Get UI ready for using drag & drop to nest a collection into another one

Nothing happen yet when it's supposed to insert the collection into
another one, that part will be handled by @dfelinto.

See gif for demo of how it works UI wise: {F500337}

Also fixed off-by-one error in utility function.

1 files changed, 9 insertions, 2 deletions

diff --git a/source/blender/editors/space_outliner/outliner_intern.h b/source/blender/editors/space_outliner/outliner_intern.h
index 731b5041bd9..4cee0e44fca 100644
--- a/source/blender/editors/space_outliner/outliner_intern.h
+++ b/source/blender/editors/space_outliner/outliner_intern.h
@@ -49,6 +49,12 @@ struct EditBone;
 struct wmKeyConfig;
 
 
+typedef enum TreeElementInsertType {
+	/* no INSERT_BEFORE needed for now */
+	TE_INSERT_AFTER,
+	TE_INSERT_INTO,
+} TreeElementInsertType;
+
 typedef enum TreeTraversalAction {
 	/* Continue traversal regularly, don't skip children. */
 	TRAVERSE_CONTINUE = 0,
@@ -63,7 +69,7 @@ typedef enum TreeTraversalAction {
  * Passing scene right now, may be better to allow some custom data.
  */
 typedef void (*TreeElementReinsertFunc)(const struct Scene *scene, struct TreeElement *insert_element,
-                                        struct TreeElement *insert_after);
+                                        struct TreeElement *insert_handle, TreeElementInsertType action);
 typedef TreeTraversalAction (*TreeTraversalFunc)(struct TreeElement *te, void *customdata);
 
 
@@ -84,8 +90,9 @@ typedef struct TreeElement {
 	TreeElementReinsertFunc reinsert;
 
 	struct {
+		TreeElementInsertType insert_type;
 		/* the element after which we may insert the dragged one (NULL to insert at top) */
-		struct TreeElement *insert_te;
+		struct TreeElement *insert_handle;
 	} *drag_data;
 } TreeElement;