Outliner: Barebones to port IDs to new Outliner tree-element code design

Continuation of work in 2e221de4ceee and 249e4df110e0 .
This prepares things so we can start porting the individual ID types to
the new code design. I already added some code for library IDs, because
they need some special handling during construction, which I didn't want
to break.

The `AbstractTreeElement::isExpandValid()` check can be removed once
types were ported and can be assumed to have a proper `expand()`
implemenation.

Also makes `TreeElementGPencilLayer` `final` which I forgot in
e0442a955bad.

1 files changed, 6 insertions, 1 deletions

diff --git a/source/blender/editors/space_outliner/outliner_tree.c b/source/blender/editors/space_outliner/outliner_tree.c
index 3696ef8dbce..f109efbaa2b 100644
--- a/source/blender/editors/space_outliner/outliner_tree.c
+++ b/source/blender/editors/space_outliner/outliner_tree.c
@@ -992,6 +992,11 @@ TreeElement *outliner_add_element(SpaceOutliner *space_outliner,
   else if (type == TSE_ID_BASE) {
     /* pass */
   }
+  else if (type == TSE_SOME_ID) {
+    if (!te->type) {
+      BLI_assert(!"Expected this ID type to be ported to new Outliner tree-element design");
+    }
+  }
   else {
     /* Other cases must be caught above. */
     BLI_assert(TSE_IS_REAL_ID(tselem));
@@ -1006,7 +1011,7 @@ TreeElement *outliner_add_element(SpaceOutliner *space_outliner,
     te->idcode = GS(id->name);
   }
 
-  if (te->type) {
+  if (te->type && outliner_tree_element_type_is_expand_valid(te->type)) {
     outliner_tree_element_type_expand(te->type, space_outliner);
   }
   else if (type == TSE_SOME_ID) {