Widgets 1.0: The widget Interface

W3C Working Draft 17 November 2009

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Latest Version:
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Marcos Cáceres, Opera Software ASA
Robin Berjon, Vodafone
Arve Bersvendsen, Opera Software ASA


This specification defines an application programming interface (API) for widgets that provides, amongst other things, functionality for accessing a widget's metadata and persistently storing data.

Status of this Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This is the 17 November 2009 Last Call Working Draft version of the "Widgets 1.0: The widget Interface" specification. The Last Call period ends on 8 December 2009. The requirements, which are listed in the specification, were addressed and specified through extensive research, and via consultation with W3C members and the public via the Working Group's mailing lists (WAF archive, WebApps archive). The purpose of this Last Call is to give external interested parties a final opportunity to publicly comment on how the API should work within widgets before the Working Group issues a call for implementations. The Working Group's goal is to make sure that vendor's requirements for an API for widgets have been effectively addressed and

This document is produced by the Web Applications WG, part of the Rich Web Clients Activity in the W3C Interaction Domain. It is expected that this document will progress along the W3C's Recommendation track. Publication as a Working Draft 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 this document as other than work in progress.

You can find the latest Editor's Draft of this document in the W3C's CVS repository, which is updated on a very regular basis. The public is encouraged to send comments to the Web Apps Working Group's public mailing list public-webapps@w3.org (archive). See W3C mailing list and archive usage guidelines. A detailed list of changes from the previous version is also available from the W3C's CVS server.

Interested parties who are not taking part in the discussions are likely to find the specification changing out from under them in incompatible ways. If you are interested in implementing this document before it eventually reaches the Candidate Recommendation stage, join the aforementioned mailing lists and take part in the discussions.

Table of Contents

1 Introduction

This section is non-normative.

This specification defines an application programming interface that enables baseline functionality for widgets, including the ability to:

1.1 The Widget Family of Specifications

This section is non-normative.

This specification is part of the Widgets 1.0 family of specifications, which together standardize widgets as a whole. The list of specifications that make up the Widgets 1.0 Family of Specifications can be found on the Working Group's wiki.

1.2 Design Goals and Requirements

This section is non-normative.

The design goals and requirements for this specification are documented in the Widgets 1.0 Requirements [Widgets-Reqs] document.

This document addresses some of the requirements relating to Application Programming Interfaces of the 30 April 2009 Working Draft of the Widgets 1.0: Requirements Document:

2 Conformance

All examples and notes in this specification are non-normative, as are all sections explicitly marked as non-normative. Everything else in this specification is normative.

The key words must, must not, should, recommended, may and optional in the normative parts of this specification are to be interpreted as described in [RFC2119].

Only user agents can claim conformance to this specification. Conformance requirements phrased as algorithms or specific steps can be implemented in any manner, so long as the end result is equivalent to what would be achieved when following the specification.

Note: Implementers can partially check their level of conformance to this specification by successfully passing the test cases of the [A&E-Test-Suite]. Passing all the tests in the test suite does not imply conformance to this specification; It only implies that the implementation conforms to aspects tested by the test suite.

3 Definitions

The following definitions are used throughout this specification. Please note that the following list is not exhaustive; other terms are defined throughout this specification.

Author script

Some code running within a widget instance (e.g., some ECMAScript).

Configuration document

A configuration document is reserved file called "config.xml" at the root of the widget package as specified in the [Widgets-Packaging] specification.


A runtime component (e.g. a device API, video decoder, etc.) that is made available by the user agent to the widget instance as a direct result of an author requesting its availability via a feature element in the widget's configuration document.


A DOM attribute is said to be getting when its value is being retrieved (e.g. by an author script).


The act of user agent processing a widget package through the Steps for Processing a Widget Package, as specified in the [Widgets-Packaging] specification.


A persistently stored name-value pair that is associated with the widget the first time the widget is initiated.

Start file

A file in the widget package to be loaded by the user agent when it instantiates the widget, as specified in the [Widgets-Packaging] specification.


A user agent implements a mentioned specification or conformance clause.

Valid IRI

A string that matches the IRI token of the [IRI] specification.


A CSS viewport. For a start file rendered on continuous media, as defined in the [CSS21] specification, a viewport is the area on which the Document of the start file is rendered by the user agent. The dimensions of a viewport excludes scrollbars, toolbars, and other user interface "chrome".

Widget Instance

A browsing context that comes into existence after initialization. The concept of a browsing context is defined in [HTML5]. Multiple widget instances can be instantiated from a single widget package. Every widget instance is universally unique, meaning it does not share any DOM attribute values or storage areas with any other widget instance.

4 User Agents

A user agent is a software implementation of this specification that also supports the [Widgets-Packaging] specification.

Note: The user agent described in this specification does not denote a "widget user agent" at large. That is, a user agent that implements all the specifications, and dependencies, defined in the Widgets 1.0: Family of Specifications. The user agent described in this specification is only concerned with the behavior of programming interfaces. A user agent needs to impose implementation-specific limits on otherwise unconstrained inputs, e.g. to prevent denial of service attacks, to guard against running out of memory, or to work around platform-specific limitations.

5 The Widget Interface

This specification uses [WebIDL] to define the application programming interfaces.

A user agent whose start file implements [HTML5]s Window interface must implement the Widget interface as the widget attribute of the window object in the manner defined by the WindowWidget interface.

[Supplemental, NoInterfaceObject]
interface WindowWidget {
  readonly attribute Widget widget;

Window implements WindowWidget;

The Widget object provides the following attributes and method:

interface Widget {
    readonly attribute DOMString     author;
    readonly attribute DOMString     authorEmail;
    readonly attribute DOMString     authorHref;
    readonly attribute DOMString     description;
    readonly attribute DOMString     id;
    readonly attribute DOMString     name;
    readonly attribute DOMString     shortName;
    readonly attribute Storage       preferences;
    readonly attribute DOMString     version;
    readonly attribute unsigned long height;
    readonly attribute unsigned long width;

    void openURL(in DOMString iri);

Note: A user agent can support the Storage interface on DOM attributes other than the preferences attribute (e.g., a user agent can to support the [WebStorage] specification’s localStorage attribute of the window object in conjunction to the preferences attribute. For the sake of interoperability across widget user agents, and where it makes sense, authors can use the preferences attribute over other APIs that provide a Storage interface.

When an object implementing the Widget interface is instantiated, if a user agent has not previously associated a storage area with the instance of a widget, then the user agent must initialize the preferences attribute.

5.1 Metadata Attribute Values

Most of the attributes of the widget interface correspond to the metadata derived from the initialization process.

When an object implementing the Widget interface is instantiated, a user agent sets the attributes identified in the left column of the configuration attributes table to the values that correspond to values in table of configuration defaults as defined in [Widgets-Packaging] (identified by the values in the right hand column).

Upon getting any of the attributes from the attributes column of the configuration attributes table, a user agent must return the corresponding value from the ‘Values in Table of Configuration Defaults’ column.

Configuration Attributes Table
Attributes Values in Table of Configuration Defaults
author author name
authorEmail author email
authorHref author href
description widget description
id widget id
name widget name
version widget version
shortName widget short name

5.1.1 Usage Example

This section is non-normative.

// Hypothetical example that emails a bug to an Author
function emailBug(bug){
    var subject  = widget.name   + " (" + widget.version + ")";
    var to       = widget.authorEmail;
    var emailURI =    "mailto:"   + to 
                    + "?subject=" + escape(subject) 
                    + "&body="    + escape(bug); 
  //use email client to send email

// Hypothetical example that generates an about box 
// using metadata from a widget's configuration document. 
function makeAboutBox(){
    var title    = "<h1>" + widget.name + "</h1>";
    var version  = "<h2>Version: " + widget.version + "</h2>";
    var id       = "<p><small>" + widget.id +"</small></p> <hr/>";
    var auth     = "<p>Created by: " + widget.author + "</p>"; 
    var homepage = "<p>My Site: "  +widget.authorHref + "</p>" 
    var desc     = "<h3>About this Widget</h3><p>"  
                  + widget.description + "</p>";
    var box      = "<div id='aboutBox'>" 
    + title + version + id + auth + desc +
    document.getElementById("console").innerHTML = box; 

5.2 The width Attributes

Upon getting the width attribute, a user agent must return an number that represents the width of the widget instance’s viewport in [CSS21] pixels.

5.2.1 Usage Example

This section is non-normative.

function getHypotenuse(){
    var x   = widget.width;  
    var y   = widget.height;  
    var hyp = Math.sqrt(x*x + y*y);
    return hyp;

5.3 The height Attribute

Upon getting the height attribute, a user agent must return a number that represents the height of the widget instance’s viewport in [CSS21] pixels.

5.3.1 Usage Example

This section is non-normative.

function getHypotenuse(){
    var x   = widget.width;  
    var y   = widget.height;  
    var hyp = Math.sqrt(x*x + y*y);
    return hyp;

5.4 The preferences Attribute

The preferences attribute allows authors to manipulate the storage area that is unique for the instance of a widget.

Upon invocation of the setItem() or removeItem() method by an author script on an item that is not a read-only item, a user agent must queue a task to fire an event with the name storage, which does not bubble and is not cancelable, and which uses the StorageEvent interface, at each Window object whose Widget object has a Storage object that is affected. The concept of ‘queue a task’ is defined in [HTML5]. Storage event and the StorageEvent interface is defined in [Storage].

Upon invocation of the setItem() or removeItem() method by an author script on a read-only item, a user agent must throw a NO_MODIFICATION_ALLOWED_ERR exception and must not fire a storage event. The NO_MODIFICATION_ALLOWED_ERR is defined in the [DOM3Core] specification.

Upon invocation of the preferences attribute's clear() method, a user agent must not remove read-only items and corresponding values from a storage area. A user agent must, however, remove other items from the storage area in the manner described in the [WebStorage] specification without throwing a NO_MODIFICATION_ALLOWED_ERR exception for items that the user agent cannot remove.

When getting the preferences attribute, if the origin of a widget instance is mutable (e.g., if the user agent allows document.domain to be dynamically changed), then the user agent must perform the preference-origin security check.

5.4.1 Preference Origin Security Check

The steps to perform the preference-origin security check are given by the following algorithm:

  1. If the Document‘s effective script origin is not the same origin as the Document’s origin, then throw a SECURITY_ERR exception and abort these steps. The concept of effective script origin and same origin are defined in [HTML5].

  2. Return the Storage object associated with that widget instance's preferences attribute.

5.4.2 Initialization of the preferences Attribute

The steps to initialize the preferences attribute are given by the following algorithm:

  1. Establish the instance of a widget for this widget and create a storage area that is unique for the origin.

  2. If the widget preferences variable of the Table of Configuration Defaults contains any preferences, then for each preference held by widget preferences:

    1. Let pref-key be the name of the preference.

    2. If the pref-key already exists in the storage area, stop processing this preference: go back to step 2 in this algorithm, and process the next preference (if any).

    3. Let pref-value be the value of the preference.

    4. Directly write pref-key and pref-value to the storage area (i.e., don't use the Storage interface to avoid firing storage events):

      1. If the user agent cannot write to the storage area (e.g., because it ran out of disk space, or the space quota was exceeded, etc.), terminate all processing of this widget. It is recommended that the user agent inform the end-user of the error.

      2. If this preference's associated readonly value is true, then flag this key as a read-only item in the storage area.

  3. Implement the Storage interface on the storage area, and make the preferences attribute a pointer to that storage area.

5.4.3 Usage Example

This section is non-normative.

<!doctype html>
<fieldset id="prefs-form">
  <legend>Game Options</legend>
  <label>Volume:  <input type="range" min="0" max="100" name="volume"/> </label> 
  <label>Difficulty:  <input type="range" min="0" max="100" name="difficulty"/> </label>

  <input type="button" value="Save" onclick="savePrefs()"/>
  <input type="button" value="load" onclick="loadPrefs()"/>


var form   = document.getElementById("prefs-form");
var fields = form.querySelectorAll("input[type='range']"); 
function loadPrefs () {
   for(i in fields){
      var field = fields[i];
      if (typeof widget.preferences[field.name] != "undefined") {
        field.value = widget.preferences[field.name]; 

function savePrefs () {
  for(var i = 0; i < fields.length; i++){
      var field = fields[i];
      widget.preferences[field.name] = field.value;

5.5 The openURL() Method

The openURL() method provides a means to defer the handling of [IRI]s, to an appropriate scheme handler for the given IRI scheme (if one is available). The openURL() method takes a valid IRI as an argument.

When the method is invoked, a user agent must behave in one of the following two ways: One, either handle the IRI passed as an argument with a supported handler for the given URI scheme; or two, if there is no such scheme handler, or the scheme is unsupported or otherwise rejected by the user agent (e.g., because of security policy or user prompting), then the user agent must act as if the method had not been invoked (i.e., return void and continue executing the author script as normal).

For security reasons, a user agent must not navigate the browsing context of a widget instance through the openURL() method. The concept of navigate is defined in [HTML5].

The user agent may inform the end-user when a request to handle the IRI via the openURL() method has failed.

It is optional for a user agent to support any URI scheme via the openURL() method.

If the argument passed to the openURL() method is not a valid IRI, the user agent should inform the end-user that the request to handle the IRI method has failed.

If a relative URI reference is used as an argument for openURL(), a user agent must act as if the openURL() method had not been invoked (i.e., return void and continue executing the author script as normal).

5.5.1 Usage Example

This section is non-normative.

//open email 

//make a phone call 


//open a web page

//send and sms


6 Storage Areas

A storage area is a data-store that is unique for the widget instance, which a user agent uses to store key-value pairs that pertain to the preferences attribute. An author script interacts with the storage area via the [WebStorage] specification's Storage interface. To facilitate the storage of preferences during the initialization of the preferences attribute, a user agent needs to have the ability to directly read and write to a storage area without invoking the methods of the [WebStorage] specification's Storage interface.

A user agent must preserve the values stored in a storage area when a widget is re-instantiated (i.e., if the widget is closed, the data is saved; when the device is rebooted and the widget is reopened, the save data is made available to the storage area).

A user agent may expire data from a storage area for security reasons, or when requested to do so by the end user, or if the user agent detects that it has limited storage space.

6.1 Read-only Items

As an extension to the [WebStorage] specification, this specification allows certain key-value pairs (items) in a storage area to be read-only: a read-only item is an item in a storage area that cannot be modified or deleted by an author script. A read-only item always represents a preference that author explicitly flagged as "read-only" in the widget's configuration document (denoted by the preference having a readonly attribute value set to true). A user agent creates read-only items when the preferences attribute is initialized.


Max Froumentin, Rune F. Halvorsen, Scott Wilson, Addison Phillips, Alexander Dreiling, Arun Ranganathan, Arthur Barstow, Bárbara Barbosa Neves, Brian Wilson, Bjoern Hoehrmann, Benoit Suzanne, Bert Bos, Boris Zbarsky, Bradford Lassey, Cameron McCormack, Cliff Schmidt, Claudio Venezia, Coach Wei, Corin Edwards, Dan Brickley, David Clarke, Dean Jackson, David Pollington, Doug Schepers, Ed Voas, Felix Sasaki, Francois Daoust, Gautam Chandna, Geir Pedersen, Gene Vayngrib, Gorm Haug Eriksen, Guido Grassel, Hari Kumar G, Ian Hickson, Jay Sweeney, Jere Käpyaho, Jim Ley, Jon Ferraiolo, Jose Manuel Cantera Fonseca, Josh Soref, Jouni Hakala, Joey Bacalhau, Kevin Lawver, Kai Hendry, Krzysztof Maczyński, Lachlan Hunt, Marc Silbey, Marcin Hanclik, Mark Baker, Mikko Pohja, Mohamed Zergaoui, Olli Immonen, Philipp Heltewig, Philip Taylor, Scott Wilson, Stephen Paul Weber.

Normative References

Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification, B. Bos, I. Hickson, T. Çelik, H. Wium Lie. W3C Candidate Recommendation 23 April 2009.
Document Object Model (DOM) Level 3 Core Specification, A. Le Hors, P. Le Hégaret, L. Wood, G. Nicol, J. Robie, M. Champion, S. Byrne, editors. World Wide Web Consortium, April 2004.
Key words for use in RFCs to Indicate Requirement Levels, RFC2119, S. Bradner. March 1997.
Internationalized Resource Identifiers (IRIs). RFC3987, M. Duerst, M. Suignard. January 2005.
Uniform Resource Identifier (URI): Generic Syntax.RFC 3986, T. Berners-Lee, R. Fielding and L. Masinter. January 2005.
Widgets 1.0: Packaging and Configuration. M. Cáceres. W3C Working Draft 29 October 2009. (Work in progress)
Web IDL. Cameron McCormack. W3C Working Draft, 19 December 2008. (Work in progress).
Web Storage. I. Hickson. W3C Working Draft, 23 April 2009. (Work in progress).
HTML5. I. Hickson. W3C Working Draft, 25 August 2009. (Work in progress).

Informative References

Widgets 1.0: The Widget Landscape (Q1 2008). M. Caceres. W3C Working Draft. 14 April 2008.
Widgets 1.0: Digital Signature
Test Suite for the Widget Interface Specification.
Widgets 1.0: Access Requests Policy. Robin Berjon. W3C Working Draft 4 August 2009 (Work in progress).
Widgets 1.0 Requirements, M. Cáceres and M. Priestley. W3C Working Draft, 30 April 2009 (Work in progress).
Widgets 1.0: Media Query Extensions. A. Bersvendsen and M. Caceres. Unpublished editors' draft.
Widgets 1.0: Updates. M. Cáceres. W3C Working Draft, 7 October 2008 (Work in progress).
Hypertext Transfer Protocol -- HTTP/1.1, RFC 2616, R. Fielding, et al. June 1999.