## Content extensions

A content extension is either a contribution of content into a known anchor (similar to an extension into an extension point), or the replacement of an existing element in a document.

### Contributing into anchors

Anchor elements (e.g. `<anchor id="my_anchor"/>`) can be specified in any user assistance XML document, and represent places at which other components are allowed to extend this document. Specifying an anchor is as simple as adding the element in your markup.

To contribute content into an anchor, you must write the content to be added in a separate XML file and hook it into the platform using one of two extension points. For help or cheat sheet content, you must use the `org.eclipse.help.contentExtension` extension point. For welcome (intro) extensions, use `org.eclipse.ui.intro.configExtension`. The table of contents markup also allows the special constructs `link` and `link_to` which perform a similar function to includes and contributions, but perform the additional operation of merging extra documents to index for searching.

For example, let's say component A's documentation provides a listing of file formats it supports. If component B extends A's support to several more formats, you can place an anchor in A's list, and extend the list from B. For example:

       <p>The following list shows the supported formats:</p>
       <ul>
          <li>Portable Network Graphics (.png)</li>
          <li>Joint Photographic Experts Group (.jpeg)</li>
          <li>Graphical Interchange Format (.gif)</li>

          <!-- Extra formats go here -->
          <anchor id="image_format_list"/>
       </ul>

Then component B can make a contribution to the anchor to add more formats to the list by specifying the following extension:

       <extension point="org.eclipse.help.contentExtension">
          <contentExtension
                file="path/to/extension.xml"/>
       </extension>

Where the file `path/to/extension.xml` might contain:

       <contentExtension>
          <contribution
                content="docs/mydoc.xhtml#mycontent"
                path="/component_a_plugin/docs/doc_with_anchor.xhtml#anchorId">
          </contribution>
       </contentExtension>

The element with the `id` attribute `mycontent` in document `docs/mydoc.xhtml` would contain the content you wish to contribute into the anchor. The result will be that when the user views the document, the extra content will appear at the anchor as though it was explicitly added into the original document.

### Replacing content

Any XML element in a user assistance document that has an `id` attribute that is unique to that document can be replaced with any other element.

As with contributions into anchors, to replace an element, you must write the new element to replace with in a separate XML file and hook it into the platform using one of two extension points. For help or cheat sheet content, you must use the `org.eclipse.help.contentExtension` extension point. For welcome (intro) extensions, use `org.eclipse.ui.intro.configExtension`.

The only difference with replacements as opposed to contributions into anchors is the name of the XML element used in the markup; use the `replacement` element instead of `contribution` (or `replacementContent` in the case of welcome). For example:

       <contentExtension>
          <replacement
                content="docs/mydoc.xml#myelement"
                path="/plugin.id/path/doc.xml#elementId">
          </replacement>
       </contentExtension>

(Note the markup is different for welcome, as specified in the `org.eclipse.ui.intro.configExtension` extension point)

### Path format

The format of the `path` attribute is `/pluginId/path/file.xml#elementId`, except welcome where it is `pageId/path/to/elementId` where the pageId is the welcome page id, and all other path segments are ids of container elements like groups, leading to the target element.