Anim: make RNA property bone_collection.parent writable

Moving a bone collection to another parent is now possible in Python by
assigning to `bone_collection.parent`.

Thanks to Sergey for the implementation.

source/blender/makesrna/intern/rna_armature.cc

@@ -266,6 +266,34 @@ static PointerRNA rna_BoneCollection_parent_get(PointerRNA *ptr)
   return RNA_pointer_create(&arm->id, &RNA_BoneCollection, parent);
 }
 
+static void rna_BoneCollection_parent_set(PointerRNA *ptr,
+                                          PointerRNA value,
+                                          struct ReportList *reports)
+{
+  using namespace blender::animrig;
+
+  BoneCollection *self = (BoneCollection *)ptr->data;
+  BoneCollection *to_parent = (BoneCollection *)value.data;
+
+  bArmature *armature = (bArmature *)ptr->owner_id;
+
+  const int from_bcoll_index = armature_bonecoll_find_index(armature, self);
+  const int from_parent_index = armature_bonecoll_find_parent_index(armature, from_bcoll_index);
+  const int to_parent_index = armature_bonecoll_find_index(armature, to_parent);
+
+  if (to_parent_index == from_bcoll_index ||
+      armature_bonecoll_is_decendent_of(armature, from_bcoll_index, to_parent_index))
+  {
+    BKE_report(reports, RPT_ERROR, "Cannot make a bone collection a decendent of itself");
+    return;
+  }
+
+  armature_bonecoll_move_to_parent(
+      armature, from_bcoll_index, -1, from_parent_index, to_parent_index);
+
+  WM_main_add_notifier(NC_OBJECT | ND_BONE_COLLECTION, nullptr);
+}
+
 static int rna_BoneCollections_active_index_get(PointerRNA *ptr)
 {
   bArmature *arm = (bArmature *)ptr->data;
@@ -2269,8 +2297,9 @@ static void rna_def_bonecollection(BlenderRNA *brna)
 
   prop = RNA_def_property(srna, "parent", PROP_POINTER, PROP_NONE);
   RNA_def_property_struct_type(prop, "BoneCollection");
-  RNA_def_property_pointer_funcs(prop, "rna_BoneCollection_parent_get", nullptr, nullptr, nullptr);
-  RNA_def_property_clear_flag(prop, PROP_EDITABLE);
+  RNA_def_property_flag(prop, PROP_EDITABLE);
+  RNA_def_property_pointer_funcs(
+      prop, "rna_BoneCollection_parent_get", "rna_BoneCollection_parent_set", nullptr, nullptr);
   RNA_def_property_ui_text(prop,
                            "Parent",
                            "Parent bone collection. Note that accessing this requires a scan of "

tests/python/bl_animation_armature.py

@@ -111,6 +111,37 @@ class BoneCollectionTest(unittest.TestCase):
         # Check the array order.
         self.assertEqual([root1, r1_child1, root2, r1_child1_001, r2_child1, r2_child2], list(bcolls.all))
 
+    def test_parent_property(self):
+        # Just to keep the rest of the code shorter.
+        bcolls = self.arm.collections
+
+        self.assertEqual([], list(bcolls), "By default an Armature should have no collections")
+
+        # Build a hierarchy.
+        root1 = bcolls.new('root1')
+        r1_child1 = bcolls.new('r1_child1', parent=root1)
+        r1_child2 = bcolls.new('r1_child2', parent=root1)
+        root2 = bcolls.new('root2')
+        r2_child1 = bcolls.new('r2_child1', parent=root2)
+        r2_child2 = bcolls.new('r2_child2', parent=root2)
+
+        # Check getting the parent.
+        self.assertEqual(root1, r1_child1.parent)
+        self.assertEqual(root1, r1_child2.parent)
+        self.assertEqual(root2, r2_child1.parent)
+        self.assertEqual(root2, r2_child2.parent)
+
+        # Move r1_child1 to be a child of root2 by assigning the parent.
+        r1_child1.parent = root2
+        self.assertEqual(root2, r1_child1.parent)
+
+        # Check the sibling order.
+        self.assertEqual([r2_child1, r2_child2, r1_child1], list(root2.children))
+
+        # Make r1_child1 a root.
+        r1_child1.parent = None
+        self.assertIsNone(r1_child1.parent)
+
     def test_bone_collection_bones(self):
         # Build a hierarchy on the armature.
         bcolls = self.arm.collections