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.

1 files changed, 10 insertions, 0 deletions

diff --git a/source/blender/makesdna/DNA_outliner_types.h b/source/blender/makesdna/DNA_outliner_types.h
index 16129768b60..7a39e0caef3 100644
--- a/source/blender/makesdna/DNA_outliner_types.h
+++ b/source/blender/makesdna/DNA_outliner_types.h
@@ -73,6 +73,16 @@ enum {
 
 /** #TreeStoreElem.types */
 typedef enum eTreeStoreElemType {
+  /**
+   * If an element is of this type, `TreeStoreElem.id` points to a valid ID and the ID-type can be
+   * received through `TreeElement.idcode` (or `GS(TreeStoreElem.id->name)`). Note however that the
+   * types below may also have a valid ID pointer (see #TSE_IS_REAL_ID()).
+   *
+   * In cases where the type is still checked against "0" (even implicitly), please replace it with
+   * an explicit check against `TSE_SOME_ID`.
+   */
+  TSE_SOME_ID = 0,
+
   TSE_NLA = 1, /* NO ID */
   TSE_NLA_ACTION = 2,
   TSE_DEFGROUP_BASE = 3,