Undo System: support for grouping steps with begin/end calls

This adds support for treating multiple undo steps as a single step
from the user perspective.

This is needed for outliner mode switching and `object.switch_object`
operator which change active object and mode in a single action.

1 files changed, 45 insertions, 0 deletions

diff --git a/source/blender/blenkernel/intern/undo_system.c b/source/blender/blenkernel/intern/undo_system.c
index 0a0f81f7829..93f70b606dc 100644
--- a/source/blender/blenkernel/intern/undo_system.c
+++ b/source/blender/blenkernel/intern/undo_system.c
@@ -580,6 +580,12 @@ bool BKE_undosys_step_push_with_type(UndoStack *ustack,
     }
   }
 
+  if (ustack->group_level > 0) {
+    /* Temporarily set skip for the active step.
+     * This is an invalid state which must be corrected once the last group ends. */
+    ustack->step_active->skip = true;
+  }
+
   undosys_stack_validate(ustack, true);
   return true;
 }
@@ -849,6 +855,45 @@ void BKE_undosys_type_free_all(void)
 /** \} */
 
 /* -------------------------------------------------------------------- */
+/** \name Undo Stack Grouping
+ *
+ * This enables skip while group-level is set.
+ * In general it's not allowed that #UndoStack.step_active have 'skip' enabled.
+ *
+ * This rule is relaxed for grouping, however it's important each call to
+ * #BKE_undosys_stack_group_begin has a matching #BKE_undosys_stack_group_end.
+ *
+ * - Levels are used so nesting is supported, where the last call to #BKE_undosys_stack_group_end
+ *   will set the active undo step that should not be skipped.
+ *
+ * - Correct begin/end is checked by an assert since any errors here will cause undo
+ *   to consider all steps part of one large group.
+ *
+ * - Calls to begin/end with no undo steps being pushed is supported and does nothing.
+ *
+ * \{ */
+
+void BKE_undosys_stack_group_begin(UndoStack *ustack)
+{
+  BLI_assert(ustack->group_level >= 0);
+  ustack->group_level += 1;
+}
+
+void BKE_undosys_stack_group_end(UndoStack *ustack)
+{
+  ustack->group_level -= 1;
+  BLI_assert(ustack->group_level >= 0);
+
+  if (ustack->group_level == 0) {
+    if (LIKELY(ustack->step_active != NULL)) {
+      ustack->step_active->skip = false;
+    }
+  }
+}
+
+/** \} */
+
+/* -------------------------------------------------------------------- */
 /** \name ID Reference Utilities
  *
  * Unfortunately we need this for a handful of places.