UI: Support easy dropping into/onto rows in new tree-view API

Adds an easy way to add drop support for tree-view rows.

Most of the work is handled by the tree-view UI code. The tree items can
simply override a few functions (`can_drop()`, `on_drop()`,
`drop_tooltip()`) to implement their custom drop behavior.

While dragging over a tree-view item that can be dropped into/onto, the
item can show a custom and dynamic tooltip explaining what's gonna
happen on drop.

This isn't used yet, but will soon be for asset catalogs.

See documentation here:
https://wiki.blender.org/wiki/Source/Interface/Views#Further_Customizations

1 files changed, 10 insertions, 0 deletions

diff --git a/source/blender/editors/interface/interface_query.c b/source/blender/editors/interface/interface_query.c
index 15d1d2f2eec..2f6bda3252d 100644
--- a/source/blender/editors/interface/interface_query.c
+++ b/source/blender/editors/interface/interface_query.c
@@ -463,6 +463,16 @@ uiBut *ui_list_row_find_from_index(const ARegion *region, const int index, uiBut
   return ui_but_find(region, ui_but_is_listrow_at_index, &data);
 }
 
+static bool ui_but_is_treerow(const uiBut *but, const void *UNUSED(customdata))
+{
+  return but->type == UI_BTYPE_TREEROW;
+}
+
+uiBut *ui_tree_row_find_mouse_over(const ARegion *region, const int x, const int y)
+{
+  return ui_but_find_mouse_over_ex(region, x, y, false, ui_but_is_treerow, NULL);
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */