Cleanup: Add & use enum value for ID Outliner element type

Code to check if the Outliner tree-element type was the general ID one
would always check against "0" (explicity or even implicitly). For
somebody unfamiliar with the code this is very confusing. Instead the
value should be given a name, e.g. through an enum.

Adds `TSE_SOME_ID` as the "default" ID tree-element type. Other types
may still represent IDs, as I explained in a comment at the definition.

There may also still be cases where the type is checked against "0". I
noted in the comment that such cases should be cleaned up if found.

5 files changed, 17 insertions, 10 deletions

diff --git a/source/blender/editors/space_outliner/tree/tree_display_libraries.cc b/source/blender/editors/space_outliner/tree/tree_display_libraries.cc
index a81ce11498a..91b690d35fa 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_libraries.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_libraries.cc
@@ -144,7 +144,7 @@ TreeElement *TreeDisplayLibraries::add_library_contents(Main &mainvar,
       if (!tenlib) {
         /* Create library tree element on demand, depending if there are any data-blocks. */
         if (lib) {
-          tenlib = outliner_add_element(&space_outliner_, &lb, lib, nullptr, 0, 0);
+          tenlib = outliner_add_element(&space_outliner_, &lb, lib, nullptr, TSE_SOME_ID, 0);
         }
         else {
           tenlib = outliner_add_element(&space_outliner_, &lb, &mainvar, nullptr, TSE_ID_BASE, 0);
@@ -168,7 +168,7 @@ TreeElement *TreeDisplayLibraries::add_library_contents(Main &mainvar,
 
         for (ID *id : List<ID>(lbarray[a])) {
           if (library_id_filter_poll(lib, id)) {
-            outliner_add_element(&space_outliner_, &ten->subtree, id, ten, 0, 0);
+            outliner_add_element(&space_outliner_, &ten->subtree, id, ten, TSE_SOME_ID, 0);
           }
         }
       }
diff --git a/source/blender/editors/space_outliner/tree/tree_display_orphaned.cc b/source/blender/editors/space_outliner/tree/tree_display_orphaned.cc
index 559cb289f3f..69ccf014642 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_orphaned.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_orphaned.cc
@@ -76,7 +76,8 @@ ListBase TreeDisplayIDOrphans::buildTree(const TreeSourceData &source_data)
     /* Add the orphaned data-blocks - these will not be added with any subtrees attached. */
     for (ID *id : List<ID>(lbarray[a])) {
       if (ID_REAL_USERS(id) <= 0) {
-        outliner_add_element(&space_outliner_, (te) ? &te->subtree : &tree, id, te, 0, 0);
+        outliner_add_element(
+            &space_outliner_, (te) ? &te->subtree : &tree, id, te, TSE_SOME_ID, 0);
       }
     }
   }
diff --git a/source/blender/editors/space_outliner/tree/tree_display_scenes.cc b/source/blender/editors/space_outliner/tree/tree_display_scenes.cc
index f377512d81e..390f81cfcd1 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_scenes.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_scenes.cc
@@ -46,7 +46,8 @@ ListBase TreeDisplayScenes::buildTree(const TreeSourceData &source_data)
 
   for (ID *id : List<ID>(source_data.bmain->scenes)) {
     Scene *scene = reinterpret_cast<Scene *>(id);
-    TreeElement *te = outliner_add_element(&space_outliner_, &tree, scene, nullptr, 0, 0);
+    TreeElement *te = outliner_add_element(
+        &space_outliner_, &tree, scene, nullptr, TSE_SOME_ID, 0);
     TreeStoreElem *tselem = TREESTORE(te);
 
     /* New scene elements open by default */
diff --git a/source/blender/editors/space_outliner/tree/tree_display_view_layer.cc b/source/blender/editors/space_outliner/tree/tree_display_view_layer.cc
index a0ebac5f451..89c9960a24f 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_view_layer.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_view_layer.cc
@@ -80,7 +80,7 @@ ListBase TreeDisplayViewLayer::buildTree(const TreeSourceData &source_data)
     /* Show objects in the view layer. */
     for (Base *base : List<Base>(view_layer_->object_bases)) {
       TreeElement *te_object = outliner_add_element(
-          &space_outliner_, &tree, base->object, nullptr, 0, 0);
+          &space_outliner_, &tree, base->object, nullptr, TSE_SOME_ID, 0);
       te_object->directdata = base;
     }
 
@@ -158,7 +158,7 @@ void TreeDisplayViewLayer::add_layer_collection_objects(ListBase &tree,
   for (CollectionObject *cob : List<CollectionObject>(lc.collection->gobject)) {
     Base *base = BKE_view_layer_base_find(view_layer_, cob->ob);
     TreeElement *te_object = outliner_add_element(
-        &space_outliner_, &tree, base->object, &ten, 0, 0);
+        &space_outliner_, &tree, base->object, &ten, TSE_SOME_ID, 0);
     te_object->directdata = base;
   }
 }
@@ -203,7 +203,7 @@ void ObjectsChildrenBuilder::object_tree_elements_lookup_create_recursive(TreeEl
       continue;
     }
 
-    if (tselem->type == 0 && te->idcode == ID_OB) {
+    if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
       Object *ob = (Object *)tselem->id;
       /* Lookup children or add new, empty children vector. */
       Vector<TreeElement *> &tree_elements = object_tree_elements_map_.lookup_or_add(ob, {});
@@ -261,8 +261,12 @@ void ObjectsChildrenBuilder::make_object_parent_hierarchy_collections()
       if (!found) {
         /* We add the child in the tree even if it is not in the collection.
          * We deliberately clear its sub-tree though, to make it less prominent. */
-        TreeElement *child_ob_tree_element = outliner_add_element(
-            &outliner_, &parent_ob_tree_element->subtree, child, parent_ob_tree_element, 0, 0);
+        TreeElement *child_ob_tree_element = outliner_add_element(&outliner_,
+                                                                  &parent_ob_tree_element->subtree,
+                                                                  child,
+                                                                  parent_ob_tree_element,
+                                                                  TSE_SOME_ID,
+                                                                  0);
         outliner_free_tree(&child_ob_tree_element->subtree);
         child_ob_tree_element->flag |= TE_CHILD_NOT_IN_COLLECTION;
         child_ob_tree_elements.append(child_ob_tree_element);
diff --git a/source/blender/editors/space_outliner/tree/tree_element_anim_data.cc b/source/blender/editors/space_outliner/tree/tree_element_anim_data.cc
index 13a25800800..5a9568ea906 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_anim_data.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element_anim_data.cc
@@ -44,7 +44,8 @@ TreeElementAnimData::TreeElementAnimData(TreeElement &legacy_te, ID &id)
 void TreeElementAnimData::expand(SpaceOutliner &space_outliner) const
 {
   /* Animation data-block itself. */
-  outliner_add_element(&space_outliner, &legacy_te_.subtree, anim_data_.action, &legacy_te_, 0, 0);
+  outliner_add_element(
+      &space_outliner, &legacy_te_.subtree, anim_data_.action, &legacy_te_, TSE_SOME_ID, 0);
 
   expand_drivers(space_outliner);
   expand_NLA_tracks(space_outliner);