Custom theme which presents the front-end UI to all users. .
Breadcrumbs are an informative device which appear on many pages on the site. Breadcrumbs provide the user a sense of location within the site and a way to logically navigate back to the homepage.
A breadcrumb is an ordered collection of crumbs, with each crumb having a title and a link.
Drupal has a built-in breadcrumbs methodology, which will attempt to build out a pathway based on the URI (e.g. /departments/housing/metrolist) defined by the pages (i.e. nodes) URL Alias.
It does not matter if the URL Alias is set manually or automatically, the value shown in the back-end editor form once the node is saved is used to build out the breadcrumb.
The Drupal core process creates the breadcrumb by scanning the path represented by the URI, and testing if a local page exists for each path element. It stops adding crumbs when a path element does not resolve.
FOR EXAMPLE an article is created with a URI (as defined in its URL Alias):
/departments/housing/boston/housing-information-in-boston.
When the page is rendered, Drupal scans the articles URI and
if we have a breadcrumb setting which stipulates that the homepage should always be shown as the first crumb, then a crumb of home with a link to https://site is created, then
checks if /departments is a valid URI. https://site/departments is a valid URI, so it creates a crumb of "departments" with a link to https://site/departments , then
checks if /departments/housing is a valid URI. https://site/departments/housing is a valid URI, so it creates a crumb of "housing" with a link to https://site/department/housing , then
checks if /departments/housing/boston is a valid URI. https://site/departments/housing/boston is NOT a valid URI - there is no page with that name on https://site so the breadcrumb scanner stops evaluating at this point, but
if we have a breadcrumb setting to display the actual page in the breadcrumb then a final crumb of housing information in boston is added, with no link (because this is the page showing).
The final breadcrumb in this instance would be HOME > DEPARTMENTS > HOUSING > HOUSING INFORMATION IN BOSTON with links on the first 3 crumbs.
Where Drupal core cannot build out its own breadcrumb trail, there is some additional custom code intended to help make a logical breadcrumb.
The custom code scans URL redirects as well as URL Aliases when building out the breadcrumbs.
Care: Redirects which are manually made on the page admin/config/search/redirect are usually considered "external" by default. Breadcrumbs which use an external link may behave unexpectedly when clicked.
Example: the breadcrumb on d8-dev.boston.gov may open a page on www.boston.gov when clicked.
Solution: Do not create redirects for internal (i.e. Drupal hosted) pages on in the admin/config/search/redirect page. Instead create redirects using the redirect function on the "advanced" tab of the editor form for a page.
Some URI paths are hard-coded to build specific breadcrumbs.
For example pages which have a URI path starting with government/cabinet . The custom code ignores the "government/cabinets" part of the path and then build the breadcrumb from the remainder of the path.