Formulas and XPath

Formulas in Orbeon Forms are expressed using XPath, a standard expression language for XML. It does not allow you to modify XML data, but it allows you to query XML data and compute values.

For those familiar with Microsoft Excel or other spreadsheet software, you can think of formulas with Orbeon Forms as what follows the "=" sign in a spreadsheet. It is similar to that, but with a slightly different syntax and set of rules.

In general you don't need to know about XPath in Form Builder, with the exception of some properties in the Control Settings and Section Settings dialogs. XPath expressions are considered an advanced feature of Form Builder, which might require some programming knowledge.

NOTE: Incorrect XPath expressions may cause the form to behave improperly, so caution must be applied.

Referring to control values from formulas

Basic usage

To refer to a control value from a formula, use the notation $foo where foo is the control name. For example:

$price * $quantity

• ​When the internal data format matters​

• ​Examples of formulas​

Resolution of repeated controls

When referring to controls that are repeated, for example within repeated grids or repeated sections, a variable can return a sequence of multiple values. Each value corresponds to a repeated control.

However, not all controls in the form are necessarily selected. Instead, the "closest" controls are selected, as follows:

• The closest enclosing repeat iteration between the location of the formula and the control identified by the variable is identified. If there is none, then this is the top-level of the form.

• Then all the controls within that iteration, or within the entire form when there is no such iteration, are selected.

For example, consider

• a repeated grid

• a decimal field called price on each row

• an integer field called quantity on each row

• a decimal text output field called row-total on each row

• a decimal text output field called total below the grid

Calculated value expression for row-total:

$price * $quantity

The row-total calculated value applies to the closest price and quantity controls, that is, those on the same row, and each row gets its own row total.

Calculated value expression for total:

sum($row-total[string() castable as xs:decimal], 0.0)

On the other hand, the total calculation is outside the repeat, and when it refers to $row-total, all row-total values are returned. Therefore, the sum applies to all the row-total values (assuming they can be cast as xs:decimal, in this example).

Renaming of controls and formulas

[SINCE Orbeon Forms 2019.1]

When a control or section or grid is renamed, dependent formulas which use the notation $foo where foo is the control name, are automatically updated.

Where do formulas appear?

Control Settings dialog

In the Control Settings dialog, XPath expressions are used to specify the following aspects of a control.

Validations and alerts

• Constraint: Boolean expression specifying whether the control is valid.

  • If this field is left blank, then the validity of the control depends on the data type and the "Required" option.

  • Otherwise, the control is valid if in addition to all the other constraint being met, the result of the Boolean expression is true().

Formulas

• Visibility:

  • Boolean expression specifying whether the control is visible.

  • If this field is left blank, then the control is always visible, unless the section is not visible.

  • Otherwise, it is visible only if the result of the boolean expression is true().

• Read-Only:

  • Boolean expression specifying whether the control is read-only (not editable).

  • If this field is left blank, then the control is editable unless the section is read-only.

  • Otherwise, the field is editable only if the result of the Boolean expression is false().

• Initial value:

  • String expression returning the initial value of the control when the form first shows. This is only applied in new mode.

  • Default: the value set into the field at design time (usually a blank value).

• Calculated Value:

  • String expression specifying a calculated value of the control which updates while the form user interacts with the form.

  • Default: the default value of the control, or the value entered by the form user.

Section/Grid Settings dialog

In the Section/Grid Settings dialog, XPath expressions are used to specify the following aspects of a section or grid:

• Visibility: Boolean expression specifying whether the section is visible or not.

  • If this field is left blank, then the section is always visible.

  • Otherwise, it is visible only if the result of the Boolean expression is true().

• Read-Only: Boolean expression specifying whether the section is shown as read-only or not.

  • If this field is left blank, then the section content is always editable.

  • Otherwise, the section content is editable only if the result of the Boolean expression is false().

[SINCE Orbeon Forms 2020.1]

In the Form Settings dialog, an XPath expression can be used to specify whether the entire form is read-only (not editable).

Actions

TODO

Referring to control values

XPath expressions may refer to the value of other controls in the page, using variables named after the name of the controls to use.

Examples

See examples of formulas.

See also

• ​Examples of formulas​

• ​Form Builder Validation​

• ​Better formulas with XPath type annotations​

• ​Formulas for summing values, done right​

• ​Control required values with formulas in Orbeon Forms 4.7​

• ​XForms Validation​