2. Writing an endpoint

Endpoint services are registered with the plone:service ZCML tags:

<plone:service
  method="GET"
  factory=".get.BreadcrumbsGet"
  for="zope.interface.Interface"
  permission="zope2.View"
  name="@breadcrumbs"
  />

They are similar in registration with the more simple BrowserViews, except you can also set the HTTP verb that they will be used for.

For the python implementation:

from plone.restapi.services import Service

class BreadcrumbsGet(Service):
    def reply(self):
        # ... pseudocode
        return extract_something_from(self.context)

Make sure that the return value is compatible with JSON serialization. You may need to convert the value. plone.restapi provides an extensible adapter-based function for this, the plone.restapi.serializer.converters.json_compatible.

A common pattern is to reuse a content expander class in your service implementation. You can do it like:

from plone.restapi.services import Service
from plone.restapi.interfaces import IExpandableElement

class BreadcrumbsGet(Service):
    def reply(self):
        # ... pseudocode
        expander = getMultiAdapter((self.context, self.request), interface=IExpandableElement, name="breadcrumbs")
        return expander(expand=True)['breadcrumbs']