OPS User Guide

XForms Reference

1. Scope

Web applications use forms to collect data from users. Orbeon PresentationServer (OPS)'s form handling capabilities are based on XForms, namely the XForms 1.0 W3C Recommendation. This section provides an introduction to XForms concepts and explains how to use XForms in your OPS application.

Note

This document is considered a work in progress. While it does cover some generic features of XForms, it focuses before all on features specific to the OPS XForms engine. For more information about XForms, please refer to the following resources:

2. Introduction to XForms

2.1. Origin, Today, and Tomorrow

XForms 1.0 has been designed by the W3C based on experience with HTML forms. It was promoted to the rank of W3C Recommendation in October 2003, and a second edition of the specification has been released in October 2005. As of December 2005, mainstream browsers (Internet Explorer, Mozilla / Firefox, Opera, Safari) do not support XForms natively, although XForms support in Mozilla is under way and plugins are available for Internet Explorer. However you can already leverage the benefits of XForms today by using a hybrid client-side / server-side XForms engine like the one provided in OPS. The OPS XForms engine transparently generates HTML forms and performs the work that would be done by an XForms-compliant browser. This way you can start leveraging XForms today, be ready for upcoming XForms-compliant browsers, and work smoothly with the mainstream browsers that are deployed in the marketplace.

For more information about the whys and therefores of server-side XForms, please read our article, Are Server-Side XForms Engines the Future of XForms? (pre-conference version and updated version).

2.2. Benefits

Compared to HTML forms, XForms offers a higher level approach to forms. The benefits are that less programming is needed (less JavaScript, and less server-side programming), so forms are easier to create and modify. As an illustration, let's consider some facets of XForms:

  1. XML Representation of Forms. XForms clearly defines how data entered by the end-user is collected: it is stored in an XML document called an XForms instance, an initially empty, "skeletal" XML instance document that defines the structure of the data you wish to collect from the user, which is afterwards filled out with information collected from the user. For example, credit card information collected on a web site can be structured as follows:

      <credit-card>  <type/>  <number/>  <expiration-month/>  <expiration-year/>  </credit-card>

    The outcome of the user filling out a form collecting this information could be this complete XML document:

      <credit-card>  <type>visa</type>  <number>1234567812345678</number>  <expiration-month>8</expiration-month>  <expiration-year>2008</expiration-year>  </credit-card>

    An application using this data to do some processing (e.g. checking the validity of the credit card) receives the above XML document. There is no need to write code to go read HTTP request parameters, or to use a framework performing this task: XForms does it all.

  2. Declarative Constraints and Validation. More often than not, there are constraints on the data that can be entered by the end-user. For instance, in the example we just considered, the card number must have 16 digits and the expiration month must be a number between 1 and 12. Traditionally code must be written to check for those constraints. And more code must be written to handle error conditions (getting back to the page displaying the form and showing the appropriate error messages). All this is done is very simple and declarative way with XForms. For instance, checking that the expiration month is valid number between 1 and 12 can be done with:

      <xforms:bind nodeset="/credit-card/expiration-month" type="xs:integer" constraint=". >= 1 and 12 >= ."/>

    An error message can be attached to the "month" text field and if the end-user enters an invalid month the XForms engine will notice that the above constraint is not met and will display the error message. You do not have to write any code for this to happen. We will see later how you go about doing this with XForms in more details.

  3. Declarative Event Handling. User interfaces need to react to user event such as mouse clicks and character entry. With most UI frameworks, developers must register event handlers and implement them in JavaScript, Java, or other traditional imperative languages. With XForms, a set of predefined event handlers and actions are available, which cover a set of useful cases without requiring understanding the complex syntax and semantic of JavaScript or Java. For example, to set a value into an XForms instance, you write:

      <xforms:setvalue ref="/credit-card/expiration-month">11</xforms:setvalue>

    Once you have learned the simple built-in XForms actions, you can combine them in sequences to obtain more complex behavior.

3. Getting Started With the OPS XForms Engine

3.1. The XForms Sandbox

The easiest way to get started with simple examples is to use the OPS XForms Sandbox. This tool allows you to upload example XForms files from your web browser and to see the results directly. You can access the XForms sandbox:

  • Online: visit this link to access the online public XForms Sandbox.

  • Locally: if this documentation is produced by your local installation of OPS, visit this link.

After submitting an XHTML + XForms file, the result, or errors, should display. If you have changed your local XForms file, reloads that page in your browser and this will upload again your local XForms file and the XForms Sandbox will run the new version. To select another file to upload use your browser quotes "back" button to return to the main XForms sandbox page.

4. Programming With XForms 1.0

4.1. XForms Model

4.1.1. Introduction

To help in our exploration of XForms we consider a specific example: an XForms Credit Card Verifier. This example displays a simple form asking for a credit card number and related information to be entered, as shown on the screenshot to the right. The information entered by the end-user is validated by a set of rules and errors are flagged in red.

First, the information contained in the form is stored in an XML document called an XForms instance, which is the skeleton or shell that will contain the data captured by the form. You define an XForms instance within an xforms:instance. In the Credit Card Verifier the unique XForms instance is declared with:

  <xforms:instance id="credit-card-instance">  <credit-card>  <type/>  <number/>  <expiration-month/>  <expiration-year/>  <verification-code/>  <valid/>  </credit-card>  </xforms:instance>

The XForms instance does not have to be empty of data: it can contain initial values for the form. Here we set the valid element to the value "false" by default:

  <xforms:instance id="credit-card-instance">  <credit-card>  <type/>  <number/>  <expiration-month/>  <expiration-year/>  <verification-code/>  <valid>false</valid>  </credit-card>  </xforms:instance>

XForms instances are always contained in an XForms model, which:

  1. Declares one or more XForms instance.

  2. Optionally, declares a set of rules attached to the XForms instances.

  3. Optionally, declares submissions.

At a minimum, the XForms instance above must be encapsulated as follows:

  <xforms:model id="main-model">  <xforms:instance id="credit-card-instance">  <credit-card>  <type/>  <number/>  <expiration-month/>  <expiration-year/>  <verification-code/>  <valid>false</valid>  </credit-card>  </xforms:instance>  </xforms:model>

Note that instances and models can have an optional id attribute. If you have only one model and one instance, the id is optional, but it becomes very convenient when more than one model or instance are used.

4.1.2. Model Item Properties

In addition to one or more XForms instances, an XForms model can declare a set of "rules", called "model item properties". Let's write a set of rules for the above Credit Card Validation form. Specifically we want to:

  1. Check that the credit card number is a number and valid according to particular credit card rules

  2. Check that the expiration month is valid (integer between 1 and 12)

  3. Check that the expiration year is valid (4 digit number)

  4. Display the "verification code" line only if the card type is Visa or MasterCard

  5. Check that the verification code is valid only for Visa or MasterCard

You describe each one of those rules with an <xforms:bind> element in the XForms model. Rules apply to elements and attributes in the XForms instance. You specify the elements and attributes each rule applies to with an XPath expression in the mandatory nodeset attribute. In addition to the nodeset attribute you want to have at least one attribute specifying the essence of the rule. We go over the all the possible attributes later in this section, but first let's see how we can express the above rules for the Credit Card Verifier form:

  1. You specify that the credit card number must be a number with:

      <xforms:bind nodeset="number" type="xs:integer"/>

    The value of the type attribute is a W3C XML Schema simple type. You can see the list of simple types in the XML Schema primer. If the end-user enters an invalid credit card number (i.e. not a number), an error will be displayed as shows in the screenshot on the right.

  2. You can also constrain the value of an element or attribute with an XPath expression in the constraint attribute. For instance you specify that the expiration month must be an integer between 1 and 12 with:

      <xforms:bind nodeset="expiration-month" constraint=". castable as xs:integer and . >= 1 and 12 >= ."/>

    Note that we have decided here not to bother checking the expiration month if no credit card number was entered.

  3. Similarly, you check that the expiration year is a 4 digit number with:

      <xforms:bind nodeset="expiration-year" constraint=". castable as xs:integer and string-length(.) = 4"/>

  4. You hide the "verification code" text field for American Express cards with:

      <xforms:bind nodeset="verification-code" relevant="../type = 'visa' or ../type = 'mastercard'"/>

    The attribute we use here is relevant. By default, everything is relevant in the XForms instance. If a "relevant" rule is specified, the XPath expression is evaluated for each node in the nodeset, and if the expression returns false, then the node is not considered relevant. When a node is not relevant, the corresponding widget is not displayed (more on this later).

  5. Finally, you check that the verification code is entered for Visa and Mastercard:

      <xforms:bind nodeset="verification-code" constraint="/credit-card/type = ('visa', 'mastercard') and . castable as xs:positiveInteger"/>

    Because the verification-code element has both a relevant and a constraint attribute, we combine them on the same xforms:bind:

      <xforms:bind nodeset="verification-code" relevant="../type = 'visa' or ../type = 'mastercard'" constraint="/credit-card/type = ('visa', 'mastercard') and . castable as xs:positiveInteger"/>

XPath expressions in xforms:bind are by default relative to the root element of the first XForms instance. This allows you to write the first constraint above:

  • Relatively to the root element of the first XForms instance:

      <xforms:bind nodeset="number" type="xs:integer"/>

  • With an absolute path in the first XForms instance:

      <xforms:bind nodeset="/credit-card/number" type="xs:integer"/>

  • Referring explicitly to the "credit-card-instance" using the instance() function:

      <xforms:bind nodeset="instance('credit-card-instance')/number" type="xs:integer"/>

Now that we have seen a few examples of model item properties, let's go over all the XForms model item properties. Model item properties can essentially be used for 3 purposes:

Validation

The purpose of validation is to determine if the content of an element or attribute in the XForms instance is valid. Invalid values can have an impact on how a form is displayed (you might want to highlight errors and show some information to help the end-user to correct the issue). Also, the XForms engine makes sure that invalid data cannot be submitted. There are 3 ways to validate the content of an element or attribute:

  • required ― You can specify in the required attribute an XPath expression that determines if a value is required. The XPath can be as simple as true(), or more complex and depend on other values entered by the end-user. By default values are not required.

  • type ― In the type attribute you can specify a W3C XML Schema simple type. The type attribute complements the required attribute, but applies separately.

    In Addition, some XML schema types have special behavior:
    • xs:date ― The input field is complemented by a pop-up calendar. The user can enter a date manually, or use the calendar to select a date in the past or in the future. The calendar is customizable by the application developer under: oxf:/config/theme/jscalendar

  • constraint ― The constraint attribute supports any XPath expression that returns a boolean value. If false() is returned, then the value is considered invalid, otherwise it is considered valid.

Calculation

The purpose of calculations is to dynamically compute values. You do this with the calculate attribute:

  • calculate ― The content of the element or attribute will be set to the result of the evaluation of the XPath expression in the calculate attribute. This way you can automatically compute some values in the XForms instance based on other values, typically entered by the end-user. By default, nodes that contain calculated values are read-only.

Visibility

In general XForms instance nodes are not read-only and are relevant, which means that if an XForms control is bound to that node (e.g. a text field), the control is displayed and is editable by the end-user. You can change this by providing XPath expressions in the readonly and relevant attributes:

  • readonly ― If the XPath expression in readonly evaluates to true, the control will be displayed in non-editable mode. Typically, in an XHTML user interface only the current value is displayed, instead of displaying a form element, like a text field.

  • relevant ― If the XPath expression in relevant evaluates to false, the control will not be displayed at all.

4.2. XForms Controls

4.2.1. Controls

XForms controls are similar to HTML form elements: they include text fields, drop down lists, checkboxes, etc. These are some differences between HTML forms elements and XForms controls:

  • The value displayed by an XForms control comes from a node of the XForms instance. When you declare a control, you bind it to a node of your XForms instance with an XPath expression in the ref attribute. For instance this text field a text field is bound to the <number> element, which a child of <credit-card>:

      <xforms:input ref="/credit-card/number"/>

  • The way a control is rendered depends on model item properties that apply to the node the control is bound to: if it is bound to an invalid node then an error can be displayed; if the control is bound to a read-only node the value is displayed in read-only mode; if the node is not relevant the control isn't be displayed at all; if the control is bound to a non-existing node, the control is considered non-relevant and is not displayed;

The table below lists all the available XForms controls and shows for each one the XML you need to use in your view, as well as an example showing that control in action.

Control XForms in the view Example
Text field

  <xforms:input ref="text"/>

XForms Controls
Password field

  <xforms:secret ref="secret"/>

XForms Controls
Text area

  <xforms:textarea ref="textarea"/>

XForms Controls
Radio buttons

  <xforms:select1 ref="carrier" appearance="full">  <xforms:item>  <xforms:label>Fedex</xforms:label>  <xforms:value>fedex</xforms:value>  </xforms:item>  <xforms:item>  <xforms:label>UPS</xforms:label>  <xforms:value>ups</xforms:value>  </xforms:item>  </xforms:select1>

XForms Controls
Single-selection lists

  <xforms:select1 ref="carrier" appearance="compact">  <xforms:item>  <xforms:label>Fedex</xforms:label>  <xforms:value>fedex</xforms:value>  </xforms:item>  <xforms:item>  <xforms:label>UPS</xforms:label>  <xforms:value>ups</xforms:value>  </xforms:item>  </xforms:select1>

XForms Controls
Combo box

  <xforms:select1 ref="payment" appearance="minimal">  <xforms:item>  <xforms:label>Cash</xforms:label>  <xforms:value>cash</xforms:value>  </xforms:item>  <xforms:item>  <xforms:label>Credit</xforms:label>  <xforms:value>credit</xforms:value>  </xforms:item>  </xforms:select1>

XForms Controls
Checkboxes

  <xforms:select ref="wrapping" appearance="full">  <xforms:choices>  <xforms:item>  <xforms:label>Hard-box</xforms:label>  <xforms:value>box</xforms:value>  </xforms:item>  <xforms:item>  <xforms:label>Gift</xforms:label>  <xforms:value>gift</xforms:value>  </xforms:item>  </xforms:choices>  </xforms:select>

XForms Controls
List

  <xforms:select ref="taste" appearance="compact">  <xforms:item>  <xforms:label>Vanilla</xforms:label>  <xforms:value>vanilla</xforms:value>  </xforms:item>  <xforms:item>  <xforms:label>Strawberry</xforms:label>  <xforms:value>strawberry</xforms:value>  </xforms:item>  </xforms:select>

XForms Controls
Trigger button

  <xforms:trigger>  <xforms:label>Add carrier</xforms:label>  </xforms:trigger>

XForms Controls
Submit button

  <xforms:submit submission="main-submission">  <xforms:label>Submit</xforms:label>  </xforms:submit>

-
Submit link

  <xforms:submit submission="main-submission" appearance="xxforms:link">  <xforms:label>Submit</xforms:label>  </xforms:submit>

-
Submit image

  <xforms:submit submission="main-submission" appearance="xxforms:image">  <xxforms:img src="images/submit.gif"/>  <xforms:label>Submit</xforms:label>  </xforms:submit>

-
Upload

  <xforms:upload ref="files/file[1]">  <xforms:filename ref="@filename"/>  <xforms:mediatype ref="@mediatype"/>  <xxforms:size ref="@size"/>  </xforms:upload>

Upload Control
Range

  <xforms:range ref="range/value">  <xforms:send submission="countries-submission" ev:event="xforms-value-changed"/>  </xforms:range>

XForms Controls

In the examples above, the labels and values for the select and select1 controls are declared in the control element with multiple <xforms:item> elements. Alternatively the label/value pairs can be pulled out from the instance. You do this with an <xforms:itemset> element (instead of <xforms:item> elements):

  <xforms:select1 ref="country" appearance="compact">  <xforms:itemset nodeset="instance('countries')/country">  <xforms:label ref="name"/>  <xforms:value ref="us-code"/>  </xforms:itemset>  </xforms:select1>

4.2.2. Label, Alert, Help, and Hint

Nested inside each XForms control element, you can specify additional elements that can alter the way the control is displayed. The table below lists those elements:

Label

By default a label is used in submit controls, as well as the single and multiple selection controls, as shown in the table above. The label element is mandatory for all controls.

Alert

In each control you can specify an error message that can be displayed if the value entered by the user triggers a validation error.

  <xforms:secret ref="secret">  <xforms:alert>Invalid password</xforms:alert>  </xforms:secret>

Hint

You can specify a hint on each control, which is displayed next to the control and becomes highlighted when the control is selected.

  <xforms:textarea ref="textarea">  <xforms:hint>Enter at least 11 characters</xforms:hint>  </xforms:textarea>

Help

If you specify a help message for a control, an icon with a question mark is displayed next to the control. A pop-up shows the help message when you position the mouse cursor over the icon.

  <xforms:input ref="date" class="xforms-date">  <xforms:label class="fixed-width">Birth date:</xforms:label>  <xforms:help>This is supposed to be a help message explaining what a birth date is. But since you already know, it mostly serves the purpose of showing how help messages can be attached to controls, and that they can be pretty long as they can be displayed on multiple lines.  </xforms:help>  </xforms:input>

In the examples above, the text displayed is directly in the <xforms:label>, <xforms:alert>, <xforms:help>, or <xforms:hint> element. Alternatively that text can come from an XForms instance with a ref attribute on any one of those elements. The ref references a node in the instant that contains the text to use. This is illustrated in the code below from the XForms Text Controls example:

  <xforms:secret ref="secret">  <xforms:alert ref="@alert"/>  </xforms:secret>

4.2.3. Upload

XForms allows you to upload files with the XForms Upload control:

  <xforms:upload ref="files/file[1]">  <xforms:filename ref="@filename"/>  <xforms:mediatype ref="@mediatype"/>  <xxforms:size ref="@size"/>  </xforms:upload>

The related section of the XForms instance can look like this:

  <files>  <file filename="" mediatype="" size="" xsi:type="xs:anyURI"/>  </files>

The file element is the element storing the result of the file upload. The result can be stored in two ways:

  • As a URL, by specifying the type xs:anyURI.
  • As Base64-encoded text, by specifying the type xs:base64Binary. Base64 is a mechanism to encode any binary data using a 65-character subset of US-ASCII. Using this mechanism allows embedding binary data into XML documents, at the typical cost of taking 50% more space than the original binary data. For more information, please refer to the RFC.
Note

It is mandatory to specify either one of xs:anyURI or xs:base64Binary.

The optional xforms:filename, xforms:mediatype, and xxforms:size (the latter being an extension) allow storing metadata about an uploaded file:

  • xforms:filename: stores the file name sent by the user agent
  • xforms:mediatype: store the media type sent by the user agent
  • xxforms:size: stores the actual size in bytes of the uploaded data

Note that the file name and the media type are provided by the user agent (typically a web browser) and are not guaranteed to be correct.

The result of a file upload can look as follows when using xs:anyURI:

  <file filename="photo.jpg" mediatype="image/jpeg" size="2345" xsi:type="xs:anyURI">file:/C:/Tomcat/temp/upload_00000005.tmp</file>

Warning
The URL stored as the value of the upload is temporary and only valid for the duration of the current request. It is only accessible from the server side, and will not be accessible from a client such as a web browser. It is not guaranteed to be a file: URL, only that it can be read with Presentation Server's URL generator.

The contents of the file can be retrieved using the URL Generator. The result will be an XML document containing a single root element containing the uploaded file in Base64-encoded text.

Note

Using the xs:anyURI type allows OPS to make sure the uploaded file does not have to reside entirely in memory. This is the preferred method for uploading large files.

The result of a file upload can look as follows when using xs:base64Binary:

  <file filename="photo.jpg" mediatype="image/jpeg" size="2345" xsi:type="xs:base64Binary">/9j/4AAQSkZJRgABAQEASABIAAD/2wBDAAQDAwQDAwQEBAQFBQQFBwsHBwYGBw4KCggLEA4RERAO EA8SFBoWEhMYEw8QFh8XGBsbHR0dERYgIh8cIhocHRz/2wBDAQUFBQcGBw0HBw0cEhASHBwcHBwc ...  </file>

In this case, the uploaded file is encoded an directly embedded into the XML instance. This is a good method to handle small files only, because the entire file is converted and stored in memory.

Make sure, in your XForms model, that you have the correct submission method and encoding:

  <xforms:submission method="post" encoding="multipart/form-data" xmlns:xforms="http://www.w3.org/2002/xforms"/>

4.3. Repeating with xforms:repeat

4.3.1. Basics

A very common requirement of user interfaces consists in repeating visual elements, such as rows in a table or entries in a list. Those repeated sections usually have an homogeneous aspect: they all have the same or a very similar structure. For example, multiple table rows will differ only in the particular content they display in their cells. An example of this is an invoice made of lines with each a description, unit price, and quantity.

XForms provides a very powerful mechanism to implement such repeated structures: the xforms:repeat element. You use xforms:repeat around XHTML elements or XForms controls. For example, to repeat a table row, you write:

  <xforms:repeat>  <xhtml:tr>...  </xhtml:tr>  </xforms:repeat>

This is not enough to be functional code: you need to indicate to the xforms:repeat element how many repetitions must be performed. This is done not by supplying a simple count value, but by binding the the element to a node-set with the nodeset attribute. Consider the following XForms instance:

  <xforms:instance id="employees-instance" xmlns:xforms="http://www.w3.org/2002/xforms">  <employees>  <employee>  <first-name>Alice</first-name>  </employee>  <employee>  <first-name>Bob</first-name>  </employee>  <employee>  <first-name>Marie</first-name>  </employee>  </employees>  </xforms:instance>

Assuming you want to produce one table row per employee, add the following nodeset attribute:

  <xforms:repeat nodeset="instance('employees-instance')/employee">  <xhtml:tr>...  </xhtml:tr>  </xforms:repeat>

This produces automatically three xhtml:tr rows. Note that we explicitly use the XForms instance() function, but you may not have to do so if that instance is already in scope. Then you display in each row the content of the first-name element for each employee:

  <xforms:repeat nodeset="instance('employees-instance')/employee">  <xhtml:tr>  <xhtml:td>  <xforms:output ref="first-name"/>  </xhtml:td>  </xhtml:tr>  </xforms:repeat>

This works because for each iteration, the context node for the ref attribute changes: during the first iteration, the context node is the first employee element of the XForms instance; during the second iteration, the second employee element, and so on.

Note
The nodeset attribute of xforms:repeat must point to a so-called homogeneous collection. Such a collection must consist of contiguous XML elements with same name and same namespace. XForms does not predict what happens if the node-set is not homogenous.

4.3.2. Deleting Rows with the xforms:delete Action

xforms:repeat may be used purely for display purposes, but it can also be used for interactively editing repeated data. This includes allowing the user to delete and insert rows. Two XForms actions are used for this purpose: xforms:delete and xforms:insert.

xforms:delete is provided with a nodeset attribute pointing to the homogenous collection into which the insertion must take place. It also has an at attribute, which contains an XPath expression returning the index of the element to delete. See how xforms:delete is used in these 3 scenarios:

  <!-- This deletes the last element of the collection -->  <xforms:delete nodeset="employees" at="last()"/>  <!-- This deletes the first element of the collection -->  <xforms:delete nodeset="employees" at="1"/>  <!-- This deletes the currently selected element of the collection (assuming the repeat id 'employee-repeat') -->  <xforms:delete nodeset="employees" at="index('employee-repeat')"/>

4.3.3. Inserting Rows with the xforms:insert Action

xforms:insert has a nodeset attribute pointing to the homogenous collection into which the insertion must take place. xforms:insert then considers the last element of that collection (and all its content if any) as a template for the new element to insert: it duplicates it and inserts it into the homogenous collection at a position you specify. The last element of an homogeneous collection therefore always acts as a template for insertions:

  <!-- This inserts a copy of the template before the last element of the collection -->  <xforms:insert nodeset="employees" at="last()" position="before"/>  <!-- This inserts a copy of the template after the last element of the collection -->  <xforms:insert nodeset="employees" at="last()" position="after"/>  <!-- This inserts a copy of the template before the first element of the collection -->  <xforms:insert nodeset="employees" at="1" position="before"/>  <!-- This inserts a copy of the template after the first element of the collection -->  <xforms:insert nodeset="employees" at="1" position="after"/>  <xforms:insert nodeset="employees" at="last()" position="after"/>  <!-- This inserts a copy of the template before the currently selected element of the collection -->  <xforms:insert nodeset="employees" at="index('employee-repeat')" position="before"/>  <!-- This inserts a copy of the template after the currently selected element of the collection -->  <xforms:insert nodeset="employees" at="index('employee-repeat')" position="after"/>

The at attribute contains an XPath expression returning the index of the element before or after which the insertion must be performed. The position element contains either after or before, and specifies whether the insertion is performed before or after the element specified by the at attribute.

It is important to note that while it is possible to delete the last element of an homogeneous collection, it becomes then impossible to insert a new element into that collection with XForms 1.0, since there is no longer a template element available in this case (save for using an XML submission with replace="instance"). This means that in general you will want to have at least one element in your collections.

In case you want the user interface to visually appear empty empty when there is "no more" elements in the collection, you can use the tip provided below, which can be used in most situations. The idea is to consider that the last element of the collection is never displayed, but always used as a template for xforms:insert:

  <xforms:instance id="employees-instance" xmlns:xforms="http://www.w3.org/2002/xforms">  <employees>  <employee>  <first-name>Alice</first-name>  </employee>  <employee>  <first-name>Bob</first-name>  </employee>  <employee>  <first-name>Marie</first-name>  </employee>  <!-- This is a template used by xforms:insert -->  <employee>  <first-name/>  </employee>  </employees>  </xforms:instance>

You do not want to display that template, however. Therefore you use an xforms:repeat element of the form:

  <xforms:repeat nodeset="instance('employees-instance')/employee[position() &lt; last()]">...  </xforms:repeat>

The position() &lt; last() condition tells xforms:repeat to consider all the elements of the collection except the last one. This causes the repetition to display zero iteration when there is one element in the collection, one iteration when there are two, etc. The xforms:insert action, on the other hand, operates on the entire collection including the last element, so that that element can be duplicated:

  <xforms:insert nodeset="employees" at="..." position="..."/>

Another solution involves using an xforms:bind element which makes the last element of the collection non-relevant. This achieves the same result, but requires extra code, so the tip above is usually preferred.

Upon submission, some care must be taken with repeat template. For example, if the first-name element above is required, and the template contains an empty value as above, submission will fail. xforsm:bind statements must then also exclude the last element of the collection:

  <xforms:bind nodeset="employee[position() &lt; last()]/first-name" required="true()"/>

Note

If you are dealing with an XML document format which requires removing the last element of a collection, you have to post-process your XForms instance to remove such extra elements, and pre-process it to add such elements when initializing your XForms instance.

4.3.4. Using xforms:trigger to Execute Actions

Insertions and deletions are typically performed when the user of the application presses a button, with the effect of adding a new repeated element before or after the currently selected element, or of deleting the currently selected element. You use an xforms:trigger control and the XPath index() function for that purpose:

  <xforms:trigger>  <xforms:label>Add</xforms:label>  <xforms:action ev:event="DOMActivate">  <xforms:insert nodeset="employees" at="index('employee-repeat')" position="after"/>  </xforms:action>  </xforms:trigger>

or:

  <xforms:trigger>  <xforms:label>Delete</xforms:label>  <xforms:action ev:event="DOMActivate">  <xforms:delete nodeset="employees" at="index('employee-repeat')"/>  </xforms:action>  </xforms:trigger>

Note that we use xforms:action as a container for xforms:insert and xforms:delete. Since there is only one action to execute, xforms:action is not necessary, but it may increase the legibility of the code. It is also possible to write:

  <xforms:trigger>  <xforms:label>Add</xforms:label>  <xforms:insert ev:event="DOMActivate" nodeset="employees" at="index('employee-repeat')" position="after"/>  </xforms:trigger>

or:

  <xforms:trigger>  <xforms:label>Delete</xforms:label>  <xforms:delete ev:event="DOMActivate" nodeset="employees" at="index('employee-repeat')"/>  </xforms:trigger>

Notice in that case how ev:event="DOMActivate" has been moved from the enclosing xforms:action to the xforms:insert and xforms:delete elements.

4.3.5. Nested Repeats

It is often desirable to nest repeat sections. Consider the following XForms instance representing a company containing departments, each containing a number of employees:

  <xforms:instance id="departments">  <departments>  <department>  <name>Research and Development</name>  <employees>  <employee>  <first-name>John</first-name>  </employee>  <employee>  <first-name>Mary</first-name>  </employee>  </employees>  </department>  <department>  <name>Support</name>  <employees>  <employee>  <first-name>Anne</first-name>  </employee>  <employee>  <first-name>Mark</first-name>  </employee>  <employee>  <first-name>Sophie</first-name>  </employee>  </employees>  </department>  </departments>  </xforms:instance>

This document clearly contains two nested sections subject to repetition:

  • Departments: a node-set containing all the department elements can be referred to with the following XPath expression: instance('departments')/department.

  • Employees: a node-set containing all the employee elements can be referred to with the following XPath expression: instance('departments')/department/employees/employee. However, if the context node of the XPath expression points to a particular department element, then the following relative XPath expression refers to all the employee elements under that department element: employees/employee.

Following the example above, here is how departments and employees can be represented in nested tables with XForms:

  <xhtml:table>  <xforms:repeat nodeset="instance('departments')/department">  <xhtml:tr>  <xhtml:td>  <xforms:output ref="name"/>  </xhtml:td>  <xhtml:td>  <xhtml:table>  <xforms:repeat nodeset="employees/employee">  <xhtml:tr>  <xhtml:td>  <xforms:output ref="first-name"/>  </xhtml:td>  </xhtml:tr>  </xforms:repeat>  </xhtml:table>  </xhtml:td>  </xhtml:tr>  </xforms:repeat>  </xhtml:table>

In the code above, the second xforms:repeat's nodeset expression is interpreted relatively to the department element of the parent xforms:repeat for each iteration of the parent's repetition. During the first iteration of the parent, the "Research and Development" department is in scope, and employees/employee refers to the two employees of that department, John and Mary. During the second iteration of the parent, the "Support" department is in scope, and employees/employee refers to the three employees of that department, Anne, Mark and Sophie.

4.4. Actions

4.4.1. Setting Instance Values with the xforms:setvalue Action

There are two ways of providing the value to set with <xforms:setvalue>. The first one specifies the value as a literal enclosed in the <xforms:setvalue> element. The second possibility uses the value attribute: the content of the attribute is an XPath expression evaluated in the context of the node the xforms:setvalue element is bound (through the ref attribute). The content of the node pointed to by the ref attribute will be set with the result of the XPath expression provided in the value attribute. The example below and uses two <xforms:setvalue>, each one providing the new value in a different way.

  <xforms:trigger xmlns:xforms="http://www.w3.org/2002/xforms">  <xforms:label>Submit</xforms:label>  <xforms:action ev:event="DOMActivate">  <xforms:setvalue ref="clicked">my-button</xforms:setvalue>  <xforms:setvalue ref="flavor" value="concat('van', 'illa')"/>  </xforms:action>  </xforms:trigger>

4.4.2. Displaying Message with the xforms:message Action

The XForms message action displays a message to the user. OPS being a server-side XForms implementation.

Typically, the content of the message element is the message to render. It can also come from the binding attributes (ref or bind), or from the linking attribute (src). The order of preference is the following:

  • Binding attributes
  • Linking attribute
  • Inline text
Note
  • The only value currently supported for the level attribute is modal. This attribute is optional.
  • When using the linking attribute (src), the value must be an absolute URL, starting with oxf:, http: or other supported protocols.

  <xforms:trigger>  <xforms:label>Test</xforms:label>  <xforms:message ev:event="DOMActivate" ref="taste"/>  </xforms:trigger>

4.5. Submission

Two properties control some aspects of XForms submission in OPS:

  <property as="xs:boolean" name="oxf.xforms.optimize-post-all" value="true"/>

If set to true (the default), OPS optimizes submissions with replace="all" by sending the response of the submission directly to the web browser. This however means that submission errors cannot be caught by XForms event handlers after OPS has started connecting to the submission URL, as should be the case following XForms 1.0. If set to false, OPS buffers the reply so that errors can be handled as per XForms 1.0. However, this solution is less efficient.

  <property as="xs:boolean" name="oxf.xforms.optimize-local-submission" value="true"/>

  • If set to true (the default), OPS optimizes "local" HTTP and HTTPS submissions, i.e. submissions performed to a URL controlled by OPS itself, by directly submitting using the Java Servlet API instead of actually using the HTTP protocol for the submission.
  • If set to false, OPS always always uses the HTTP or HTTPS protocol, which is less efficient. In this case, it is possible to specify the xxforms:post method instead of the post method on the xforms:submission element to force an optimized local submission.

5. Formatting

5.1. Rationale

It is usually recommended to use native XML types within XForms instances, as this guarantees interoperability and maintainability. For example, a date of January 10, 2005 is stored in ISO format as: 2005-10-01. However it is often necessary to format such values on screen in a user-readable format, like "January 10, 2005", "10 janvier 2005", or "10. Januar 2005".

OPS provides an extension attribute, xxforms:format, for that purpose. xxforms:format must contain an XPath 2.0 expression. In your XPath expression you can use all the XPath 2.0 functions, including those for date manipulation (external documentation). However since XPath 2.0 functions don't provide any facility for date and time formatting, you can in this attribute also use the following XSLT 2.0 functions:

The XPath expression is evaluated by the XForms engine whenever the value bound to the xforms:input control changes and needs to be updated on screen. It is evaluated in the context of the instance node bound to the control. This means that the current value of the control can be accessed with ".". Often the value must be converted, for example to a date, in which case the conversion can be done with XPath 2.0 type casts such as xs:date(.).

5.2. xforms:input

When using xforms:input and a bound xs:date type, you can control the formatting of the date using the xxforms:format extension attribute on the xforms:input control. For example:

  <xforms:input ref="date" xxforms:format="format-date(xs:date(.), '[MNn] [D], [Y]', 'en', (), ())"/>

5.3. xforms:output

When using xforms:output, you can control the formatting of the date using the xxforms:format extension attribute on the xforms:input control.

  <xforms:output ref="date" xxforms:format="format-date(xs:date(.), '[MNn] [D], [Y]', 'en', (), ())"/>  <xforms:output ref="size" xxforms:format="format-number(., '###,##0')"/>

5.4. Default Formatting

For both xforms:input and xforms:output, if the bound node is of type xs:date, xs:dateTime or xs:time, and if no xxforms:format attribute is present on the control, formatting is based on properties. If the properties are missing, a built-in default formatting is used. The default properties, as well as the built-in defaults, are as follows:

  <property as="xs:string" name="oxf.xforms.format.date" value="if (. castable as xs:date) then format-date(xs:date(.), '[FNn] [MNn] [D], [Y]', 'en', (), ()) else ."/>  <property as="xs:string" name="oxf.xforms.format.dateTime" value="if (. castable as xs:dateTime) then format-dateTime(xs:dateTime(.), '[FNn] [MNn] [D], [Y] [H01]:[m01]:[s01] UTC', 'en', (), ()) else ."/>  <property as="xs:string" name="oxf.xforms.format.time" value="if (. castable as xs:time) then format-time(xs:time(.), '[H01]:[m01]:[s01] UTC', 'en', (), ()) else ."/>

They produce results as follows:

  • 2004-01-07 is displayed as Wednesday January 7, 2004

  • 2004-01-07T04:38:35.123 is displayed as Wednesday January 7, 2004 04:38:35 UTC

  • 04:38:35.123 is displayed as 04:38:35 UTC

Note that with the condition in the XPath expressions, a value which cannot be converted to the appropriate type is simply displayed as is.

6. XForms Instance Initialization

6.1. Rationale

An XForms page often needs to contain initial data when first loaded. The data may come from a database, a form submitted on a previous page, etc. There are several ways to achieve this with OPS.

6.2. Page Flow Definitions

Within your page flow, you define a page model and either a static page view:

  <page id="..." path-info="..." model="my-page-model.xpl" view="my-page-view.xhtml"/>

Or a dynamic XSLT page view:

  <page id="..." path-info="..." model="my-page-model.xpl" view="my-page-view.xsl"/>

The page model is in charge of producing an XML document which is then going to be used by the page view to initialize the XForms instance. As always with OPS, the page model produces this document on its data output, and the page view can access this document on its data input, as shown in the following sections. This mechanism is described in details in the PFC documentation.

6.3. Using XInclude

Following the MVC architecture, the PFC page model generates an XML document which contains an XForms instance. A static PFC page view then includes this instance using xi:include, as follows:

  <html>  <head>  <title>Summary</title>  <xforms:model>  <xforms:instance id="document-infos-instance">  <!-- This is where the XML document produced by the page model is included -->  <xi:include href="input:data"/>  </xforms:instance>...  </xforms:model>  </head>  <body>...  </body>  </html>

The use of the URI input:data instructs XInclude processing to dynamically include the data output of the page view, which is produced on the data output of the page model. Note that it is possible to use the instance input, which then refers to the current XML submission.

6.4. Using XSLT

It is also possible to use a dynamic XSLT page view to perform the inclusion of the instance. XSLT is more flexible than XInclude, but less efficient at runtime. This is an example:

  <html xsl:version="2.0">  <head>  <title>Summary</title>  <xforms:model>  <xforms:instance id="document-infos-instance">  <!-- This is where the XML document produced by the page model is included -->  <xsl:copy-of select="doc('input:data')/*"/>  </xforms:instance>...  </xforms:model>  </head>  <body>...  </body>  </html>

Note the use of xsl:version="2.0" on the root element of the document, which instructs the PFC to process the page view as an XSLT stylesheet.

The use of the XPath doc() function with a URI input:data instructs XSLT processing to dynamically include the data output of the page view, which is produced on the data output of the page model.

Note
It is possible to use XInclude instructions in a dynamic XSLT page view as well. In this case, it is important to note that XInclude instructions are processed before XSLT instructions, i.e. the result of XInclude instructions is an XSLT stylesheet, which is then executed.
Note
Using XSLT for page views has an impact for debugging, as the output of XSLT transformations do not contain valuable location information. For performance and ease of debugging reasons, we recommend using static XHTML views with XInclude whenever possible.

7. Relative Paths

7.1. Rationale

XForms documents can refer to external resources using URIs in the following circumstances:

  • External Instances. The xforms:instance element can have an src attribute linking to an external instance definition.

  • Submission. The xforms:submission element must refer to an action URI.

  • Load Action. The xforms:load action must refer to an URI that must be loaded upon execution.

  • Image Mediatype. The xforms:output control may refer to an image URI.

  • Message, Label, Help, Hint, and Alert. xforms:label, xforms:help, xforms:hint, and xforms:alert may use an src attribute to refer to external content.

    Note
    The XForms 1.1 draft of November 15, 2004 removes linking attributes from actions and metadata elements and "the src attribute is not available to XForms 1.1 message, label, help, hint, alert elements."

URIs are resolved relatively to a base URI. The base URI is, by default, the external URL used to display the XForms page, with special handling of the servlet context, if necessary. It is also possible to override this behavior by adding xml:base attributes on xforms:load or any of its ancestor elements.

7.2. External XForms Instances

Referring to external XForms instances is done with the src attribute on the xforms:instance element:

  <xforms:instance src="instance.xml"/>

This feature allows for improved modularity by separating an XForms instance definition from an XHTML page. It also allows for producing XForms instances dynamically.

The following assumes that OPS runs in the /ops servlet context:

Base URI
(External URL or xml:base attributes)
Initial URI
(src attribute)
Resolved URI Comment

The following URI is loaded in a servlet:

http://a.org/ops/page

http://b.com/instance http://b.com/instance Absolute URLs are left untouched.
/new-instance http://a.org/ops/new-instance Absolute paths resolve against the current servlet context.
admin/instance http://a.org/ops/admin/instance The relative path resolves against the original URL.

The following path is loaded in a portlet:

/example/page

http://b.com/instance http://b.com/instance Absolute URLs are left untouched.
/new-instance /new-instance The absolute path is used as is. The XForms instance is loaded from the portlet. The developer must make sure that the path resolves to a PFC entry producing XML.
admin/instance /example/admin/instance The relative path is resolved against the original path. The XForms instance is loaded from the portlet. The developer must make sure that the path resolves to a PFC entry producing XML.

7.3. XForms Submisssion

Specifying a submission URL is done with the action attribute on the xforms:submission element:

  <xforms:submission action="/submission" ref="..."/>

The following assumes that OPS runs in the /ops servlet context:

Base URI
(External URL or xml:base attributes)
Initial URI
(action attribute)
XForms Init 1 Resolved URI Comment

The following URI is loaded in a servlet:

http://a.org/ops/page

http://b.com/submission N/A http://b.com/submission The absolute URL is left untouched. The XForms submission is performed on the absolute URL.
/new-submission N/A http://a.org/ops/new-submission Absolute paths resolve against the current servlet context.
admin/submission N/A http://a.org/ops/admin/submission The relative path resolves against the original URL.

The following path is loaded in a portlet:

/example/page

http://b.com/submission N/A http://b.com/submission The absolute URL is left untouched. The XForms submission is performed on the absolute URL.
/new-submission Yes /new-submission The absolute path is used as is. The XForms submission is performed on the portlet.
No http://a.org/ops/new-submission The absolute path resolves against the current servlet context. The submission is performed on the web application.
admin/submission Yes /example/admin/submission The relative path is resolved against the original path. The XForms submission is performed on the portlet.
No http://a.org/ops/ example/admin/submission The relative path resolves against the original path, then against the the current servlet context. The submission is performed on the web application.

1 If "yes", this means the submission is performed during XForms initialization, for example upon an xforms-ready event. If "no", this means that the submission is performed after XForms initialization, for example upon the user activating a trigger.

7.4. XForms Load Action

The xforms:load action can refer to a resource to load either through the resource attribute or using a single-node binding retrieving the URI from an XForms instance. In both cases, the value of the URI is resolved relatively to the base URI.

The following assumes that OPS runs in the /ops servlet context:

Base URI
(External URL or xml:base attributes)
Initial URI
(resource or Single-Node Binding)
show
f:url-type
Resolved URI Comment

The following URI is loaded in a servlet:

http://a.org/ops/page

http://b.com/software/ replace http://b.com/software/ The absolute URL is left untouched. The new page replaces the existing page.
new The absolute URL is left untouched. A new window or tab opens for the new page.
/new-page replace http://a.org/ops/new-page Absolute paths resolve against the current servlet context. The new page replaces the existing page.
new Absolute paths resolve against the current servlet context. A new window or tab opens for the new page.
admin/main-page replace http://a.org/ops/admin/main-page The new page replaces the existing page.
new A new window or tab opens for the new page.

The following path is loaded in a portlet:

/example/page

http://b.com/software/ replace http://b.com/software/ This causes the application to load a page outside of the portlet, replacing the entire portal.
new This causes the application to load a page in a new window outside of the portlet.
/new-page replace /new-page The resulting path is loaded within the portlet.
replace
f:url-type="resource"
http://a.org/ops/new-page The resulting path is loaded in the same window outside the portal.
new http://a.org/ops/new-page The resulting path is loaded in a new window.
admin/main-page replace /example/admin/main-page The resulting path is loaded within the portlet.
replace
f:url-type="resource"
undefined undefined
new undefined undefined

7.5. Image Mediatype for xforms:output

When an xforms:output control refers to an image URI, as documented below, the resulting value is resolved relatively to the base URI.

8. XForms and Services

8.1. Introduction

XForms 1.0 allows an XForms page to perform submissions of XForms instances and to handle a response. In most cases, both the submitted XForms instance and the response are XML documents.

Note
It is possible to submit an XForms instance with the HTTP GET method. In that case, some information contained in the XML document is lost, as the structure of the instance, attributes, and namespace prefixes among others, are not passed to the submission.

The XForms submission feature practically allows forms to call XML services. Those services are accessible through an XML API, which means that a request is performed by sending an XML document to the service, and a response consists of an XML document as well.

9. Extensions

9.1. XForms 1.1 Extensions

9.1.1. Media Type for xforms:output

In XForms 1.0, xforms:output is used to display text. However, based on a proposal in a draft version of XForms 1.1, OPS supports a mediatype attribute on that element.

Image Types

For the <xforms:output> control to display an image, you need to:

  • Have a mediatype attribute on the <xforms:output>. That attribute must refer to an image, such as image/* or image/jpeg.

  • Use the value attribute on <xforms:output> or bind to the control to a node without type or with an xs:anyURI type.

The resulting value is interpreted a URI pointing to an image. The image will display in place of the xforms:output. When a single-node binding is used, it is possible to dynamically change the image pointed to. For example:

  <xforms:output mediatype="image/*" value="'/images/moon.jpg'"/>

  <xforms:model>  <xforms:instance>  <image-uri/>  </xforms:instance>  <xforms:bind nodeset="image-uri" type="xs:anyURI"/>  </xforms:model>...  <xforms:output mediatype="image/*" ref="image-uri"/>

Note
It is not yet possible to directly embed image data in an XForms instance using the xs:base64Binary type.
HTML Type

When an xforms:output control has a mediatype attribute with value text/html, the value of the node to which the control is bound is interpreted as HTML content. Consider the following XForms instance:

  <xforms:instance id="my-instance">  <form>  <html-content>This is in &lt;b&gt;bold&lt;/b&gt;!  </html-content>  </form>  </xforms:instance>

You bind an xforms:output control to the html-content node as follows:

  <xforms:output ref="instance('my-instance')/html-content" mediatype="text/html"/>

This will display the result as HTML, as expected: "This is in in bold!". If the mediatype is not specified, the result would be instead: "This is in in <b>bold</b>!". In the XForms instance, the HTML content must be escaped as text. On the other hand, the following content will not work as expected:

  <xforms:instance>  <form>  <html-content>This is in in<b>bold</b>!  </html-content>  </form>  </xforms:instance>

Note
When using a mediatype="text/html", an HTML <div> element will be generated by the XForms engine to hold the HTML data. As in HTML a <div> cannot be embedded into a <p>, if you have a <xforms:output mediatype="text/html"> control, you should not put that control into a <xhtml:p>.

9.1.2. origin Attribute on xforms:insert Action

Based on a proposal in a draft version of XForms 1.1, OPS supports an origin attribute on the xforms:insert action. This attribute allows specifying the source node to use as template. This allows storing templates separately from the node-set specified by the nodeset attribute. For example:

  <xforms:insert nodeset="address" at="last()" position="after" origin="instance('template-instance')"/>

The template copied in this case comes from an XForms instance:

  <xforms:instance id="template-instance">  <address>  <street>  <number/>  <name-1/>  <name-2/>  </street>  <apt/>  <city/>  <state/>  <zip/>  </address>  </xforms:instance>

9.2. XPath Extension Functions

OPS implements some extension functions which can be used from XPath expressions in XForms documents.

9.2.1. XSLT 2.0 Functions

When using XPath 2.0, the following functions from XSLT 2.0 are also available:

9.2.2. OPS Functions

The following functions are implemented:

  • xxforms:call-xpl($xplURL as xs:string, $inputNames as xs:string*, $inputElements as element()*, $outputNames as xs:string+).

    This function lets you call an XPL pipeline.

    1. The first argument, $XPLurl, is the URL of the pipeline. It must be an absolute URL.
    2. The second argument, $inputNames, is a sequence of strings, each one representing the name of an input of the pipeline that you want to connect.
    3. The third argument, $inputElements, is a sequence of elements to be used as input for the pipeline. The $inputNames and $inputElements sequences must have the same length. For each element in $inputElements, a document is created and connected to an input of the pipeline. Elements are matched to input name by position, for instance the element at position 3 of $inputElements is connected to the input with the name specified at position 3 in $inputNames.
    4. The fourth argument, $outputNames, is a sequence of output names to read.
    5. The function returns a sequence of elements corresponding the output of the pipeline. The returned sequence will have the same length as $outputNames and will correspond to the pipeline output with the name specified on $outputNames based on position.

    The example below shows a call to the xxforms:call-xpl function:

    xxforms:call-xpl ('oxf:/examples/sandbox/xpath/run-xpath.xpl', ('input', 'xpath'), (instance('instance')/input, instance('instance')/xpath), 'formatted-output')/*, 'html')

9.2.3. eXForms Functions

eXForms is a suggested set of extensions to XForms 1.0, grouped into different modules. OPS supports the exf:mip module, which includes the following functions:

  • exf:relevant()

  • exf:readonly()

  • exf:required()

eXForms functions live in the http://www.exforms.org/exf/1-0 namespace, usually bound to the prefix exf.

10. State Handling

10.1. Rationale

The OPS XForms engine requires keeping processing state while operating on an XForms page. Such state includes the current values of XForms instances, selected repeated elements, and more. With OPS, XForms state information can be handled in one of two ways:

  • Client-side: in this case, static initial state information is sent along with the initial HTML page, and dynamic state is exchanged over the wire between the client browser and the OPS XForms server when necessary.

    Benefits of the approach:

    • The OPS server is entirely stateless. It only requires memory while processing a client request. It can be restarted without consequence for the XForms engine.

    • State information does not expire as long as the user keeps the application page open in the web browser.

    Drawbacks of the approach:

    • Resulting HTML pages are larger. In particular, the size of state data grows when XForms instances grow, regardless of whether many XForms controls are bound to instance data.

    • More data circulates between the client browser and the OPS XForms server.

    Note
    OPS compresses and encrypts XForms state information sent to the client.
  • Server-side: in this case, state information is stored on the server, in association with an application session. Only very little state information circulates between client and server.

    Benefits of the approach:

    • Resulting HTML page are smaller. HTML pages increase in size as more XForms controls are used, but they don't increase in size proportionally to the size of XForms instances.

    • Small amounts of data circulate between the client browser and the OPS XForms server.

    • This means that very large XForms instances can be processed without any impact on the amount of data that is transmitted between the client and the server.

    Drawbacks of the approach:

    • The OPS XForms server needs to be stateful. It uses server memory to store state information in a session even when no request is being processed. The server must be configured to determine how much state information is kept in a session, how long session take to expire, etc. This creates additional demand for resources on the server and complicates the task of tuning the server.

    • State information can become unavailable when sessions expire or when the server is restarted (unless you setup the server to persist session information). When state information becomes unavailable for a page, that page will no longer function unless it is reloaded.

      Note
      With most servlet containers, it is possible to configure session handling to passivate sessions out of the application server memory to a persistent store. It is this way possible to partially alleviate the drawback above by making sure that a very large number of active but idle sessions can be kept, with a minimum impact on application server memory. It is this way also possible to make sure that sessions survive a servlet container restart.
    Note
    OPS ensures that it is possible to open multiple client browser windows showing the same page within the same session.

10.2. Configuring State Handling

State handling can be configured globally for all pages, or locally for each individual page served. Global configuration is performed in properties.xml with the oxf.xforms.state-handling property. When missing or set to client, state is stored client-side. When set to session, state is stored server-side in a session. For example:

  <!-- Store state in the session -->  <property as="xs:string" name="oxf.xforms.state-handling" value="session"/>

The global configuration can be overridden for each page by setting the xxforms:state-handling attribute in the page. This attribute can be set for example on the root element of the XHTML page, or on the first xforms:model element. Only the first such attribute encountered by the XForms engine is used:

  <xforms:model xxforms:state-handling="client">...</xforms:model>

When storing state in a session, the maximum size of the data to be stored for each user can be selected using the oxf.xforms.cache.session.size property. The size is specified in bytes:

  <!-- Allow a maximum of 500 KB of state information for each user -->  <property as="xs:integer" name="oxf.xforms.cache.session.size" value="500000"/>

Whether state information is kept client-side or server-side, a property controls whether the XForms engine should try to optimize state reconstruction by using a cache. This property should usually be set to true:

  <!-- This should usually be set to "true" -->  <property as="xs:boolean" name="oxf.xforms.cache.document" value="true"/>

11. JavaScript Integration

11.1. Rationale

While XForms gets you a long way towards creating a dynamic user-friendly user interface, there are some dynamic behaviors of the user interface that cannot be implemented with XForms, and that you might want to implement directly in JavaScript. We describe here how your JavaScript code can interact with XForms and in particular how you can have access to the value of an XForms control.

11.2. Accessing The Value XForms Controls

In JavaScript, you obtain a reference to the element representing the control with var control = document.getElementById('myControl') where myControl is the id of the XForms control (i.e. <xforms:input id="myControl">. You can read the current value of the control with control.value and set the value of the control assigning a value to control.value, for instance: control.value = 42. When you do such an assignment the value of the node in the instance is updated, the xforms-value-changed event thrown, and all the standard XForms processing happens (validation, recalculation, etc).

Note
Currently, accessing the value of a control is only supported for <xforms:input>. If you want to access the value of another control, a workaround consists in adding an <xforms:input> with style="display: none" bound to the same node as that control, and accessing the <xforms:input>'s id to change the value of the node and therefore indirectly the other control's value.