OSLC Core Guidance: User Interface Previewing of Resources (early drafts)

This wiki topic is a record of the evolution of the Ui Preview Guidance. Please see OslcCoreUiPreview for the current approved guidance. Comments and issues on the specification may be logged at OslcCoreUiPreviewISSUES.

Introduction

UI Preview is a technique that can be used to show a user a brief preview of a resource when displaying a link to the resource and to show more information when the user's mouse lingers over the resource. For example, when a lifecycle resource containing links to other resources is being presented to a user, a descriptive title can be displayed to represent the link. Since the descriptive title alone might not provide the user with sufficient context to decide whether the link is worth navigating at that moment, the user may linger over the link and cause the user interface to provide additional rich previews revealing even more information about the linked resource (in advance of actually navigating the link).

This specification describes a mechanism that enables client applications, given only the URI of a resource, to discover helpful information for presenting in-context links and previews of a linked resource. The approach (1) acknowledges that a lifecycle resource may contain links to resources of arbitrary types, and (2) does not require the client to know much about the particular details of the linked resources.

Terminology

The following terms are used in discussions of previews:

• Preview Consumer - a web application displaying a link to a lifecycle resource.

• Preview Provider - a Service Provider that offers Consumers information for displaying a link to a resource and rich previews of the resource.

Conceptual Model

To enable previews of a target resource, a Provider needs to supply the Consumer with a Preview Resource. The Consumer fetches the Preview Resource by sending a GET request to the target resource's URI. If the Provider supports the preview mechanism, it responds with a representation of the Preview Resource with properties that are helpful to the Consumer in displaying the link and previews of the target resource.

Preview Resource representations MUST be in RDF/XML format. The preview media-type ( application/x-oslc-preview+xml) MUST be included in HTTP accept and response headers for Preview Resource requests.

Preview Resources

Resource: Preview

• Name: Preview
• Suggested Prefix: oslc
• Namespace URI: http://open-services.net/xmlns/oslc#
• Type URI: http://open-services.net/xmlns/oslc#Preview
• Version String: oslc-core-1.0
• Properties:

Property	Value-type	Occurs	Description
dc:title	XML Literal	at-most-one	Title which may be used in the display of the resource link. The value SHOULD include only content that is valid inside an XHTML <span> element. Providers SHOULD include a dc:title property with an informative label for the resource. The title is typically shown to a user as a hyperlink. For a resource with no obvious title, Providers SHOULD omit the dc:title property.
dc:name	XML Literal	at-most-one	Name or abbreviated title which may be used in the display of the resource link. The value SHOULD include only content that is valid inside an XHTML <span> element. Providers SHOULD include an abbreviated title for the resource when possible. The abbreviated title is typically shown to a user as a hyperlink in presentations where visual space is limited. As a general guideline, the length of the abbreviated title SHOULD be 5 characters or less. A user-visible identifier that ordinarily appears in the dc:title, such as a defect number, makes for a good dc:name value. When a resource has no obvious identifier or handle, Providers SHOULD omit the dc:name property.
olsc:icon	Resource	at-most-one	URI of an image which may be used in the display of the resource link. The image SHOULD be 16x16 pixels in size.
oslc:smallPreview	Local Inline Resource of type oslc:DetailedPreview	at-most-one	URI and sizing properties for an HTML document to be used for a small preview.
oslc:largePreview	Local Inline Resource of type oslc:DetailedPreview	at-most-one	URI and sizing properties for an HTML document to be used for a large preview.

Resource: DetailedPreview

• Name: DetailedPreview
• Suggested Prefix: oslc
• Namespace URI: http://open-services.net/xmlns/oslc#
• Type URI: http://open-services.net/xmlns/oslc#DetailedPreview
• Version String: oslc-core-1.0
• Properties:

Property	Value-type	Occurs	Description
olsc:document	Resource	at-most-one	The URI of an HTML document to be used for the preview.
oslc:hintWidth	String	at-most-one	Provider recommended width of the preview. Valid values are defined by the width property in the W3C Cascading Style Sheets Specification (CSS 2.1).
oslc:hintHeight	String	at-most-one	Provider recommended height of the preview Valid values are defined by the height property in the W3C Cascading Style Sheets Specification (CSS 2.1).
oslc:initialHeight	String	at-most-one	Provider recommended initial height of the preview. The presence of this property indicates that the preview supports dynamically computing its size. Valid values are defined by the height property in the W3C Cascading Style Sheets Specification (CSS 2.1).

Preview Provider Responsibilities

Providers that support UI preview will examine the HTTP Accept header for each incoming GET and HEAD request for a resource. If the header indicates that the client has requested a preview, and if a preview is available for the resource, the Provider responds with a UI Preview resource representation. The following constraints apply to providers that implement UI Preview:

• Providers that implement UI Previews need not provide a preview for every resource. Providers SHOULD provide previews the resources that other applications or resources ordinarily link to. This is left to individual Providers to decide.
• If the resource does not exist or is not accessible to the requesting Consumer, the Provider SHOULD return the same 4xx status code that it would have returned had the Consumer instead requested the default representation of the resource.
• If the resource exists and is accessible to the Consumer, but does not have a Preview Resource (in other words, does not support the preview mechanism), the Provider SHOULD respond with a status code of 406 Not Acceptable ( Section 14.1 of HTTP 1.1 specification says that a server "SHOULD" respond with a 406 status code if it cannot serve up the media-type specified in the accept header).
  • Otherwise, the Provider SHOULD respond with a representation of a Preview Resource and a response header including a 200 OK status code and the preview media-type. The representation MUST follow the guidelines for RDF/XML resource representations as outlined in the OSLC Core Specification Guidelines (for example representations, see Sample Representations of Preview Resources).
• Providers SHOULD include a Vary field in the response header when returning a Preview Resource. Experience has shown that for subsequent requests on the URL, some browser caching algorithms ignore the media-type and return a cached representation of the URL request (which may be the default target resource representation or the Preview Resource representation). Use of the Vary field allows a cache to properly interpret future requests on the resource and informs the browser that the cacheable response is subject to server-driven negotiation (see Section 14.44 of the HTTP 1.1 specification).
• Providers MAY consult the Accept-Language field on requests for a Preview Resource (and on requests for rich preview documents) and return a resource representation for the requested natural language.

Guidelines and Responsibilities for Preview Consumers

Though Consumer implementations will vary, tool users will benefit if the preview mechanism is used consistently. Previews are a valuable added benefit to tool users and not a necessary element for users to perform tasks. As such, Consumers should gracefully handle unsuccessful attempts to use the preview mechanism; unsuccessful attempts should never result in failure in the way a Consumer application operates or performs. Consumers SHOULD anticipate that the preview mechanism may not be supported for a linked resource, or that the the preview mechanism may not work due to security or service availability issues, or even that preview capability once supported by a Provider has been phased out.

Important: In order to properly recognize when a Provider supports the preview mechanism, Consumers MUST perform two checks on the response from a Preview Resource request. Only a response header including a 200 OK status code and the preview media-type assures the Consumer that the preview protocol is understood by the Provider. Though Providers should respond with a 406 Not Acceptable status code when they do not recognize the preview media-type, Consumers cannot rely on this behavior. Some servers simply ignore an unrecognized media-type and respond with 200 OK and a default representation of the resource. If either the status code or media-type conditions are not met, then Consumers MUST conclude that the Provider does not support the preview mechanism.

The following guidance is suggested for Consumer display of a resource link and rich previews. For purposes of this discussion, assume source resource A has a link to target resource B:

Navigation to the target resource

At any time, the user SHOULD be able to gesture that they would like to navigate to the target resource, regardless of how the link to the target resource happens to be rendered.

Default way to display the link to the target resource

The default display of the link SHOULD be based on information that is stored locally in source resource A (or that is generally known to the Consumer). Generally, link text can be derived from property values of resource A, and potentially from a string property value of the link to the target resource B, if such a property exists. Because these property values are part of the representation of the source resource A, they are available without consulting the target resource, and will be available even if the target resource B cannot be fetched. When available, Consumers SHOULD display this string (as opposed to the URI) when presenting resource A to indicate a potential navigation to resource B. This is the basic presentation of a link to target resource B.

The default display of the link from A to B is visible to any user authorized to access resource A. The use of Preview Resource information described below is only viable for users who are also authorized to access resource B.

Displaying the link using the Preview Resource title, name, and icon

A Preview Resource may contain a more accurate and slightly richer label for a target resource ( dc:title element), a short-form title for the resource ( dc:name), and a corresponding image ( oslc:icon), all of which may be based on the current state of the target resource. If this becomes known to the Consumer, the Consumer SHOULD assume that this information is better and use it to improve and replace the default link display. When available, dc:name MAY be used instead of dc:title in presentations where visual space is severely limited.

Consumers SHOULD NOT fetch Preview Resources when there is perfectly usable default display information available. When designing a Consumer application, consideration should be given to the potential benefits of obtaining an improved title and icon for the linked resource against the costs of preemptively fetching the Preview Resource in terms of added load on servers and networks (which might only be apparent to users on slow networks or heavily loaded servers).

Consumers SHOULD be wary of material obtained from non-trusted sources; in particular, the Consumer SHOULD check that the dc:title and dc:name property values do not contain HTML markup beyond the specified set of simple elements.

Displaying a small preview

If a user mouses or hovers over a displayed link, the Consumer SHOULD determine whether the target resource has a small preview (oslc:smallPreview). If it does, the Consumer SHOULD present the small preview document in a hovering widget. Since preview documents may contain arbitrary content, including HTML and scripts, Consumers MUST use iframes if embedding the preview document inside another HTML-based presentation.

The Consumer SHOULD NOT attempt to prefetch a Preview Resource just to have the preview URIs in hand so that the hover can come up faster. There is a low chance that the user will make a gesture that would call for the display of a small preview. It would generally be a poor tradeoff to increase overall system load just to decrease UI latency for low probability eventualities.

Displaying a large preview

If the user then gestures that they want to see a bit more of the resource, the Consumer should determine whether the target resource has a large preview ( oslc:largePreview). If it does, the Consumer SHOULD present the large preview document in a stationary widget that permits further user interaction. Again, since preview documents may contain arbitrary content, including HTML and scripts, Consumers MUST use iframes if embedding the preview document inside another HTML-based presentation.

Preview sizing

Small and large previews are designed to be sized statically by the client. The Preview Resource MAY include oslc:hintHeight and/or oslc:hintWidth properties for each of the previews. When the resource does not supply preferred sizing for a preview, the Consumer SHOULD use a reasonably generous default value large enough to display previews without scroll bars or truncation. When the resource supplies preferred sizing, the Consumer SHOULD use the preferred value instead.

Small and large previews MAY also support dynamic resizing. For a preview that supports dynamic resizing, the Preview Resource MUST include the oslc:initialHeight property to indicate the initial height. A Provider MAY use dynamic resizing previews on some of their resources but not others. Moreover, dynamic resizing applies to the small and large previews independently.

The communication of dynamic preview size information happens within a web browser. JavaScript code running in the preview iframe sends a message that is received and acted upon by JavaScript code running in the iframe's parenting window. This cross-frame communication is done using HTML 5 postMessage. While many major web browsers support HTML 5 postMessage, support is not yet universal. In browsers that do not support postMessage, there is no way to implement dynamic resizing. In this case, everything falls back on static sizing.

A dynamic preview resize message consists of the characters "oslc-preview-height:" followed by the representation of the desired height of the preview. Whitespace characters are not allowed.

Example showing desired preview height of 277 pixels:

           oslc-preview-height:277px 

The source of the event MUST be the preview's window.

Provders SHOULD make a preview dynamically resizeable where that suits their needs, but are under no obligation to do so. A Provider that chooses to offer dynamic resizing for a particular resource preview MUST (a) supply a static oslc:initialHeight value, and (b) send one or more preview resize messages when the preview is shown in an iframe. When the preview is shown in a web browser that does not support HTML 5 postMessage (e.g., Internet Explorer 7), everything falls back on static sizing.

JavaScript snippet for a typical resource preview to report a preview resize event to its parent window:

var newPreviewHeight = ...; // desired height of preview (hard!)

if (window.postMessage && window.parent)  {
    var PREVIEW_RESIZE_MESSAGE_PREFIX = "oslc-preview-height:";
    var resizeMessage = PREVIEW_RESIZE_MESSAGE_PREFIX + newPreviewHeight;
    window.parent.postMessage(resizeMessage, "*");
 }

Note that postMessage for a preview resize message MUST be executed in the context of the preview's window so that the source of the event is set correctly.

Warning: Reliably computing the height of a document (newPreviewHeight above) is non-trivial owing to severe incompatibilities between browsers. Caveat emptor.

Consumers SHOULD honor a preview's ability to dynamic resize, but are under no obligation to do so. A Consumer that chooses to honor dynamic resizing for a particular preview MUST (a) use the oslc:initialHeight value instead of any static height, and (b) register a handler to receive preview resize messages sent by the preview and adjust the size of the preview accordingly. Since a preview is allowed to resize itself any number of times, the Consumer MUST keep a handler registered and react appropriately each time it received a preview resize message from that preview. When the preview is not shown in an iframe, or shown in a browser that does not support HTML 5 postMessage, everything falls back on static sizing.

Consumers that honor dynamic preview resizing SHOULD make an effort to minimize the amount of resizing behavior that the user actually sees. Consumers SHOULD locally cache the last known dynamic height for recently shown preview URLs, and use a cached height for a preview in preference to its oslc:initialHeight value.

JavaScript snippet for a typical Consumer to show a preview in an iframe:

var previewURL = ...; // from oscl:document attribute for preview 
var previewPreferredHeight = ...; // from oslc:hintHeight property for preview,or empty string if none 
var previewPreferredWidth = ...; // from oslc:hintWidth property for preview, or empty string if none 
var previewInitialHeight = ...; // from oslc:initialHeight property for preview, or empty string if none  

var previewFrame = document.createElement("iframe");  // create iframe for preview 
previewFrame.src = previewURL;
previewFrame.height = (previewPreferredHeight || DEFAULT_PREVIEW_HEIGHT); 
previewFrame.width = (previewPreferredWidth || DEFAULT_PREVIEW_WIDTH);  

if (window.postMessage && previewInitialHeight) {
   previewFrame.height = previewInitialHeight;

   var handlePreviewResize = function(event) { 
        if (event.source !== previewFrame.contentWindow) return; 
        var data = event.data;  
        var PREVIEW_RESIZE_MESSAGE_PREFIX = "oslc-preview-height:"; 
        if (data && data.indexOf(PREVIEW_RESIZE_MESSAGE_PREFIX) === 0) {    
            var newHeight = data.substring(PREVIEW_RESIZE_MESSAGE_PREFIX.length);   
            previewFrame.height = newHeight;    
         } 
    } 
   dojo.connect(window, "onmessage", window, handlePreviewResize);
} 
document.body.appendChild(previewFrame); // make preview iframe visible

Sample Representations of Preview Resources

Below is a sample preview resource representation that complies with the OSLC Core Specification v1.0 (DRAFT 20100419).

Preview resource representation: RDF/XML

<?xml version="1.0" encoding="UTF-8"?>

<oslc:Preview
   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
   xmlns:dc="http://purl.org/dc/terms/"
   xmlns:oslc="http://open-services.net/xmlns/oslc#"
   rdf:about="http://example.com/bugs/2314">

   <dc:title> 12345: &lt;s&gt;Null pointer exception during startup&lt;/s&gt; </dc:title>
   <dc:name> 12345 </dc:name>
   <oslc:icon rdf:resource="http://example.com/icons/defect.jpg" />

   <oslc:smallPreview>
      <oslc:DetailedPreview>
         <oslc:document rdf:resource="http://example.com/bugs/12345?hover=small" />
      </oslc:DetailedPreview>
   </oslc:smallPreview>


   <oslc:largePreview>
      <oslc:DetailedPreview>
         <oslc:document rdf:resource="http://example.com/bugs/12345?hover=large" />
         <oslc:hintWidth> 60em </oslc:hintWidth>
         <oslc:hintHeight> 20em </oslc:hintHeight>
      </oslc:DetailedPreview>
   </oslc:largePreview>

 </oslc:Preview>

Click HERE to see RDF/XML validation results.

References

• CSS - Cascading Style Sheets Level 2 Revision 2 v2.1
• Dublin Core - Dublin Core Metadata Element Set, Version 1.1
• HTML5 Posting Message - W3C HTML5 postMessage
• HTTP - Hyper-text Transfer Protocol (HTTP/1.1)
• RDF/XML Concepts - RDF/XML Concepts and Abstract Syntax
• RDF/XML Syntax - RDF / XML Syntax Specification (Revised)
• Strawman Proposal - IBM Jazz Compact Rendering 1.0
• Window Object - Window Object 1.0 Window.name
• XSD Datatypes - XML Schema Part 2: Datatypes Second Edition