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.

2 files changed, 9 insertions, 2 deletions

diff --git a/source/blender/editors/include/UI_interface.h b/source/blender/editors/include/UI_interface.h
index b6e33d6ed0d..12fdda2092c 100644
--- a/source/blender/editors/include/UI_interface.h
+++ b/source/blender/editors/include/UI_interface.h
@@ -2777,7 +2777,9 @@ void UI_interface_tag_script_reload(void);
 
 bool UI_tree_view_item_is_active(const uiTreeViewItemHandle *item);
 bool UI_tree_view_item_matches(const uiTreeViewItemHandle *a, const uiTreeViewItemHandle *b);
-bool UI_tree_view_item_can_drop(const uiTreeViewItemHandle *item_, const struct wmDrag *drag);
+bool UI_tree_view_item_can_drop(const uiTreeViewItemHandle *item_,
+                                const struct wmDrag *drag,
+                                const char **r_disabled_hint);
 char *UI_tree_view_item_drop_tooltip(const uiTreeViewItemHandle *item, const struct wmDrag *drag);
 bool UI_tree_view_item_drop_handle(uiTreeViewItemHandle *item_, const struct ListBase *drags);
 bool UI_tree_view_item_can_rename(const uiTreeViewItemHandle *item_handle);
diff --git a/source/blender/editors/include/UI_tree_view.hh b/source/blender/editors/include/UI_tree_view.hh
index f565c80193f..fb0304060c9 100644
--- a/source/blender/editors/include/UI_tree_view.hh
+++ b/source/blender/editors/include/UI_tree_view.hh
@@ -362,8 +362,13 @@ class AbstractTreeViewItemDropController {
 
   /**
    * Check if the data dragged with \a drag can be dropped on the item this controller is for.
+   * \param r_disabled_hint: Return a static string to display to the user, explaining why dropping
+   *                         isn't possible on this item. Shouldn't be done too aggressively, e.g.
+   *                         don't set this if the drag-type can't be dropped here; only if it can
+   *                         but there's another reason it can't be dropped.
+   *                         Can assume this is a non-null pointer.
    */
-  virtual bool can_drop(const wmDrag &drag) const = 0;
+  virtual bool can_drop(const wmDrag &drag, const char **r_disabled_hint) const = 0;
   /**
    * Custom text to display when dragging over a tree item. Should explain what happens when
    * dropping the data onto this item. Will only be used if #AbstractTreeViewItem::can_drop()