Rigify: unhide the rig before generation if it is hidden.

Rig generation fails if the target rig that is to be overwritten
is hidden, either directly or via collections. To improve usability,
try unhiding the object, and fail with an appropriate message if
it doesn't work.

In addition, slightly refactor code that selects a suitable
unhidden collection to use for generation.

This started as D11355 by @Mets, but was somewhat refactored.

rigify/generate.py

@@ -65,6 +65,17 @@ class Generator(base_generate.BaseGenerator):
         return rig_module.Rig
 
 
+    def __switch_to_usable_collection(self, obj, fallback=False):
+        collections = filter_layer_collections_by_object(self.usable_collections, obj)
+
+        if collections:
+            self.layer_collection = collections[0]
+        elif fallback:
+            self.layer_collection = self.view_layer.layer_collection
+
+        self.collection = self.layer_collection.collection
+
+
     def __create_rig_object(self):
         scene = self.scene
         id_store = self.id_store
@@ -79,33 +90,36 @@ class Generator(base_generate.BaseGenerator):
 
         obj = None
 
+        # Try existing object if overwriting
         if meta_data.rigify_generate_mode == 'overwrite':
             obj = meta_data.rigify_target_rig
 
-            if not obj and name in scene.objects:
-                obj = scene.objects[name]
-
             if obj:
                 self.rig_old_name = obj.name
 
                 obj.name = name
                 obj.data.name = obj.name
 
-                rig_collections = filter_layer_collections_by_object(self.usable_collections, obj)
-                self.layer_collection = (rig_collections + [self.layer_collection])[0]
-                self.collection = self.layer_collection.collection
-
             elif name in bpy.data.objects:
                 obj = bpy.data.objects[name]
 
+        # Create a new object if not found
         if not obj:
             obj = bpy.data.objects.new(name, bpy.data.armatures.new(name))
             obj.display_type = 'WIRE'
+
+        # If the object is already added to the scene, switch to its collection
+        if obj.name in self.context.scene.collection.all_objects:
+            self.__switch_to_usable_collection(obj)
+        else:
+            # Otherwise, add to the selected collection or the metarig collection if unusable
+            if (self.layer_collection not in self.usable_collections
+                or self.layer_collection == self.view_layer.layer_collection):
+                self.__switch_to_usable_collection(self.metarig, True)
+
             self.collection.objects.link(obj)
 
-        elif obj.name not in self.collection.objects:  # rig exists but was deleted
-            self.collection.objects.link(obj)
-
+        # Configure and remember the object
         meta_data.rigify_target_rig = obj
         obj.data.pose_position = 'POSE'
 
@@ -113,6 +127,23 @@ class Generator(base_generate.BaseGenerator):
         return obj
 
 
+    def __unhide_rig_object(self, obj):
+        # Ensure the object is visible and selectable
+        obj.hide_set(False, view_layer=self.view_layer)
+        obj.hide_viewport = False
+
+        if not obj.visible_get(view_layer=self.view_layer):
+            raise Exception('Could not generate: Target rig is not visible')
+
+        obj.select_set(True, view_layer=self.view_layer)
+
+        if not obj.select_get(view_layer=self.view_layer):
+            raise Exception('Could not generate: Cannot select target rig')
+
+        if self.layer_collection not in self.usable_collections:
+            raise Exception('Could not generate: Could not find a usable collection.')
+
+
     def __create_widget_group(self):
         new_group_name = "WGTS_" + self.obj.name
         wgts_group_name = "WGTS_" + (self.rig_old_name or self.obj.name)
@@ -373,17 +404,17 @@ class Generator(base_generate.BaseGenerator):
 
         self.usable_collections = list_layer_collections(view_layer.layer_collection, selectable=True)
 
-        if self.layer_collection not in self.usable_collections:
-            metarig_collections = filter_layer_collections_by_object(self.usable_collections, self.metarig)
-            self.layer_collection = (metarig_collections + [view_layer.layer_collection])[0]
-            self.collection = self.layer_collection.collection
-
         bpy.ops.object.mode_set(mode='OBJECT')
 
         #------------------------------------------
         # Create/find the rig object and set it up
         obj = self.__create_rig_object()
 
+        self.__unhide_rig_object(obj)
+
+        # Select the chosen working collection in case it changed
+        self.view_layer.active_layer_collection = self.layer_collection
+
         # Get rid of anim data in case the rig already existed
         print("Clear rig animation data.")