Anim: add operator for creating a new Action Slot to the dope sheet header

Add new operator `anim.slot_new_for_object`, which creates a new action
slot for the active object. Since this slot is created on the action
used by the object, it requires that this action exists and is a
layered/slotted action (which by definition includes empty actions).

Pull Request: https://projects.blender.org/blender/blender/pulls/126934

scripts/startup/bl_ui/space_dopesheet.py

@@ -253,7 +253,7 @@ class DOPESHEET_HT_editor_buttons:
                         layout.template_search(
                             adt, "action_slot",
                             adt, "action_slots",
-                            new="",
+                            new="anim.slot_new_for_object",
                             unlink="anim.slot_unassign_object",
                         )
 

source/blender/editors/animation/anim_ops.cc

@@ -718,6 +718,58 @@ static void ANIM_OT_scene_range_frame(wmOperatorType *ot)
 /** \name Slots
  * \{ */
 
+static bool slot_new_for_object_poll(bContext *C)
+{
+  using namespace blender;
+
+  Object *object = CTX_data_active_object(C);
+  if (!object) {
+    return false;
+  }
+  animrig::Action *action = animrig::get_action(object->id);
+  if (!action) {
+    CTX_wm_operator_poll_msg_set(
+        C, "Creating a new Action Slot is only possible when an Action is already assigned");
+    return false;
+  }
+  return action->is_action_layered();
+}
+
+static int slot_new_for_object_exec(bContext *C, wmOperator * /*op*/)
+{
+  using namespace blender;
+
+  Object *object = CTX_data_active_object(C);
+  animrig::Action *action = animrig::get_action(object->id);
+  BLI_assert_msg(action, "The poll function should have ensured the Action is not NULL");
+
+  animrig::Slot &slot = action->slot_add_for_id(object->id);
+  { /* Assign the newly created slot. */
+    const bool ok = action->assign_id(&slot, object->id);
+    BLI_assert_msg(ok, "Assigning a slot that was made for this ID should always work");
+    UNUSED_VARS_NDEBUG(ok);
+  }
+
+  DEG_relations_tag_update(CTX_data_main(C));
+  WM_event_add_notifier(C, NC_ANIMATION | ND_ANIMCHAN, nullptr);
+  return OPERATOR_FINISHED;
+}
+
+static void ANIM_OT_slot_new_for_object(wmOperatorType *ot)
+{
+  /* identifiers */
+  ot->name = "New Slot";
+  ot->idname = "ANIM_OT_slot_new_for_object";
+  ot->description = "Create a new Slot for this object, on the Action already assigned to it";
+
+  /* api callbacks */
+  ot->exec = slot_new_for_object_exec;
+  ot->poll = slot_new_for_object_poll;
+
+  /* flags */
+  ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;
+}
+
 static bool slot_unassign_object_poll(bContext *C)
 {
   Object *object = CTX_data_active_object(C);
@@ -873,6 +925,7 @@ void ED_operatortypes_anim()
 
   WM_operatortype_append(ANIM_OT_keying_set_active_set);
 
+  WM_operatortype_append(ANIM_OT_slot_new_for_object);
   WM_operatortype_append(ANIM_OT_slot_unassign_object);
   WM_operatortype_append(ANIM_OT_convert_legacy_action);
 }