Move SnippetViewSet.get_edit_handler() to ModelViewSet

However, keep the logic for falling back to extracting panel definitions
from the model class as snippets-specific. ModelViewSets are likely used
with models that are more low-level and thus we want developers to
explicitly define the fields that are editable in the admin, just like
how Django's ModelForm works.

docs/reference/viewsets.md

@@ -77,6 +77,7 @@ Viewsets are Wagtail's mechanism for defining a group of related admin views wit
    Used in place of :attr:`form_fields` to indicate that all of the model's fields except the ones listed here should appear in the create / edit forms. Either ``form_fields`` or ``exclude_form_fields`` must be supplied (unless :meth:`get_form_class` is being overridden).
 
    .. automethod:: get_form_class
+   .. automethod:: get_edit_handler
 
    .. autoattribute:: menu_label
 
@@ -195,7 +196,6 @@ Viewsets are Wagtail's mechanism for defining a group of related admin views wit
    .. autoattribute:: chooser_viewset_class
    .. automethod:: get_queryset
    .. automethod:: get_edit_handler
-   .. automethod:: get_form_class
    .. automethod:: get_index_template
    .. automethod:: get_index_results_template
    .. automethod:: get_create_template

wagtail/admin/viewsets/model.py

@@ -10,6 +10,7 @@ from wagtail.admin.admin_url_finder import (
     ModelAdminURLFinder,
     register_admin_url_finder,
 )
+from wagtail.admin.panels.group import ObjectList
 from wagtail.admin.views import generic
 from wagtail.admin.views.generic import history, usage
 from wagtail.models import ReferenceIndex
@@ -151,6 +152,7 @@ class ModelViewSet(ViewSet):
 
     def get_add_view_kwargs(self, **kwargs):
         return {
+            "panel": self._edit_handler,
             "form_class": self.get_form_class(),
             "template_name": self.create_template_name,
             **kwargs,
@@ -158,6 +160,7 @@ class ModelViewSet(ViewSet):
 
     def get_edit_view_kwargs(self, **kwargs):
         return {
+            "panel": self._edit_handler,
             "form_class": self.get_form_class(for_update=True),
             "template_name": self.edit_template_name,
             **kwargs,
@@ -507,6 +510,11 @@ class ModelViewSet(ViewSet):
         """
         Returns the form class to use for the create / edit forms.
         """
+        # If an edit handler is defined, use it to construct the form class.
+        if self._edit_handler:
+            return self._edit_handler.get_form_class()
+
+        # Otherwise, use Django's modelform_factory.
         fields = self.get_form_fields()
         exclude = self.get_exclude_form_fields()
 
@@ -535,6 +543,37 @@ class ModelViewSet(ViewSet):
         """
         return getattr(self, "exclude_form_fields", None)
 
+    def get_edit_handler(self):
+        """
+        Returns the appropriate edit handler for this ``ModelViewSet`` class.
+        It can be defined either on the model itself or on the ``ModelViewSet``,
+        as the ``edit_handler`` or ``panels`` properties. If none of these are
+        defined, it will return ``None`` and the form will be constructed as
+        a Django form using :meth:`get_form_class` (without using
+        :ref:`forms_panels_overview`).
+        """
+        if hasattr(self, "edit_handler"):
+            edit_handler = self.edit_handler
+        elif hasattr(self, "panels"):
+            panels = self.panels
+            edit_handler = ObjectList(panels)
+        elif hasattr(self.model, "edit_handler"):
+            edit_handler = self.model.edit_handler
+        elif hasattr(self.model, "panels"):
+            panels = self.model.panels
+            edit_handler = ObjectList(panels)
+        else:
+            return None
+        return edit_handler.bind_to_model(self.model)
+
+    @cached_property
+    def _edit_handler(self):
+        """
+        An edit handler that has been bound to the model class,
+        to be used across views.
+        """
+        return self.get_edit_handler()
+
     @property
     def url_finder_class(self):
         return type(

wagtail/snippets/views/snippets.py

@@ -13,8 +13,7 @@ from django.utils.translation import gettext_lazy
 
 from wagtail import hooks
 from wagtail.admin.checks import check_panels_in_model
-from wagtail.admin.panels.group import ObjectList
-from wagtail.admin.panels.model_utils import extract_panel_definitions_from_model_class
+from wagtail.admin.panels import ObjectList, extract_panel_definitions_from_model_class
 from wagtail.admin.ui.components import MediaContainer
 from wagtail.admin.ui.side_panels import PreviewSidePanel
 from wagtail.admin.ui.tables import (
@@ -548,6 +547,12 @@ class WorkflowHistoryDetailView(
 class SnippetViewSet(ModelViewSet):
     """
     A viewset that instantiates the admin views for snippets.
+
+    All attributes and methods from
+    :class:`~wagtail.admin.viewsets.model.ModelViewSet` are available.
+
+    For more information on how to use this class,
+    see :ref:`wagtailsnippets_custom_admin_views`.
     """
 
     #: The model class to be registered as a snippet with this viewset.
@@ -667,9 +672,6 @@ class SnippetViewSet(ModelViewSet):
             self, "menu_item_is_registered", bool(self.menu_hook)
         )
 
-        # This edit handler has been bound to the model and is used for the views.
-        self._edit_handler = self.get_edit_handler()
-
     @cached_property
     def url_prefix(self):
         # SnippetViewSet historically allows overriding the URL prefix via the
@@ -731,14 +733,12 @@ class SnippetViewSet(ModelViewSet):
 
     def get_add_view_kwargs(self, **kwargs):
         return super().get_add_view_kwargs(
-            panel=self._edit_handler,
             preview_url_name=self.get_url_name("preview_on_add"),
             **kwargs,
         )
 
     def get_edit_view_kwargs(self, **kwargs):
         return super().get_edit_view_kwargs(
-            panel=self._edit_handler,
             preview_url_name=self.get_url_name("preview_on_edit"),
             workflow_history_url_name=self.get_url_name("workflow_history"),
             confirm_workflow_cancellation_url_name=self.get_url_name(
@@ -1257,31 +1257,19 @@ class SnippetViewSet(ModelViewSet):
 
     def get_edit_handler(self):
         """
-        Returns the appropriate edit handler for this ``SnippetViewSet`` class.
-        It can be defined either on the model itself or on the ``SnippetViewSet``,
-        as the ``edit_handler`` or ``panels`` properties. Falls back to
-        extracting panel / edit handler definitions from the model class.
+        Like :meth:`ModelViewSet.get_edit_handler()
+        <wagtail.admin.viewsets.model.ModelViewSet.get_edit_handler>`,
+        but falls back to extracting panel definitions from the model class
+        if no edit handler is defined.
         """
-        if hasattr(self, "edit_handler"):
-            edit_handler = self.edit_handler
-        elif hasattr(self, "panels"):
-            panels = self.panels
-            edit_handler = ObjectList(panels)
-        elif hasattr(self.model, "edit_handler"):
-            edit_handler = self.model.edit_handler
-        elif hasattr(self.model, "panels"):
-            panels = self.model.panels
-            edit_handler = ObjectList(panels)
-        else:
-            exclude = self.get_exclude_form_fields()
-            panels = extract_panel_definitions_from_model_class(
-                self.model, exclude=exclude
-            )
-            edit_handler = ObjectList(panels)
-        return edit_handler.bind_to_model(self.model)
+        edit_handler = super().get_edit_handler()
+        if edit_handler:
+            return edit_handler
 
-    def get_form_class(self, for_update=False):
-        return self._edit_handler.get_form_class()
+        exclude = self.get_exclude_form_fields()
+        panels = extract_panel_definitions_from_model_class(self.model, exclude=exclude)
+        edit_handler = ObjectList(panels)
+        return edit_handler.bind_to_model(self.model)
 
     def register_chooser_viewset(self):
         viewsets.register(self.chooser_viewset)

wagtail/test/testapp/views.py

@@ -11,6 +11,7 @@ from django.utils.translation import gettext_lazy
 from wagtail.admin import messages
 from wagtail.admin.auth import user_passes_test
 from wagtail.admin.filters import WagtailFilterSet
+from wagtail.admin.panels import FieldPanel
 from wagtail.admin.ui.tables import BooleanColumn, UpdatedAtColumn
 from wagtail.admin.views.generic import DeleteView, EditView, IndexView
 from wagtail.admin.viewsets.base import ViewSet, ViewSetGroup
@@ -206,7 +207,6 @@ class FeatureCompleteToyViewSet(ModelViewSet):
     url_prefix = "feature-complete-toy"
     menu_label = "Feature Complete Toys"
     icon = "media"
-    exclude_form_fields = ()
     template_prefix = "customprefix/"
     index_template_name = "tests/fctoy_index.html"
     list_display = ["name", BooleanColumn("is_cool"), UpdatedAtColumn()]
@@ -220,6 +220,11 @@ class FeatureCompleteToyViewSet(ModelViewSet):
     inspect_view_enabled = True
     inspect_view_fields = ["strid", "release_date"]
 
+    panels = [
+        FieldPanel("name"),
+        FieldPanel("release_date"),
+    ]
+
 
 class FCToyAlt1ViewSet(ModelViewSet):
     model = FeatureCompleteToy