= Work with the Handlebars Templates

Antora combines the Handlebars templates with the converted AsciiDoc content and other UI model data to make the pages in the site.
These "`logic-less`" templates are mostly HTML with some special mustache tags sprinkled in where content should be inserted.

== What do the templates do?

The layout templates, which are stored in [.path]_src/layouts/_, provide the main page structure.
The partial templates, in [.path]_src/partials/_, fill in different regions of a page, such as the navigation and footer.

The templates read from a model that's populated by Antora.
The model can be accessed by enclosing path expressions in mustaches, which are `{{` and `}}` or `{{{` and `}}}` (e.g., `+{{{page.title}}}+`).
The double mustaches escape the value for HTML, whereas triple mustaches insert the value as is.

WARNING: If the mustaches are preceded by a backslash (e.g, `\{{`), the expression will be disabled.
This often comes up when constructing URLs.
To avoid this problem, you should use forward slashes in URLs instead of backslashes.

When `{{` is immediately followed by `>`, that invokes a partial (from the partials directory) and inserts the result (e.g., `+{{> head}}+`.
In other words, that's not a model reference like the other mustache expressions.

=== Template variables

CAUTION: This model is not final.
Variable names and purposes may change.

Here's an overview of the available UI model:

.Variables available to the Handlebars templates (top-level variables in bold)
[#template-variables-ref,cols="1m,2"]
|===
| Name | Description

s| [[site]]site
| Information about the site.

| site.url
| The base URL of the site, if specified in the playbook.
If a site start page is defined, the visitor will be redirected from this location to the start page.

| site.path
| The pathname (i.e., subpath) of the site.url under which the site is hosted (e.g., /docs).
This value is empty if site.url is not defined, has no path segment, or matches /.
Can be dropped from the site.url value using a helper (e.g., `deleteSuffix site.url site.path`)
Can be prepended to `page.url` to create a root-relative URL for a page (e.g., `+{{{site.path}}}{{{page.url}}}+`).
(since Antora 2.1)

| site.title
| The title of the site.

| site.homeUrl
| The URL that points directly to the start (aka home) page of the site.

| site.components
| A map of all the components in the site, keyed by component name.
Properties of each component include name, title, url, latest, and versions.
Properties of each version include name (since 2.3), version, displayVersion, prerelease (if set), title, url, asciidoc (since 2.3), and navigation.
The navigation property on each version provides access to the navigation menu for that component version.

| site.ui
| Information about the site UI.

| site.ui.defaultLayout
| The default page layout used for this site.

| site.ui.url
| The absolute base URL of the UI.

s| [[page]]page
| Information about the current page.

| page.title
| The page title in HTML format (often used as the primary heading).
This value may include inline HTML elements and XML character references.

| page.contents
| The main article content in HTML format.
Sourced from AsciiDoc and converted to HTML by the Asciidoctor processor.

| page.attributes
| Any AsciiDoc document attribute prefixed with `page-`.
The `page-` prefix is dropped from the name used in this model.
For example, the value of the document attribute named `page-support-phone` can be accessed via the UI model using `page.attributes.support-phone`.
Page attributes can be defined per page in the AsciiDoc document header (e.g., `:page-support-phone: +1 212-555-1234`) or globally in the playbook under the key `asciidoc.attributes`.
The `page-` prefix is used to isolate page-related attributes from the numerous other document attributes in AsciiDoc.

| page.author
| The first author of the document, if one or more authors are specified in the AsciiDoc header.

| page.description
| The text of the description attribute in the AsciiDoc header, if specified.

| page.keywords
| A comma-separated list of keywords defined in the AsciiDoc header, if specified.

| page.component
| Information about the component for the current page.
Properties include name, title, url, latest, and versions.

| page.componentVersion
| Information about the component version for the current page.
Properties include name (since 2.3), version, displayVersion, prerelease (if set), title, url, and asciidoc (since 2.3).

| page.module
| The name of the module for the current page.

| page.relativeSrcPath
| The path of the current page relative to the pages directory in the current module (since 2.3).

| page.version
| The name of the version for the current page.

| page.displayVersion
| The name of the display version for the current page.

| page.versions
| All versions of the current page, including the current page.
Each entry has the properties url, string, and missing.

| page.latest
| The entry from `page.versions` that corresponds to the latest version available of this page.
The latest page may not come from the latest version of the component (if the page is missing in that version).
This property is not set if this page is in a versionless component.

| page.breadcrumbs
| An array of breadcrumb items that represent the current selection in the navigation tree.
Includes text-only and external items.

| page.navigation
| The hierarchical navigation menu for the component version of the current page.
Each navigation item contains the property `content` as well as the optional properties `url` and (child) `items`.

| page.url
| The URL for the current page.
This value is a root-relative path.
It's often used as the base URL to generate relative URLs from this page.

| page.canonicalUrl
| The canonical URL for the current page.
The canonicalUrl is only set if site.url is set to an absolute URL and the page's component has at least one non-prerelease version.
If there are multiple versions of the component, the canonical URL is the qualified URL of the most recent, non-prerelease version of the page.
If there's only a single version of the component, the canonical URL is the qualified URL of the current page.

| page.editUrl
| The URL to edit the current page (i.e., activates the web-based editor on the git host).
This value is derived automatically for the hosts github.com, gitlab.com, pagure.io, and bitbucket.org, even if the repository is private.
If the host is not recognized, or you want to customize the value, you can use the `edit_url` key on the content source in the playbook.

The default UI shows an "Edit this Page" link that points to this URL unless the repository is private (i.e., `page.origin.private` is truthy) or `page.fileUri` is set.
You can force this link to be shown by setting the environment variable `FORCE_SHOW_EDIT_PAGE_LINK` (e.g., `FORCE_SHOW_EDIT_PAGE_LINK=true`) or by customizing the logic in the UI template.

| page.fileUri
| The local file:// URI to edit the current page if the page originates from the local filesystem (i.e., the worktree).

If this property is set, the default UI shows an "Edit this Page" link that points to this URI (instead of the `page.editUrl` value) unless the `CI` environment variable is set (e.g., `CI=true`).
When the `CI` environment variable is set, the default UI ignores this property (since linking to a local file:// URI in a published site doesn't make any sense).

| page.origin
| Information about the content source from which the current page was taken.
Properties include url, reftype (since 3.1), refname, branch, tag, refhash (since 2.3), startPath, worktree, webUrl, fileUriPattern, editUrlPattern, private.

| page.origin.private
| This value will be `auth-required` if the git server requests authentication credentials, otherwise `auth-embedded` if credentials are embedded in the content source URL in the playbook, otherwise unset.
In the default UI, if this value is truthy, the "Edit this Page" link is not shown.
A quick way to force this property to be truthy (even if the repository is public) is to begin the content source URL in the playbook with empty credentials, as in `\https://@`.
Then, the "Edit the Page" link will not appear.

| page.home
| Indicates whether the current page is the start (aka home) page of the site.

| page.layout
| The page layout for the current page.

| page.next
| The next reachable page in the navigation tree (skips past text-only and external items).

| page.previous
| The previous reachable page in the navigation tree (skips past text-only and external items).

| page.parent
| The parent page in the navigation tree (skips past text-only and external items).

s| env
| The map of environment variables (sourced from `process.env`).

s| siteRootPath
| The relative path to the root of the published site.
If a site start page is defined, the visitor will be redirected from this location to the start page.
Can be used as a fallback when a site URL is not set.
This value is _root-relative_ in the 404 page template, which is required for the 404 page to work correctly when served by the web server. 

s| uiRootPath
| The relative path to the root directory of the UI.
This value is _root-relative_ in the 404 page template, which is required for the 404 page to work correctly when served by the web server. 

s| antoraVersion
| The version of Antora used to build the site (specifically the version of the @antora/page-composer package).
|===

This model is likely to grow over time.

== Modify a template

Let's consider the case when you want to add a new meta tag inside the HTML head.

First, make sure you have set up the project and created a development branch.
Next, open the file [.path]_templates/partials/head.hbs_ and add your tag.

[source,html]
----
<meta class="swiftype" name="title" data-type="string" content="{{page.title}}">
----

Each template file has access to the template model, which exposes information about the current page through variable names.
The variables currently available are listed in <<template-variables-ref>>.

Save the file, commit it to git, push the branch, and allow the approval workflow to play out.
