Use an extensible registry for comparison classes

wagtail/admin/compare.py

@@ -1,13 +1,18 @@
 import difflib
 
 from bs4 import BeautifulSoup
+from django.core.exceptions import ImproperlyConfigured
+from django.db import models
 from django.utils.encoding import force_str
 from django.utils.html import escape, format_html, format_html_join
 from django.utils.safestring import mark_safe
 from django.utils.text import capfirst
 from django.utils.translation import gettext_lazy as _
+from taggit.managers import TaggableManager
 
 from wagtail.core import blocks
+from wagtail.core.fields import RichTextField
+from wagtail.utils.registry import ModelFieldRegistry
 
 
 def text_from_html(val):
@@ -15,6 +20,32 @@ def text_from_html(val):
     return BeautifulSoup(force_str(val), "html5lib").getText()
 
 
+comparison_class_registry = ModelFieldRegistry()
+
+
+def register_comparison_class(
+    field_class, to=None, comparison_class=None, exact_class=False
+):
+    """
+    Define parameters for form fields to be used by WagtailAdminModelForm for a given
+    database field.
+    """
+
+    if comparison_class is None:
+        raise ImproperlyConfigured(
+            "register_comparison_class must be passed a 'comparison_class' keyword argument"
+        )
+
+    if to and field_class != models.ForeignKey:
+        raise ImproperlyConfigured(
+            "The 'to' argument on register_comparison_class is only valid for ForeignKey fields"
+        )
+
+    comparison_class_registry.register(
+        field_class, to=to, value=comparison_class, exact_class=exact_class
+    )
+
+
 class FieldComparison:
     is_field = True
     is_child_relation = False
@@ -56,6 +87,10 @@ class TextFieldComparison(FieldComparison):
         return diff_text(self.val_a, self.val_b).to_html()
 
 
+register_comparison_class(models.CharField, comparison_class=TextFieldComparison)
+register_comparison_class(models.TextField, comparison_class=TextFieldComparison)
+
+
 class RichTextFieldComparison(TextFieldComparison):
     def htmldiff(self):
         return diff_text(
@@ -63,6 +98,9 @@ class RichTextFieldComparison(TextFieldComparison):
         ).to_html()
 
 
+register_comparison_class(RichTextField, comparison_class=RichTextFieldComparison)
+
+
 def get_comparison_class_for_block(block):
     if hasattr(block, "get_comparison_class"):
         return block.get_comparison_class()
@@ -346,6 +384,9 @@ class TagsFieldComparison(M2MFieldComparison):
         return tag.slug
 
 
+register_comparison_class(TaggableManager, comparison_class=TagsFieldComparison)
+
+
 class ForeignObjectComparison(FieldComparison):
     def get_objects(self):
         model = self.field.related_model

wagtail/admin/edit_handlers.py

@@ -7,7 +7,6 @@ from django.conf import settings
 from django.contrib.auth import get_user_model
 from django.core.exceptions import FieldDoesNotExist, ImproperlyConfigured
 from django.core.signals import setting_changed
-from django.db.models.fields import CharField, TextField
 from django.dispatch import receiver
 from django.forms.formsets import DELETION_FIELD_NAME, ORDERING_FIELD_NAME
 from django.forms.models import fields_for_model
@@ -16,12 +15,10 @@ from django.utils.functional import cached_property
 from django.utils.safestring import mark_safe
 from django.utils.translation import gettext_lazy
 from modelcluster.models import get_serializable_data_for_fields
-from taggit.managers import TaggableManager
 
 from wagtail.admin import compare, widgets
 from wagtail.admin.forms.comments import CommentForm, CommentReplyForm
 from wagtail.admin.templatetags.wagtailadmin_tags import avatar_url, user_display_name
-from wagtail.core.fields import RichTextField
 from wagtail.core.models import COMMENTS_RELATION_NAME, Page
 from wagtail.core.utils import camelcase_to_underscore
 from wagtail.utils.decorators import cached_classmethod
@@ -550,20 +547,16 @@ class FieldPanel(EditHandler):
             if field.choices:
                 return compare.ChoiceFieldComparison
 
+            comparison_class = compare.comparison_class_registry.get(field)
+            if comparison_class:
+                return comparison_class
+
             if field.is_relation:
-                if isinstance(field, TaggableManager):
-                    return compare.TagsFieldComparison
-                elif field.many_to_many:
+                if field.many_to_many:
                     return compare.M2MFieldComparison
 
                 return compare.ForeignObjectComparison
 
-            if isinstance(field, RichTextField):
-                return compare.RichTextFieldComparison
-
-            if isinstance(field, (CharField, TextField)):
-                return compare.TextFieldComparison
-
         except FieldDoesNotExist:
             pass