LayerCollections: Add a way to prevent their resync with Collection hierarchy.

This is an easy & safe, yet not-so-nice way to address the
LayerCollections vs. Collections hierarchy resync problem.

Currently this resync is enforced everytime something changes in the
Collections hierarchy, which is extremely inneficient, and can even
produce 'loss' of LayerCollection data during complex Collection
processes.

Current example is during Library Overrides resync process. New code:
 * Makes resync significantly faster (between 10 and 15%).
 * Fixes 'disappearing' layer collections settings on sub-collections'
   layers.

NOTE: This is not a proper fix for the underlying issue. However,
implementing and testing the 'lazy update' solution as proposed by
{T73411} requires a significant amount of time (especially in testing
and tracking all places where code would need to ensure LayerCollections
are up-to-date), which is not possible currently.

Differential Revision: https://developer.blender.org/D11889

1 files changed, 3 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_layer.h b/source/blender/blenkernel/BKE_layer.h
index 404f344919c..c8af1a91725 100644
--- a/source/blender/blenkernel/BKE_layer.h
+++ b/source/blender/blenkernel/BKE_layer.h
@@ -98,6 +98,9 @@ struct LayerCollection *BKE_layer_collection_from_index(struct ViewLayer *view_l
                                                         const int index);
 int BKE_layer_collection_findindex(struct ViewLayer *view_layer, const struct LayerCollection *lc);
 
+void BKE_layer_collection_resync_forbid(void);
+void BKE_layer_collection_resync_allow(void);
+
 void BKE_main_collection_sync(const struct Main *bmain);
 void BKE_scene_collection_sync(const struct Scene *scene);
 void BKE_layer_collection_sync(const struct Scene *scene, struct ViewLayer *view_layer);