convenience, Orbeon Forms support an extension MIP: xxf:default. It works like the standard calculate, except that it is evaluated only once, just before the first evaluation of the calculate expressions if any.
<xf:bind ref="username" xxf:default="xxf:get-request-header('MY_USER')"/>
Forcing recalculation of initial values with the recalculate action
<xf:recalculate> supports an extension attribute, xxf:defaults, which, when set to true, forces the re-evaluation of initial values before performing the recalculation.
<xf:recalculate xxf:defaults="true"/>
The xxf:defaults attribute is an AVT and can include XPath expressions between curly brackets:
<xf:recalculate xxf:defaults="{instance()/status = 'dirty'}"/>
Evaluation of initial values upon insert
See Evaluation of initial values upon insert.
Whitespace processing
The xxf:whitespace MIP
[SINCE Orbeon Forms 2016.1]
The xxf:whitespace MIP controls whitespace trimming. When the MIP is absent, there is no whitespace trimming.
xxf:whitespace can take one of two values:
preserve: there is no change to the value's whitespace (the default).
trim: leading and trailing whitespace is removed.
NOTE: The value is static and cannot be the result of an XPath expression.
When using trim, all leading and trailing characters which are spaces, including non-breakable spaces, or ISO control characters, are removed.
Example:
<xf:bind
    id="first-name-bind"
    ref="first-name"
    xxf:whitespace="trim"/>
Processing model
xxf:whitespace processing takes place during XForms recalculate processing before xxf:default and calculate processing. This means that calculations have access to data which already had whitespace processed.
The result of xxf:default and calculate follows the xxf:whitespace MIP. This means that at the time of revalidation and refresh, all values including calculated values have up to date whitespace.
Deferred rebuild, recalculate and revalidate
XForms provides actions to force a given model's rebuild, recalculate, revalidate. As XForms specifies it, these actions have an immediate effect.
Orbeon Forms provides an extension on those actions to defer the behavior of those actions by just setting the appropriate flags defined in the XForms specification, but not actually running the actions immediately. The actions will run as needed the next time the flags are checked.
<xf:rebuild     xxf:deferred="true"/>
<xf:recalculate xxf:deferred="true"/>
<xf:revalidate  xxf:deferred="true"/>
Static appearance for read-only controls
Sometimes, read-only controls don't appear very nicely in web browsers. For example, a combo box will appear grayed out. It may be be hard to read, and there is not much point showing a combo box since the user can't interact with it. Furthermore, with some browsers, like IE 6 and earlier, it is not even possible to make disabled controls appear nicer with CSS. In order to make read-only versions of forms look nicer, Orbeon Forms supports a special extension attribute that allows you to produce a "static" appearance for read-only controls. You enable this on your first XForms model:
<xf:model xxf:readonly-appearance="static">
    ...
</xf:model>
The attribute takes one of two values: static or dynamic (the default). When using the value static, read-only controls do not produce disabled HTML form controls. This has one major limitation: you can't switch a control back to being read-write once it is displayed as read-only.
You can also set the xxf:readonly-appearance attribute directly on individual XForms controls.
See Form Runner's Preview mode for an example of this feature in action.
Tips
Making an entire instance read-only
You often want to present a form without allowing the user to enter data. An easy solution is to use the readonly MIP in the model. By making for example the root element of an instance read-only, all the controls bound to any node of that instance will appear read-only (because the read-only property is inherited in an instance):
<xf:instance>
    <form>
        ...
    </form>
</xf:instance>
<xf:bind ref="instance()" readonly="true()"/>
See also
​XForms Validation​
​XForms Model bind variables​
​Better formulas with XPath type annotations​
​Formulas for summing values, done right​
​Control required values with formulas in Orbeon Forms 4.7​
​Evaluation of initial values upon insert​
​Grid component​
Blog post: Required fields: more subtle than you might think​
Attribute Value Templates (AVTs)
Validation