Outliner: Port collection elements to new tree-element code design

No user visible changes expected.

Part of #96713, continuation of work started in 249e4df110 and 2e221de4ce.
Refer to these for a motivation and design overview.

Adds a new tree-element class for collection IDs.

Pull Request: https://projects.blender.org/blender/blender/pulls/108943

source/blender/editors/space_outliner/CMakeLists.txt

@@ -57,6 +57,7 @@ set(SRC
   tree/tree_element_driver.cc
   tree/tree_element_gpencil_layer.cc
   tree/tree_element_id.cc
+  tree/tree_element_id_collection.cc
   tree/tree_element_id_curve.cc
   tree/tree_element_id_gpencil_legacy.cc
   tree/tree_element_id_library.cc
@@ -83,6 +84,7 @@ set(SRC
   tree/tree_element_driver.hh
   tree/tree_element_gpencil_layer.hh
   tree/tree_element_id.hh
+  tree/tree_element_id_collection.hh
   tree/tree_element_id_curve.hh
   tree/tree_element_id_gpencil_legacy.hh
   tree/tree_element_id_library.hh

source/blender/editors/space_outliner/outliner_tree.cc

@@ -557,6 +557,7 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
     case ID_TE:
     case ID_LS:
     case ID_GD_LEGACY:
+    case ID_GR:
       BLI_assert_msg(0, "ID type expected to be expanded through new tree-element design");
       break;
     case ID_OB: {
@@ -674,14 +675,6 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
       }
       break;
     }
-    case ID_GR: {
-      /* Don't expand for instances, creates too many elements. */
-      if (!(te->parent && te->parent->idcode == ID_OB)) {
-        Collection *collection = (Collection *)id;
-        outliner_add_collection_recursive(space_outliner, collection, te);
-      }
-      break;
-    }
     case ID_CV: {
       Curves *curves = (Curves *)id;
       if (outliner_animdata_test(curves->adt)) {

source/blender/editors/space_outliner/tree/tree_element_id.cc

@@ -20,6 +20,7 @@
 
 #include "../outliner_intern.hh"
 #include "common.hh"
+#include "tree_element_id_collection.hh"
 #include "tree_element_id_curve.hh"
 #include "tree_element_id_gpencil_legacy.hh"
 #include "tree_element_id_library.hh"
@@ -57,6 +58,8 @@ std::unique_ptr<TreeElementID> TreeElementID::createFromID(TreeElement &legacy_t
       return std::make_unique<TreeElementIDLineStyle>(legacy_te, (FreestyleLineStyle &)id);
     case ID_GD_LEGACY:
       return std::make_unique<TreeElementIDGPLegacy>(legacy_te, (bGPdata &)id);
+    case ID_GR:
+      return std::make_unique<TreeElementIDCollection>(legacy_te, (Collection &)id);
     case ID_OB:
     case ID_MA:
     case ID_LT:
@@ -66,7 +69,6 @@ std::unique_ptr<TreeElementID> TreeElementID::createFromID(TreeElement &legacy_t
     case ID_SCR:
     case ID_WO:
     case ID_SPK:
-    case ID_GR:
     case ID_NT:
     case ID_BR:
     case ID_PA:

source/blender/editors/space_outliner/tree/tree_element_id_collection.cc

@@ -0,0 +1,37 @@
+/* SPDX-FileCopyrightText: 2023 Blender Foundation
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
+
+/** \file
+ * \ingroup spoutliner
+ */
+
+#include "DNA_ID.h"
+#include "DNA_collection_types.h"
+#include "DNA_listBase.h"
+
+#include "../outliner_intern.hh"
+
+#include "tree_element_id_collection.hh"
+
+namespace blender::ed::outliner {
+
+TreeElementIDCollection::TreeElementIDCollection(TreeElement &legacy_te, Collection &collection)
+    : TreeElementID(legacy_te, collection.id), collection_(collection)
+{
+}
+
+bool TreeElementIDCollection::isExpandValid() const
+{
+  return true;
+}
+
+void TreeElementIDCollection::expand(SpaceOutliner &space_outliner) const
+{
+  /* Don't expand for instances, creates too many elements. */
+  if (!(legacy_te_.parent && legacy_te_.parent->idcode == ID_OB)) {
+    outliner_add_collection_recursive(&space_outliner, &collection_, &legacy_te_);
+  }
+}
+
+}  // namespace blender::ed::outliner

source/blender/editors/space_outliner/tree/tree_element_id_collection.hh

@@ -0,0 +1,25 @@
+/* SPDX-FileCopyrightText: 2023 Blender Foundation
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
+
+/** \file
+ * \ingroup spoutliner
+ */
+
+#pragma once
+
+#include "tree_element_id.hh"
+
+namespace blender::ed::outliner {
+
+class TreeElementIDCollection final : public TreeElementID {
+  Collection &collection_;
+
+ public:
+  TreeElementIDCollection(TreeElement &legacy_te, Collection &collection);
+
+  void expand(SpaceOutliner &) const override;
+  bool isExpandValid() const override;
+};
+
+}  // namespace blender::ed::outliner