Allow changing the target slot of a viewlet, via configuration, instead of ZCML registration.

README.rst

@@ -8,3 +8,14 @@ local utility providing IViewletSettingsStorage (CMFPlone does this). The
 viewlet manager in manager.OrderedViewletManager can then get the filter and
 order settings. These settings can be configured by 3rd party products and
 TTW to customize the viewlets per skin.
+
+Additional viewlets
+===================
+
+A manager renders more than the viewlets registered for it. Subscription
+adapters for ``(context, request, view, manager)`` providing
+``interfaces.IAdditionalViewlets`` contribute ``(name, viewlet)`` pairs through
+their ``viewlets()`` method. This lets an add-on decide at runtime which
+manager shows a viewlet, for example from configuration stored in the
+database. Contributed viewlets are ordered, hidden and managed like registered
+ones. A registered viewlet wins over a contributed one of the same name.

news/+additional-viewlets.feature

@@ -0,0 +1,2 @@
+Add ``IAdditionalViewlets`` subscribers to contribute viewlets to ordered viewlet managers they are not registered for.
+[MrTango]

src/plone/app/viewletmanager/interfaces.py

@@ -21,5 +21,15 @@ def setHidden(name, skinname, hidden):
         given viewletmanager (name) and skin name."""
 
 
+class IAdditionalViewlets(Interface):
+    """Contributes viewlets to a manager they are not registered for.
+
+    Looked up as subscription adapters for (context, request, view, manager).
+    """
+
+    def viewlets():
+        """Return an iterable of (name, viewlet) pairs."""
+
+
 class IViewletManagementView(Interface):
     pass

src/plone/app/viewletmanager/manager.py

@@ -3,6 +3,7 @@
 from Acquisition.interfaces import IAcquirer
 from logging import getLogger
 from operator import itemgetter
+from plone.app.viewletmanager.interfaces import IAdditionalViewlets
 from plone.app.viewletmanager.interfaces import IViewletManagementView
 from plone.app.viewletmanager.interfaces import IViewletSettingsStorage
 from Products.Five import BrowserView
@@ -15,9 +16,13 @@
 from zope.component import getUtility
 from zope.component import queryMultiAdapter
 from zope.component import queryUtility
+from zope.component import subscribers
 from zope.contentprovider.interfaces import IContentProvider
 from zope.interface import implementer
 from zope.interface import providedBy
+from zope.interface.interfaces import ComponentLookupError
+from zope.location.interfaces import ILocation
+from zope.security.interfaces import Unauthorized
 from zope.viewlet.interfaces import IViewlet
 from ZPublisher import Retry
 
@@ -30,6 +35,51 @@ class BaseOrderedViewletManager:
     # See https://github.com/plone/plone.app.viewletmanager/issues/5
     _exceptions_handled_elsewhere = (ConflictError, KeyboardInterrupt, Retry)
 
+    def _additional_viewlets(self):
+        required = (self.context, self.request, self.__parent__, self)
+        for source in subscribers(required, IAdditionalViewlets):
+            yield from source.viewlets()
+
+    def available_viewlets(self):
+        """All (name, viewlet) pairs of this manager, before filtering.
+
+        Registered viewlets win over contributed ones of the same name.
+        """
+        viewlets = dict(
+            getAdapters(
+                (self.context, self.request, self.__parent__, self), IViewlet
+            )
+        )
+        for name, viewlet in self._additional_viewlets():
+            viewlets.setdefault(name, viewlet)
+        return list(viewlets.items())
+
+    def update(self):
+        # zope.viewlet's update() with available_viewlets() as the source.
+        self._ViewletManagerBase__updated = True
+        viewlets = self.sort(self.filter(self.available_viewlets()))
+        self.viewlets = []
+        for name, viewlet in viewlets:
+            if ILocation.providedBy(viewlet):
+                viewlet.__name__ = name
+            self.viewlets.append(viewlet)
+        self._updateViewlets()
+
+    def __getitem__(self, name):
+        try:
+            return super().__getitem__(name)
+        except ComponentLookupError:
+            pass
+        for viewlet_name, viewlet in self._additional_viewlets():
+            if viewlet_name != name:
+                continue
+            if not guarded_hasattr(viewlet, "render"):
+                raise Unauthorized(
+                    f"You are not authorized to access the provider called `{name}`."
+                )
+            return viewlet
+        raise ComponentLookupError(f"No provider with name `{name}` found.")
+
     def filter(self, viewlets):
         """Filter the viewlets.
 
@@ -131,9 +181,7 @@ def render(self):
 
         if is_managing:
             # if we are in the managing view, then fetch all viewlets again
-            viewlets = getAdapters(
-                (self.context, self.request, self.__parent__, self), IViewlet
-            )
+            viewlets = self.available_viewlets()
 
             # sort them first
             viewlets = self.sort(viewlets)
@@ -211,9 +259,13 @@ def _getOrder(self, manager_name):
         manager = queryMultiAdapter(
             (self.context, self.request, self), IContentProvider, manager_name
         )
-        viewlets = getAdapters(
-            (manager.context, manager.request, manager.__parent__, manager), IViewlet
-        )
+        if isinstance(manager, BaseOrderedViewletManager):
+            viewlets = manager.available_viewlets()
+        else:
+            viewlets = getAdapters(
+                (manager.context, manager.request, manager.__parent__, manager),
+                IViewlet,
+            )
         order_by_name = storage.getOrder(manager_name, skinname)
         # first get the known ones
         name_map = dict(viewlets)

src/plone/app/viewletmanager/tests/manager.rst

@@ -147,3 +147,78 @@ We can also hide viewlets.
     >>> manager.update()
     >>> manager.render()
     'third\nsecond'
+
+Additional viewlets
+-------------------
+
+A manager also renders viewlets that are not registered for it. Any
+subscription adapter for ``(context, request, view, manager)`` providing
+``IAdditionalViewlets`` can contribute ``(name, viewlet)`` pairs, for example
+viewlets whose manager assignment is kept in the database instead of ZCML.
+
+    >>> storage.setHidden('left', 'Plone Default', ())
+    >>> storage.setOrder('left', 'Plone Default', ())
+
+    >>> class FourthViewlet(BaseViewlet):
+    ...     name = u"fourth"
+
+    >>> from plone.app.viewletmanager.interfaces import IAdditionalViewlets
+    >>> from zope.component import provideSubscriptionAdapter
+    >>> @implementer(IAdditionalViewlets)
+    ... class ExtraViewlets:
+    ...
+    ...     def __init__(self, context, request, view, manager):
+    ...         self.required = (context, request, view, manager)
+    ...
+    ...     def viewlets(self):
+    ...         yield 'fourth', FourthViewlet(*self.required)
+    ...         yield 'second', FourthViewlet(*self.required)
+
+    >>> provideSubscriptionAdapter(
+    ...     ExtraViewlets,
+    ...     (Interface, IDefaultBrowserLayer, IBrowserView, ILeftColumn),
+    ...     IAdditionalViewlets)
+
+The contributed viewlet renders along with the registered ones. A name that is
+already registered for the manager keeps its registered viewlet.
+
+    >>> manager.update()
+    >>> manager.render()
+    'first\nfourth\nsecond\nthird'
+
+Order and visibility come from the storage, exactly as for registered
+viewlets.
+
+    >>> storage.setOrder('left', 'Plone Default',
+    ...                  ('third', 'fourth', 'first', 'second'))
+    >>> manager.update()
+    >>> manager.render()
+    'third\nfourth\nfirst\nsecond'
+
+    >>> storage.setHidden('left', 'Plone Default', ('fourth',))
+    >>> manager.update()
+    >>> manager.render()
+    'third\nfirst\nsecond'
+
+    >>> storage.setHidden('left', 'Plone Default', ())
+
+The manager hands out a contributed viewlet by name, too.
+
+    >>> manager['fourth'].render()
+    'fourth'
+    >>> manager.get('missing') is None
+    True
+
+The management view moves contributed viewlets like any other.
+
+    >>> from zope.contentprovider.interfaces import IContentProvider
+    >>> provideAdapter(
+    ...     LeftColumn,
+    ...     (Interface, IDefaultBrowserLayer, IBrowserView),
+    ...     IContentProvider, name='left')
+
+    >>> from plone.app.viewletmanager.manager import ManageViewlets
+    >>> manage = ManageViewlets(content, request)
+    >>> manage.moveBelow('left', 'fourth', 'second')
+    >>> storage.getOrder('left', 'Plone Default')
+    ('third', 'first', 'second', 'fourth')