form: use subclass of BoundFormField that overrides is_in_error

Instead of using isinstance(self.widget, SubFormWidget) in
BoundFormField.is_in_error, we now implement a BoundSubFormField
class that inherits from BoundFormField. It is meant to be instantianted
for subforms fields only.

This class overrides is_in_error() to perform the additional checks in
the forms' sub-fields.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

subiquitycore/ui/form.py

@@ -111,27 +111,6 @@ class _Validator(WidgetWrap):
         self.field.validate()
 
 
-class FormField(abc.ABC):
-
-    next_index = 0
-    takes_default_style = True
-    caption_first = True
-
-    def __init__(self, caption=None, help=None):
-        self.caption = caption
-        self.help = help
-        self.index = FormField.next_index
-        FormField.next_index += 1
-
-    @abc.abstractmethod
-    def _make_widget(self, form):
-        pass
-
-    def bind(self, form):
-        widget = self._make_widget(form)
-        return BoundFormField(self, form, widget)
-
-
 class WantsToKnowFormField(object):
     """A marker class."""
     def set_bound_form_field(self, bff):
@@ -161,16 +140,8 @@ class BoundFormField(object):
             widget.set_bound_form_field(self)
 
     def is_in_error(self) -> bool:
-        """ Tells whether this field is in error. We will also check if any of
-        the enabled subform reports an error.
-        """
-        if self.in_error:
-            return True
-
-        if not self._enabled or not isinstance(self.widget, SubFormWidget):
-            return False
-
-        return self.widget.form.has_validation_error()
+        """ Tells whether this field is in error. """
+        return self.in_error
 
     def _build_table(self):
         widget = self.widget
@@ -310,6 +281,42 @@ class BoundFormField(object):
             row.enabled = val
 
 
+class BoundSubFormField(BoundFormField):
+    def is_in_error(self):
+        """ Tells whether this field is in error. We will also check if the
+        subform (if enabled) reports an error.
+        """
+        if super().is_in_error():
+            return True
+
+        if not self._enabled:
+            return False
+
+        return self.widget.form.has_validation_error()
+
+
+class FormField(abc.ABC):
+
+    next_index = 0
+    takes_default_style = True
+    caption_first = True
+    bound_field_class = BoundFormField
+
+    def __init__(self, caption=None, help=None):
+        self.caption = caption
+        self.help = help
+        self.index = FormField.next_index
+        FormField.next_index += 1
+
+    @abc.abstractmethod
+    def _make_widget(self, form):
+        pass
+
+    def bind(self, form):
+        widget = self._make_widget(form)
+        return self.bound_field_class(self, form, widget)
+
+
 def simple_field(widget_maker):
     class Field(FormField):
         def _make_widget(self, form):
@@ -517,7 +524,7 @@ class Form(object, metaclass=MetaForm):
             narrow_rows=narrow_rows)
 
     def has_validation_error(self) -> bool:
-        """ Tells if any field (or subformfield) is in error. """
+        """ Tells if any field is in error. """
         return any(map(lambda f: f.is_in_error(), self._fields))
 
     def validated(self):
@@ -554,6 +561,7 @@ class SubFormWidget(WidgetWrap):
 class SubFormField(FormField):
 
     takes_default_style = False
+    bound_field_class = BoundSubFormField
 
     def __init__(self, form_cls, caption=None, help=None):
         super().__init__(caption=caption, help=help)