UI: support persistent state during number/slider interaction

Support for begin/update/end callbacks allowing state to be cached
and reused while dragging a number button or slider.

This is done using `UI_block_interaction_set` to set callbacks.

- Dragging multiple buttons at once is supported,
  passing multiple unique events into the update function.

- Update is only called once even when multiple buttons are edited.

- The update callback can detect the difference between click & drag
  actions so situations to support skipping cache creation and
  freeing for situations where it's not beneficial.

Reviewed by: Severin, HooglyBoogly

Ref D11861

1 files changed, 3 insertions, 0 deletions

diff --git a/source/blender/editors/interface/interface_intern.h b/source/blender/editors/interface/interface_intern.h
index a05afb6e542..40a9c67c630 100644
--- a/source/blender/editors/interface/interface_intern.h
+++ b/source/blender/editors/interface/interface_intern.h
@@ -511,6 +511,9 @@ struct uiBlock {
   uiBlockHandleFunc handle_func;
   void *handle_func_arg;
 
+  /** Custom interaction data. */
+  uiBlockInteraction_CallbackData custom_interaction_callbacks;
+
   /** Custom extra event handling. */
   int (*block_event_func)(const struct bContext *C, struct uiBlock *, const struct wmEvent *);