XML Events

An Events Syntax for XML

W3C Working Draft 26 October 2001

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Shane McCarron, Applied Testing and Technology, Inc.
Steven Pemberton, CWI
T. V. Raman, IBM


The XML Events module defined in this specification provides XML languages with the ability to uniformly integrate event listeners and associated event handlers with Document Object Model (DOM) Level 2 event interfaces [DOM2]. The result is to provide an interoperable way of associating behaviors with document-level markup.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. The latest status of this document series is maintained at the W3C.

This is the "Last Call Working Draft" of "XML Events", for review by W3C members and other interested parties. This document has been produced by the HTML Working Group (member only) as part of the W3C HTML Activity. The Last Call review period ends on 2359Z on 30 November 2001. Please send review comments before the review period ends to www-html-editor@w3.org (archive).

Publication of this document does not imply endorsement by the W3C membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite a W3C Working Draft as anything other than a "work in progress". A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR.

Changes since the last version

This document has changed in minor ways since the last public version. Reviewers can see a diff-marked version to understand the details of the changes.


1. Introduction

This section is informative.

An event is the representation of some asynchronous occurrence (such as a mouse click on the presentation of the element, or an arithmetical error in the value of an attribute of the element, or any of unthinkably many other possibilities) that gets associated with an element (targetted at it) in an XML document.

In the DOM model of events [DOM2], the general behavior is that when an event occurs it is dispatched by passing it down the document tree in a phase called capture to the element where the event occurred (called its target), where it then may be passed back up the tree again in the phase called bubbling. In general an event can be responded to at any element in the path (an observer) in either phase by causing an action, and/or by stopping the event, and/or by cancelling the default action for the event at the place it is responded to. The following diagram illustrates this:

Event flow in DOM2
Event flow in DOM2: an event targetted at an element (marked 'target') in the tree passes down the tree from the root to the target in the phase called 'capture'. If the event type allows it, the event then travels back up the tree by the same route in a phase called 'bubbling'. Any node in the route, including the root node and the target, may be an 'observer': that is to say, a handler may be attached to it that is activated when the event passes through in either phase. A handler can only listen for one phase. To listen for both you have to attach two handlers.

An action is some way of responding to an event; a handler is some specification for such an action, for instance using scripting or some other method. A listener is a binding of such a handler to an event targetting some element in a document.

HTML [HTML4] binds events to an element by encoding the event name in an attribute name, such that the value of the attribute is the action for that event at that element. This method has two main disadvantages: firstly it hardwires the events into the language, so that to add a new event, you have to make a change to the language, and secondly it forces you to mix the content of the document with the specifications of the scripting and event handling, rather than allowing you to separate them out.

The process of defining a new version of HTML identified the need for an extensible event specification method. The design requirements were the following:

The DOM specifies an event model that provides the following features:

Element listener and its attributes defined in this specification is the method of binding a DOM level 2 event at an element to an event handler and encapsulates various aspects of the DOM level 2 event interface, thereby providing markup-level specification of the actions to be taken during the various phases of event propagation.

This document neither specifies particular events, nor mandates any particular methods of specifying actions. These definitions are left to any markup language using the facilities described here.

2. The XML Events Module

This section is normative.

This specification defines a module called XML Events. The XML Events module uses the XML Namespaces [NAME] identifier http://www.w3.org/2001/xml-events.

Examples in this document that use the namespace prefix "ev" all assume an xmlns declaration xmlns:ev="http://www.w3.org/2001/xml-events" somewhere suitable in the document involved. All examples are informative.

The remainder of this section describes the elements and attributes in this module, the semantics, and provides an abstract module definition as required in [XHTMLMOD].

The XML Events Module supports the following element and attributes:

Element Attributes Minimal Content Model
listener event (NMTOKEN),
observer (IDREF),
target (IDREF),
handler (URI),
phase ("capture" | "default"*),
propagate ("stop" | "continue"*),
defaultAction ("cancel" | "perform"*),
id (ID)

Implementation: DTD

2.1. The listener Element

Element listener supports a subset of the DOM's EventListener interface. It is used to declare event listeners and register them with specific nodes in the DOM, and has the following attributes:

The required event attribute specifies the event type for which the listener is being registered. As specified by [DOM2], the value of the attribute should be an XML Name [XML].
The optional observer attribute specifies the id of the element with which the event listener is to be registered. If this attribute is not present, the observer is the element that the event attribute is on (see later under "Attaching Attributes Directly to the Observer Element"), or the parent of that element (see later under "Attaching Attributes Directly to the Handler Element").
The optional target attribute specifies the id of the target element of the event (i.e., the node that caused the event). If this attribute is present, only events that match both the event and target attributes will be processed by the associated event handler. Clearly because of the way events propagate, the target element should be a descendent node of the observer element.

Use of this attribute requires care; for instance, if you specify

<listener event="click" observer="para1"
     target="link1" handler="#clicker"/>

with reference to the node

<a id="link1" href="doc.html">The <em>draft</em> document</a>

and the user happens to click on the word "draft", the <em> element, and not the <a>, will be the target, and so the handler will not be activated; to catch all mouseclicks on the <a> element and its children, use observer="link1", and no target attribute.

The optional handler attribute specifies the URI of an element that defines the action that should be performed if the event reaches the observer. (This specification does not mandate what form that element should take: see further the section "Event Handlers"). If this attribute is not present, the handler is the element that the event attribute is on (see later under "Attaching Attributes Directly to the Handler Element").
The optional phase attribute specifies when (during which DOM 2 event propagation phase) the listener will be activated by the desired event.
Listener is activated during capturing phase.
Listener is activated during bubbling or target phase.

The default behavior is phase="default".

Note that not all events bubble, in which case with phase="default" you can only handle the event by making the event's target the observer.

The optional propagate attribute specifies whether after processing all listeners at the current node, the event is allowed to continue on its path (either in the capture or the bubble phase).
event propagation stops
event propagation continues (unless stopped by other means, such as scripting, or by another listener).

The default behavior is propagate="continue".

The optional defaultAction attribute specifies if after processing of all listeners for the event at the current element, the default action for the event (if any) should be performed or not. For instance, the default action for a mouse click on an <a> element in XHTML is to traverse the link. Note that this is only useful when the observer is the <a> element, and not some parent element.
if the event type is cancellable, the default action is cancelled
the default action is performed (unless cancelled by other means, such as scripting, or by another listener).

The default value is defaultAction="perform".

Note that not all events are cancellable, in which case this attribute is ignored.

The optional id attribute is a document-unique identifier. The value of this identifier is often used to manipulate the element through a DOM interface.

Note that observer = "<element-id>" and event = "<event-type>" have identical behavior to the begin = "<element-id>.<event-type>" attribute in SMIL EventTiming [SMIL20].

2.1.1. Examples of listener usage

  1. This example attaches the handler in the element at "#doit" that will get activated when the event called activate occurs on the element with id="button1", or any of its children. The activation will occur during bubbling, or if the event happened on the observer element itself, when the event reaches the element (phase target).

    <listener event="activate" observer="button1" handler="#doit"/>
  2. This attaches the handler at #overflow-handler that will get activated when the event overflow occurs on the element with id="expr1" and bubbles up to the element with id="prog1".

    <listener event="overflow" observer="prog1" target="expr1"
  3. This attaches the handler at #popup that will get activated whenever an activate event occurs at the element with id="embargo" or any of its children. Since it will be activated during the capture phase, and propagation is stopped, this will have the effect (regardless of what the handler does) of preventing any child elements of the embargo element seeing any activate events.

    <listener event="activate" observer="embargo" handler="#popup"
            phase="capture" propagate="stop"/>
  4. This attaches a handler from another document.

    <listener event="activate" observer="image1"

2.2. Attaching Attributes Directly to the Observer Element

All the attributes from the listener element with the exception of id may be used as global attributes, as defined in Namespaces in XML [NAME], to attach the attributes to other elements.

Note that this means that the <listener> element is strictly speaking redundant, since the following

<anyelement ev:event="ev:click" ev:observer="button1" ev:handler="#clicker"/>

would have the same effect as

<ev:listener event="click" observer="button1" handler="#clicker"/>

Nonetheless, for utility the <listener> element has been retained.

If the observer attribute is omitted (but not the handler attribute), then the element that the other attributes are attached to is the observer element.

2.2.1. Examples of Using Attributes Attached to an Observer Element

  1. This first example will attach the handler identified by "#popper" to the <a> element, and cancel the default action for the event.

    <a href="doc.html" ev:event="ev:activate" ev:handler="#popper"
       ev:defaultAction="cancel">The document</a>
  2. This will attach the handler at #handle-overflow for the event overflow to the current element.

    <div ev:event="overflow" ev:handler="#handle-overflow"> ... </div>

2.3. Attaching Attributes Directly to the Handler Element

If, when attaching the global attributes to an element, the handler attribute is omitted then the element that the other attributes are attached to is the handler element.

Note that, since the observer and target attributes are IDREFs, in this case the handler and observer/target elements must be in the same document (while in other cases, since the handler attribute is a URI, the handler element may be in another document).

If the observer attribute is also omitted, then the parent of the handler element is the observer element.

2.3.1. Examples of Using Attributes Attached to a Handler Element

  1. In this case the element is the handler for the submit event on the element with id="form1".

    <script type="application/x-javascript"
            ev:event="ev:submit" ev:observer="form1">
       return docheck(event);
  2. In this case the <action> element is the handler for event q-submit, and the observer is the questionnaire element.

    <questionnaire submissionURL="/q/tally">
        <action ev:event="q-submit">
  3. The <script> element is the handler for event click; the <img> element is the observer.

    <img src="button.gif" alt="OK">
        <script ev:event="ev:activate" type="application/x-javascript">
  4. The <onevent> element is the handler for event enterforward. The <card> element is the observer.

        <onevent ev:event="enterforward">
            <go href="/url"/>
  5. The <catch> element is the handler for the nomatch event. The observer is the <field> element.

    <form id="launch_missiles">
      <field name="password">
        <prompt>What is the code word?</prompt>
          <rule id="root" scope="public">rutabaga</rule>
        <help>It is the name of an obscure vegetable.</help>
        <catch ev:event="nomatch">
          <prompt>Security violation!</prompt>
          <submit next="apprehend_felon" namelist="user_id"/>
        <goto next="#get_city"/>
  6. This example shows three handlers for different events. The observer for all three is the <secret> element.

    <secret ref="/login/password">
        <caption>Please enter your password</caption>
        <info ev:event="help">
            Mail help@example.com in case of problems
        <info ev:event="hint">
            A pet's name
        <info ev:event="alert">
            This field is required

2.4. Summary of Observer and Handler Attribute Defaulting

The following table summarises which elements play the role of observer or handler if the relevant attribute is omitted.

The effect of omitted observer and handler attributes
Handler present Handler omitted
Observer present (As declared) Element is handler
Observer omitted Element is observer Element is handler
Parent is observer

2.5. Event Handlers

This specification does not require an XML application that uses XML Events to use any particular method for specifying handlers. However, the examples, particularly those in the section on attaching the attributes directly to the handler, are intended to give examples of how they could be specified.

It is however recognized that two methods are likely to occur often: scripting (such as HTML's <script> element) and declarative markup using XML elements (such as WML's <onevent> element). A companion specification will provide markup to support these methods.

2.6. The Basic XML Events Profile

The Basic XML Events Profile allows restrictions on the usage of the XML Events Module in order to make processing easier on small devices.

The Basic Profile allows the following restrictions on the use of listener element and its attributes, and on the use of the attributes from the listener element as global attributes.

  1. External Event Handlers

    The ability to process external event handlers is not required. When the 'handler' attribute on the listener element is used, or when the global 'handler' attribute is used, the handler specified in the value of that attribute should be within the current document.

    For example, the following is allowed:

    <listener event="click" target="#button1" handler="#clicker"/>

    while the following is not required to be processed:

    <listener event="click" target="#button1" handler="doc2.html#clicker"/>
  2. Ordering of Event Bindings

    The binding of an event handler to an observer may be required to be lexically before the end of the observer element. In other words, a <listener> binding to an observer may not occur after the closing tag of the observer element, and an event handler carrying the attributes to bind it to an observer may also not occur after the closing tag of the observer element.

3. Naming Event Types

This section is normative.

This specification does not normatively specify how language designers should name events (i.e., the values used in the event attribute).

To identify event types from other namespaces, qualified names, as defined in [SCHEMA], should be used.

<listener event="smil:repeatEvent" observer="mm1" handler="synch1"/> 
<img src="button.gif" alt="ok" ev:event="ev:activate" handler="#act"/>

A number of event types are defined in [DOM2], to which you should refer for their semantics.

Those events types, and the names you should use to refer to them are:

User interface events:
focusIn, focusOut, activate
Mouse events:
click, mousedown, mouseup, mouseover, mousemove, mouseout
Key events:
Mutation events:
subtreeModified, nodeInserted, nodeRemoved, nodeRemovedFromDocument, nodeInsertedIntoDocument, attrModified, characterDataModified
HTML events:
load, unload, abort, error, select, change, submit, reset, focus, blur, resize, scroll

All these event types are in the XML Events namespace.

A. DTD Implementation

This appendix is normative.

The DTD implementation of XML Events conforms to the requirements defined in [XHTMLMOD]. Consequently, it provides a Qualified Names sub-module, and individual module files for each of the XML Events modules defined in this recommendation.

A.1. Qualified Names Module

Note that this module defines the Parameter Entity %xml-events-attrs.qname. This entity is intended to be used in the attribute lists of elements in any host language that permits the use of event attributes on elements in its own namespace. In this case the Host Language driver should set a parameter entity XML-EVENTS.prefixed to INCLUDE and a parameter entity XML-EVENTS.prefix to a value that is the prefix for the XML Events attributes.

<!-- ....................................................................... -->
<!-- XML Events Qname Module  ............................................ -->
<!-- file: xml-events-qname-1.mod

     This is XML Events - the Events Module for XML,
     a definition of access to the DOM events model.

     Copyright 2000-2001 W3C (MIT, INRIA, Keio), All Rights Reserved.

     This DTD module is identified by the PUBLIC and SYSTEM identifiers:

       PUBLIC "-//W3C//ENTITIES XML Events Qnames 1.0//EN"
       SYSTEM "http://www.w3.org/TR/xml-events/DTD/xml-events-qname-1.mod"

     ....................................................................... -->

<!-- XML Events Qname (Qualified Name) Module

     This module is contained in two parts, labeled Section 'A' and 'B':

       Section A declares parameter entities to support namespace-
       qualified names, namespace declarations, and name prefixing
       for XML Events and extensions.

       Section B declares parameter entities used to provide
       namespace-qualified names for all XML Events element types:

         %listener.qname;   the xmlns-qualified name for <listener>

     XML Events extensions would create a module similar to this one.
     Included in the XML distribution is a template module
     ('template-qname-1.mod') suitable for this purpose.

<!-- Section A: XML Events XML Namespace Framework :::::::::::::::::::: -->

<!-- 1. Declare a %XML-EVENTS.prefixed; conditional section keyword, used
        to activate namespace prefixing. The default value should
        inherit '%NS.prefixed;' from the DTD driver, so that unless
        overridden, the default behaviour follows the overall DTD
        prefixing scheme.
<!ENTITY % NS.prefixed "IGNORE" >
<!ENTITY % XML-EVENTS.prefixed "%NS.prefixed;" >

<!-- 2. Declare a parameter entity (eg., %XML-EVENTS.xmlns;) containing
        the URI reference used to identify the XML Events namespace
<!ENTITY % XML-EVENTS.xmlns  "http://www.w3.org/2001/xml-events" >

<!-- 3. Declare parameter entities (eg., %XML.prefix;) containing
        the default namespace prefix string(s) to use when prefixing
        is enabled. This may be overridden in the DTD driver or the
        internal subset of an document instance. If no default prefix
        is desired, this may be declared as an empty string.

     NOTE: As specified in [XMLNAMES], the namespace prefix serves
     as a proxy for the URI reference, and is not in itself significant.
<!ENTITY % XML-EVENTS.prefix  "" >

<!-- 4. Declare parameter entities (eg., %XML-EVENTS.pfx;) containing the
        colonized prefix(es) (eg., '%XML-EVENTS.prefix;:') used when
        prefixing is active, an empty string when it is not.
<!ENTITY % XML-EVENTS.pfx  "%XML-EVENTS.prefix;:" >
<!ENTITY % XML-EVENTS.pfx  "" >

<!-- declare qualified name extensions here ............ -->
<!ENTITY % xml-events-qname-extra.mod "" >

<!-- 5. The parameter entity %XML-EVENTS.xmlns.extra.attrib; may be
        redeclared to contain any non-XML Events namespace declaration
        attributes for namespaces embedded in XML. The default
        is an empty string.  XLink should be included here if used
        in the DTD.
<!ENTITY % XML-EVENTS.xmlns.extra.attrib "" >

<!-- Section B: XML Qualified Names ::::::::::::::::::::::::::::: -->

<!-- 6. This section declares parameter entities used to provide
        namespace-qualified names for all XML Events element types.

<!ENTITY % xml-events.listener.qname  "%XML-EVENTS.pfx;listener" >

<!-- The following defines a PE for use in the attribute sets of elements in
     other namespaces that want to incorporate the XML Event attributes. Note
     that in this case the XML-EVENTS.pfx should always be defined. -->

<!ENTITY % xml-events.attrs.qname
   "%XML-EVENTS.pfx;event            NMTOKEN      #REQUIRED
    %XML-EVENTS.pfx;observer         IDREF        #IMPLIED
    %XML-EVENTS.pfx;target           IDREF        #IMPLIED
    %XML-EVENTS.pfx;handler          %URI;        #IMPLIED
    %XML-EVENTS.pfx;phase            (capture|default) #IMPLIED
    %XML-EVENTS.pfx;propagate        (stop|continue) #IMPLIED
    %XML-EVENTS.pfx;defaultAction    (cancel|perform) #IMPLIED"

<!-- end of xml-events-qname-1.mod -->

A.2. XML Events Module

<!-- ...................................................................... -->
<!-- XML Events Module .................................................... -->
<!-- file: xml-events-1.mod

     This is XML Events - the Events Module for XML.
     a redefinition of access to the DOM events model.

     Copyright 2000-2001 W3C (MIT, INRIA, Keio), All Rights Reserved.

     This DTD module is identified by the PUBLIC and SYSTEM identifiers:

       PUBLIC "-//W3C//ENTITIES XML Events 1.0//EN"
       SYSTEM "http://www.w3.org/TR/xml-events/DTD/xml-events-1.mod"

     ....................................................................... -->

<!-- XML Events defines the listener element and its attributes -->

<!ENTITY % xml-events.listener.content "EMPTY" >

<!ELEMENT %xml-events.listener.qname; %xml-events.listener.content;>
<!ATTLIST %xml-events.listener.qname;
    id               ID           #IMPLIED
    event            NMTOKEN      #REQUIRED
    observer         IDREF        #IMPLIED
    target           IDREF        #IMPLIED
    handler          %URI;        #IMPLIED
    phase            (capture|default) #IMPLIED
    propagate        (stop|continue) #IMPLIED
    defaultAction    (cancel|perform) #IMPLIED

<!-- end of xml-events-1.mod -->

B. References

This appendix is normative.

B.1. Normative References

"Document Object Model (DOM) Level 2 Core Specification", Wood L., et al. W3C Recommendation. See http://www.w3.org/TR/2000/REC-DOM-Level-2-Core-20001113.
"Extensible Markup Language (XML) 1.0". W3C Recommendation. See http://www.w3.org/TR/2000/REC-xml-20001006
"Namespaces in XML", Bray T., et al., W3C Recommendation. See http://www.w3.org/TR/1999/REC-xml-names-19990114
"XML Schema Part 2: Datatypes" , Paul V. Biron, et al., W3C Recommendation. See http://www.w3.org/TR/xmlschema-2/.

B.2. Other References

"HTML 4.01 Specification", Raggett D., et al., W3C Recommendation. See http://www.w3.org/TR/html4/.
"Synchronized Multimedia Integration Language (SMIL 2.0)". Ayars J., et al. W3C Recommendation. See http://www.w3.org/TR/2001/REC-smil20-20010807
"XHTML™ 1.0: The Extensible HyperText Markup Language". Pemberton S., et al. W3C Recommendation. See http://www.w3.org/TR/2000/REC-xhtml1-20000126
"Modularization of XHTML™", Altheim M., et al. W3C Recommendation. See http://www.w3.org/TR/xhtml-modularization

C. Acknowledgments

This section is informative.

This document was originally edited by Ted Wugofski (Openwave).

Special acknowledgments to: Mark Baker (Sun Microsystems), Wayne Carr (Intel Corporation), Warner ten Kate (Philips Electronics), Patrick Schmitz, and Peter Stark (Ericsson) for their significant contributions to the evolution of this specification.

At the time of publication, the members of the W3C HTML Working Group were:

List will be inserted when this document becomes a Recommendation.