Admin: fix and improve UI for downsample factors

This UI has always been cumbersome, but recently broke entirely for
custom factors do to Django changes. Fix this and simplify the
interaction between field and widget here.

This is deceptively difficult because:
- MultiValueField and MultiWidget assume list (and *sometimes* tuple)
types are already decompressed. Downsample factors are a list of
Integer3D, and have to be so elsewhere for use as an ArrayField.
- SimpleArrayField also makes assumptions about coupling with its
default widget.
- The SimpleArrayFields used are of Integer3DFormField, which is another
MultiValueField/MultiWidget of list type.

See discussion in #1827.

django/applications/catmaid/fields.py

@@ -292,6 +292,10 @@ def formfield(self, **kwargs):
     def planar_default(num_zoom_levels):
         return [Integer3D(2**l, 2**l, 1) for l in range(num_zoom_levels + 1)]
 
+    @staticmethod
+    def is_value(value):
+        return isinstance(value, list) and all([isinstance(l, Integer3D) for l in value])
+
 # ------------------------------------------------------------------------
 
 class Integer3DFormField(forms.MultiValueField):
@@ -355,14 +359,19 @@ def __init__(self, *args, **kwargs):
                 widget=forms.RadioSelect),
             forms.IntegerField(label='Number of zoom levels'),
             SimpleArrayField(
-                Integer3DFormField(),
+                # Must be disabled for Django to decompress str values during `clean`.
+                Integer3DFormField(disabled=True),
                 label='Factors array',
                 delimiter='|',
                 max_length=kwargs['max_length']),
         )
         del kwargs['max_length']
         del kwargs['base_field']
         super(DownsampleFactorsFormField, self).__init__(fields, *args, **kwargs)
+        # Because SimpleArrayField does not strictly adhere to Django conventions,
+        # our widget must have access to its field so that `prepare_value` can
+        # be used to convert the array to a string.
+        self.widget.array_field = self.fields[2]
 
     def compress(self, data_list):
         if data_list:
@@ -375,5 +384,8 @@ def compress(self, data_list):
                 return data_list[2]
         return None
 
-    def prepare_value(self, value):
-        return (value, self.fields[2].prepare_value(value))
+    def clean(self, value):
+        if DownsampleFactorsField.is_value(value):
+            value = self.widget.decompress(value)
+
+        return super().clean(value)

django/applications/catmaid/templates/catmaid/widgets/downsamplefactors.html

@@ -11,7 +11,11 @@
 				<label for="widget.subwidgets.1.attrs.id">Number of zoom levels</label>
 			{% endif %}
 			{% if index == 2 %}
-				{% include widget.subwidgets.1.template_name with widget=widget.subwidgets.2 %}
+				{% include widget.subwidgets.2.template_name with widget=widget.subwidgets.2 %}
+				<label for="widget.subwidgets.2.attrs.id" class="help">
+					Delimit dimension factors by <code>,</code> and scale levels by <code>|</code>.
+					Example: <code>1,1,1|2,2,2|4,4,4</code>
+				</div>
 			{% endif %}
 		</li>
 	{% endfor %}

django/applications/catmaid/widgets.py

@@ -139,16 +139,17 @@ def __init__(self, attrs=None, **kwargs):
                 attrs, **kwargs)
 
     def decompress(self, value):
-        if value is None or value[0] is None:
+        if value is None:
             return [0, None, None]
-        elif value[0] == catmaid.fields.DownsampleFactorsField.planar_default(len(value[0]) - 1):
-            return [1, len(value[0]) - 1, value[1]]
+        elif value == catmaid.fields.DownsampleFactorsField.planar_default(len(value) - 1):
+            return [1, len(value) - 1, self.array_field.prepare_value(value)]
         else:
-            return [2, len(value[0]) - 1, value[1]]
+            return [2, len(value) - 1, self.array_field.prepare_value(value)]
 
     def get_context(self, name, value, attrs):
         # Django doesn't play well with MultiValueFields/MultiWidgets whose
         # normalization type is a list, so will not try to decompress list
         # values by default.
-        value = self.decompress(value)
+        if catmaid.fields.DownsampleFactorsField.is_value(value):
+            value = self.decompress(value)
         return super(DownsampleFactorsWidget, self).get_context(name, value, attrs)