Outliner: Refactor how lazy-building of children is done

Makes the lazy-building (where children are only built when the parent
isn't collapsed) more generic, so more display modes can use it. So far
this was hardcoded for the "Data API" display mode.

This will be used to work around a big performance issue with the
Library Overrides Hierachies view in a complex production file, see
following commit.

4 files changed, 25 insertions, 4 deletions

diff --git a/source/blender/editors/space_outliner/tree/tree_display.cc b/source/blender/editors/space_outliner/tree/tree_display.cc
index 6ab497b3fbb..fe4937829d6 100644
--- a/source/blender/editors/space_outliner/tree/tree_display.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display.cc
@@ -50,4 +50,9 @@ bool AbstractTreeDisplay::supportsModeColumn() const
   return false;
 }
 
+bool AbstractTreeDisplay::is_lazy_built() const
+{
+  return false;
+}
+
 }  // namespace blender::ed::outliner
diff --git a/source/blender/editors/space_outliner/tree/tree_display.hh b/source/blender/editors/space_outliner/tree/tree_display.hh
index f8e35655c26..84eafcc3dd6 100644
--- a/source/blender/editors/space_outliner/tree/tree_display.hh
+++ b/source/blender/editors/space_outliner/tree/tree_display.hh
@@ -84,6 +84,15 @@ class AbstractTreeDisplay {
    */
   virtual bool supportsModeColumn() const;
 
+  /**
+   * Some trees may want to skip building children of collapsed parents. This should be done if the
+   * tree type may become very complex, which could cause noticeable slowdowns.
+   * Problem: This doesn't address performance issues while searching, since all elements are
+   * constructed for that. Trees of this type have to be rebuilt for any change to the collapsed
+   * state of any element.
+   */
+  virtual bool is_lazy_built() const;
+
  protected:
   /** All derived classes will need a handle to this, so storing it in the base for convenience. */
   SpaceOutliner &space_outliner_;
@@ -232,6 +241,8 @@ class TreeDisplayDataAPI final : public AbstractTreeDisplay {
   TreeDisplayDataAPI(SpaceOutliner &space_outliner);
 
   ListBase buildTree(const TreeSourceData &source_data) override;
+
+  bool is_lazy_built() const override;
 };
 
 }  // namespace blender::ed::outliner
diff --git a/source/blender/editors/space_outliner/tree/tree_display_data.cc b/source/blender/editors/space_outliner/tree/tree_display_data.cc
index bfeb8ce2bdc..3d9b927fbf1 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_data.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_data.cc
@@ -42,4 +42,9 @@ ListBase TreeDisplayDataAPI::buildTree(const TreeSourceData &source_data)
   return tree;
 }
 
+bool TreeDisplayDataAPI::is_lazy_built() const
+{
+  return true;
+}
+
 }  // namespace blender::ed::outliner
diff --git a/source/blender/editors/space_outliner/tree/tree_element_rna.cc b/source/blender/editors/space_outliner/tree/tree_element_rna.cc
index 6dd5ec84041..9e1f22b49d6 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_rna.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element_rna.cc
@@ -124,7 +124,7 @@ void TreeElementRNAStruct::expand(SpaceOutliner &space_outliner) const
     }
   }
   else if (tot) {
-    legacy_te_.flag |= TE_LAZY_CLOSED;
+    legacy_te_.flag |= TE_PRETEND_HAS_CHILDREN;
   }
 }
 
@@ -172,7 +172,7 @@ void TreeElementRNAProperty::expand(SpaceOutliner &space_outliner) const
             &space_outliner, &legacy_te_.subtree, &pptr, &legacy_te_, TSE_RNA_STRUCT, -1);
       }
       else {
-        legacy_te_.flag |= TE_LAZY_CLOSED;
+        legacy_te_.flag |= TE_PRETEND_HAS_CHILDREN;
       }
     }
   }
@@ -189,7 +189,7 @@ void TreeElementRNAProperty::expand(SpaceOutliner &space_outliner) const
       }
     }
     else if (tot) {
-      legacy_te_.flag |= TE_LAZY_CLOSED;
+      legacy_te_.flag |= TE_PRETEND_HAS_CHILDREN;
     }
   }
   else if (ELEM(proptype, PROP_BOOLEAN, PROP_INT, PROP_FLOAT)) {
@@ -207,7 +207,7 @@ void TreeElementRNAProperty::expand(SpaceOutliner &space_outliner) const
       }
     }
     else if (tot) {
-      legacy_te_.flag |= TE_LAZY_CLOSED;
+      legacy_te_.flag |= TE_PRETEND_HAS_CHILDREN;
     }
   }
 }