Fix `on_drag_start` handler not getting ID when dragging from Outliner

We would first invoke the dragging, and then set the drag data (like the
ID or the dragged modifier), so the `wmDropBox.on_drag_start()` handler
wouldn't be able to access this. This broke dragging some IDs from the
Outliner, noticed in D15333.

It's now possible to first create/request drag data, extend it, and then
invoke the actual dragging. The normal function to start dragging
returns `void` now instead of `wmDrag *`, so the drag data can't easily
be modified after starting anymore.

1 files changed, 16 insertions, 2 deletions

diff --git a/source/blender/windowmanager/WM_api.h b/source/blender/windowmanager/WM_api.h
index f337dd9d89a..44c5b86857d 100644
--- a/source/blender/windowmanager/WM_api.h
+++ b/source/blender/windowmanager/WM_api.h
@@ -1214,10 +1214,24 @@ int WM_operator_flag_only_pass_through_on_press(int retval, const struct wmEvent
 /* Drag and drop. */
 
 /**
- * Note that the pointer should be valid allocated and not on stack.
+ * Start dragging immediately with the given data.
+ * Note that \a poin should be valid allocated and not on stack.
  */
-struct wmDrag *WM_event_start_drag(
+void WM_event_start_drag(
     struct bContext *C, int icon, int type, void *poin, double value, unsigned int flags);
+/**
+ * Create and fill the dragging data, but don't start dragging just yet (unlike
+ * #WM_event_start_drag()). Must be followed up by #WM_event_start_prepared_drag(), otherwise the
+ * returned pointer will leak memory.
+ *
+ * Note that \a poin should be valid allocated and not on stack.
+ */
+wmDrag *WM_drag_data_create(
+    struct bContext *C, int icon, int type, void *poin, double value, unsigned int flags);
+/**
+ * Invoke dragging using the given \a drag data.
+ */
+void WM_event_start_prepared_drag(struct bContext *C, wmDrag *drag);
 void WM_event_drag_image(struct wmDrag *, struct ImBuf *, float scale);
 void WM_drag_free(struct wmDrag *drag);
 void WM_drag_data_free(int dragtype, void *poin);