UI: Support disabled-hint for dropping in the tree-view API

A tree-view item's drop controller can now return a message for the user
explaining why dropping isn't possible with the dropped data. This is then
displayed in red text next to the cursor.

This isn't actually used yet, the follow up commit will do that.

1 files changed, 11 insertions, 8 deletions

diff --git a/source/blender/editors/space_file/asset_catalog_tree_view.cc b/source/blender/editors/space_file/asset_catalog_tree_view.cc
index 68df4cc0544..a7762008c47 100644
--- a/source/blender/editors/space_file/asset_catalog_tree_view.cc
+++ b/source/blender/editors/space_file/asset_catalog_tree_view.cc
@@ -106,11 +106,11 @@ class AssetCatalogDropController : public ui::AbstractTreeViewItemDropController
   explicit AssetCatalogDropController(AssetCatalogTreeView &tree_view,
                                       AssetCatalogTreeItem &catalog_item);
 
-  bool can_drop(const wmDrag &drag) const override;
+  bool can_drop(const wmDrag &drag, const char **r_disabled_hint) const override;
   std::string drop_tooltip(const wmDrag &drag) const override;
   bool on_drop(const wmDrag &drag) override;
 
-  static bool has_droppable_item(const wmDrag &drag);
+  static bool has_droppable_item(const wmDrag &drag, const char **r_disabled_hint);
   static bool drop_into_catalog(const AssetCatalogTreeView &tree_view,
                                 const wmDrag &drag,
                                 CatalogID catalog_id,
@@ -131,7 +131,7 @@ class AssetCatalogTreeViewUnassignedItem : public ui::BasicTreeViewItem {
   struct DropController : public ui::AbstractTreeViewItemDropController {
     DropController(AssetCatalogTreeView &tree_view);
 
-    bool can_drop(const wmDrag &drag) const override;
+    bool can_drop(const wmDrag &drag, const char **r_disabled_hint) const override;
     std::string drop_tooltip(const wmDrag &drag) const override;
     bool on_drop(const wmDrag &drag) override;
   };
@@ -320,12 +320,12 @@ AssetCatalogDropController::AssetCatalogDropController(AssetCatalogTreeView &tre
 {
 }
 
-bool AssetCatalogDropController::can_drop(const wmDrag &drag) const
+bool AssetCatalogDropController::can_drop(const wmDrag &drag, const char **r_disabled_hint) const
 {
   if (drag.type != WM_DRAG_ASSET_LIST) {
     return false;
   }
-  return has_droppable_item(drag);
+  return has_droppable_item(drag, r_disabled_hint);
 }
 
 std::string AssetCatalogDropController::drop_tooltip(const wmDrag &drag) const
@@ -377,10 +377,12 @@ bool AssetCatalogDropController::drop_into_catalog(const AssetCatalogTreeView &t
   return true;
 }
 
-bool AssetCatalogDropController::has_droppable_item(const wmDrag &drag)
+bool AssetCatalogDropController::has_droppable_item(const wmDrag &drag,
+                                                    const char **r_disabled_hint)
 {
   const ListBase *asset_drags = WM_drag_asset_list_get(&drag);
 
+  *r_disabled_hint = nullptr;
   /* There needs to be at least one asset from the current file. */
   LISTBASE_FOREACH (const wmDragAssetListItem *, asset_item, asset_drags) {
     if (!asset_item->is_external) {
@@ -421,12 +423,13 @@ AssetCatalogTreeViewUnassignedItem::DropController::DropController(AssetCatalogT
 {
 }
 
-bool AssetCatalogTreeViewUnassignedItem::DropController::can_drop(const wmDrag &drag) const
+bool AssetCatalogTreeViewUnassignedItem::DropController::can_drop(
+    const wmDrag &drag, const char **r_disabled_hint) const
 {
   if (drag.type != WM_DRAG_ASSET_LIST) {
     return false;
   }
-  return AssetCatalogDropController::has_droppable_item(drag);
+  return AssetCatalogDropController::has_droppable_item(drag, r_disabled_hint);
 }
 
 std::string AssetCatalogTreeViewUnassignedItem::DropController::drop_tooltip(