Anim: when creating action slot by keying, name it after last-used slot

When a new Action slot is created by keying a property, it is now
named after the last-assigned slot. This is in support of the
following scenario:

- Action `A` is assigned, with slot `Legacy Slot`.
- The slot is renamed to `Main Light`, because that's what being
  animated by it.
- Animator wants to try an alternative animation, and unassigns the
  Action.
- Animator starts keying the light, which creates Action `B` and a
  slot.
- This slot is now also named `Main Light`, independently of the
  actual name of the light being animated.
- Animator can switch between actions `A` and `B`, and because the
  slots have the same name, the auto-assignment Just Works™.

Pull Request: https://projects.blender.org/blender/blender/pulls/131600

source/blender/animrig/ANIM_action.hh

@@ -210,7 +210,11 @@ class Action : public ::bAction {
   Slot &slot_add_for_id_type(ID_Type idtype);
 
   /**
-   * Create a new slot, named after the given ID, and limited to the ID's type.
+   * Create a new slot suitable for the ID's type.
+   *
+   * The slot will be named after `animated_id.adt.last_slot_identifier`, defaulting to the ID's
+   * name when that is not set. This is done so that toggling Actions works transparently, when
+   * toggling between `this` and the Action last assigned to the ID.
    *
    * Note that this assigns neither this Action nor the new Slot to the ID. This function
    * merely initializes the Slot itself to suitable values to start animating this ID.

source/blender/animrig/intern/action.cc

@@ -529,13 +529,28 @@ Slot &Action::slot_add_for_id_type(const ID_Type idtype)
 Slot &Action::slot_add_for_id(const ID &animated_id)
 {
   Slot &slot = this->slot_add();
-
   slot.idtype = GS(animated_id.name);
-  this->slot_identifier_define(slot, animated_id.name);
 
+  /* Determine the identifier for this slot, prioritising transparent
+   * auto-selection when toggling between Actions. That's why the last-used slot
+   * identifier is used here, and the ID name only as fallback. */
+  const AnimData *adt = BKE_animdata_from_id(&animated_id);
+  const StringRefNull last_slot_identifier = adt ? adt->last_slot_identifier : "";
+
+  StringRefNull slot_identifier = last_slot_identifier;
+  if (slot_identifier.is_empty()) {
+    slot_identifier = animated_id.name;
+  }
+
+  this->slot_identifier_define(slot, slot_identifier);
   /* No need to call anim.slot_identifier_propagate() as nothing will be using
    * this brand new Slot yet. */
 
+  /* The last-used slot might have had a different ID type through some quirk (changes to linked
+   * data, for example). So better ensure that the identifier prefix is correct on this new slot,
+   * instead of relying for 100% on the old one. */
+  slot.identifier_ensure_prefix();
+
   return slot;
 }
 

source/blender/animrig/intern/action_test.cc

@@ -302,6 +302,22 @@ TEST_F(ActionLayersTest, add_slot)
     EXPECT_STREQ(cube->id.name, slot.identifier);
     EXPECT_EQ(ID_OB, slot.idtype);
   }
+
+  { /* Creating a Slot for a specific ID that already had a slot assigned before should name it
+     * after that previous slot. This should also ensure that the first two characters are actually
+     * correct for the ID type. */
+    AnimData *adt = BKE_animdata_ensure_id(&cube->id);
+    STRNCPY_UTF8(adt->last_slot_identifier, "$$Kübuš 😹");
+    Slot &slot = action->slot_add_for_id(cube->id);
+    EXPECT_EQ(DNA_DEFAULT_ACTION_LAST_SLOT_HANDLE + 3, action->last_slot_handle);
+    EXPECT_EQ(DNA_DEFAULT_ACTION_LAST_SLOT_HANDLE + 3, slot.handle);
+
+    EXPECT_STREQ("Kübuš 😹", slot.identifier + 2)
+        << "The last-assigned slot name should be reused";
+    EXPECT_STREQ("OBKübuš 😹", slot.identifier)
+        << "The ID type encoded in the slot identifier should be correct";
+    EXPECT_EQ(ID_OB, slot.idtype);
+  }
 }
 
 TEST_F(ActionLayersTest, add_slot__reset_idroot)