Allow setting custom scene root for behavior trees

- Adds new argument to `BehaviorTree.instantiate()`
- Adds `BTPlayer.set_scene_root_hint()` method

bt/behavior_tree.cpp

@@ -77,13 +77,13 @@ void BehaviorTree::copy_other(const Ref<BehaviorTree> &p_other) {
 	root_task = p_other->get_root_task();
 }
 
-Ref<BTInstance> BehaviorTree::instantiate(Node *p_agent, const Ref<Blackboard> &p_blackboard, Node *p_instance_owner) const {
+Ref<BTInstance> BehaviorTree::instantiate(Node *p_agent, const Ref<Blackboard> &p_blackboard, Node *p_instance_owner, Node *p_custom_scene_root) const {
 	ERR_FAIL_COND_V_MSG(root_task == nullptr, nullptr, "BehaviorTree: Instantiation failed - BT has no valid root task.");
 	ERR_FAIL_NULL_V_MSG(p_agent, nullptr, "BehaviorTree: Instantiation failed - agent can't be null.");
 	ERR_FAIL_NULL_V_MSG(p_instance_owner, nullptr, "BehaviorTree: Instantiation failed -- instance owner can't be null.");
 	ERR_FAIL_NULL_V_MSG(p_blackboard, nullptr, "BehaviorTree: Instantiation failed - blackboard can't be null.");
-	Node *scene_root = p_instance_owner->get_owner();
+	Node *scene_root = p_custom_scene_root ? p_custom_scene_root : p_instance_owner->get_owner();
-	ERR_FAIL_NULL_V_MSG(scene_root, nullptr, "BehaviorTree: Instantiation failed - can't get scene root, because instance_owner not owned by a scene node. Hint: Try my_player.set_owner(get_owner()).");
+	ERR_FAIL_NULL_V_MSG(scene_root, nullptr, "BehaviorTree: Instantiation failed - unable to establish scene root. This is likely due to the instance owner not being owned by a scene node and custom_scene_root being null.");
 	Ref<BTTask> root_copy = root_task->clone();
 	root_copy->initialize(p_agent, p_blackboard, scene_root);
 	return BTInstance::create(root_copy, get_path(), p_instance_owner);
@@ -119,7 +119,7 @@ void BehaviorTree::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("get_root_task"), &BehaviorTree::get_root_task);
 	ClassDB::bind_method(D_METHOD("clone"), &BehaviorTree::clone);
 	ClassDB::bind_method(D_METHOD("copy_other", "other"), &BehaviorTree::copy_other);
-	ClassDB::bind_method(D_METHOD("instantiate", "agent", "blackboard", "instance_owner"), &BehaviorTree::instantiate);
+	ClassDB::bind_method(D_METHOD("instantiate", "agent", "blackboard", "instance_owner", "custom_scene_root"), &BehaviorTree::instantiate, DEFVAL(Variant()));
 
 	ADD_PROPERTY(PropertyInfo(Variant::STRING, "description", PROPERTY_HINT_MULTILINE_TEXT), "set_description", "get_description");
 	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "blackboard_plan", PROPERTY_HINT_RESOURCE_TYPE, "BlackboardPlan", PROPERTY_USAGE_DEFAULT | PROPERTY_USAGE_EDITOR_INSTANTIATE_OBJECT), "set_blackboard_plan", "get_blackboard_plan");

bt/behavior_tree.h

@@ -59,7 +59,7 @@ public:
 
 	Ref<BehaviorTree> clone() const;
 	void copy_other(const Ref<BehaviorTree> &p_other);
-	Ref<BTInstance> instantiate(Node *p_agent, const Ref<Blackboard> &p_blackboard, Node *p_instance_owner) const;
+	Ref<BTInstance> instantiate(Node *p_agent, const Ref<Blackboard> &p_blackboard, Node *p_instance_owner, Node *p_custom_scene_root = nullptr) const;
 
 	BehaviorTree();
 	~BehaviorTree();

bt/bt_player.cpp

@@ -48,8 +48,9 @@ void BTPlayer::_load_tree() {
 	ERR_FAIL_COND_MSG(!behavior_tree->get_root_task().is_valid(), "BTPlayer: Initialization failed - behavior tree has no valid root task.");
 	Node *agent = GET_NODE(this, agent_node);
 	ERR_FAIL_NULL_MSG(agent, vformat("BTPlayer: Initialization failed - can't get agent with path '%s'.", agent_node));
-	Node *scene_root = get_owner();
+	Node *scene_root = scene_root_hint ? scene_root_hint : get_owner();
-	ERR_FAIL_NULL_MSG(scene_root, "BTPlayer: Initialization failed - can't get scene root (make sure the BTPlayer's owner property is set).");
+	ERR_FAIL_COND_MSG(scene_root == nullptr,
+			"BTPlayer: Initialization failed - unable to establish scene root. This is likely due to BTPlayer not being owned by a scene node. Check BTPlayer.set_scene_root_hint().");
 	bt_instance = behavior_tree->instantiate(agent, blackboard, this);
 	ERR_FAIL_COND_MSG(bt_instance.is_null(), "BTPlayer: Failed to instantiate behavior tree.");
 #ifdef DEBUG_ENABLED
@@ -81,6 +82,15 @@ void BTPlayer::set_bt_instance(const Ref<BTInstance> &p_bt_instance) {
 	behavior_tree.unref();
 }
 
+void BTPlayer::set_scene_root_hint(Node *p_scene_root) {
+	ERR_FAIL_NULL_MSG(p_scene_root, "BTPlayer: Failed to set scene root hint - scene root is null.");
+	if (bt_instance.is_valid()) {
+		ERR_PRINT("BTPlayer: Scene root hint shouldn't be set after the behavior tree is instantiated. This change will not affect the current behavior tree instance.");
+	}
+
+	scene_root_hint = p_scene_root;
+}
+
 void BTPlayer::set_behavior_tree(const Ref<BehaviorTree> &p_tree) {
 	if (Engine::get_singleton()->is_editor_hint()) {
 		if (behavior_tree.is_valid() && behavior_tree->is_connected(LW_NAME(plan_changed), callable_mp(this, &BTPlayer::_update_blackboard_plan))) {
@@ -231,6 +241,8 @@ void BTPlayer::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("get_bt_instance"), &BTPlayer::get_bt_instance);
 	ClassDB::bind_method(D_METHOD("set_bt_instance", "bt_instance"), &BTPlayer::set_bt_instance);
 
+	ClassDB::bind_method(D_METHOD("set_scene_root_hint", "scene_root"), &BTPlayer::set_scene_root_hint);
+
 	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "behavior_tree", PROPERTY_HINT_RESOURCE_TYPE, "BehaviorTree"), "set_behavior_tree", "get_behavior_tree");
 	ADD_PROPERTY(PropertyInfo(Variant::NODE_PATH, "agent_node"), "set_agent_node", "get_agent_node");
 	ADD_PROPERTY(PropertyInfo(Variant::INT, "update_mode", PROPERTY_HINT_ENUM, "Idle,Physics,Manual"), "set_update_mode", "get_update_mode");

bt/bt_player.h

@@ -43,6 +43,7 @@ private:
 	UpdateMode update_mode = UpdateMode::PHYSICS;
 	bool active = true;
 	Ref<Blackboard> blackboard;
+	Node *scene_root_hint;
 
 	Ref<BTInstance> bt_instance;
 
@@ -79,6 +80,8 @@ public:
 	Ref<BTInstance> get_bt_instance() { return bt_instance; }
 	void set_bt_instance(const Ref<BTInstance> &p_bt_instance);
 
+	void set_scene_root_hint(Node *p_scene_root);
+
 	BTPlayer();
 	~BTPlayer();
 

doc/source/classes/class_behaviortree.rst

@@ -55,17 +55,17 @@ Methods
 .. table::
    :widths: auto
 
-   +-----------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+   +-----------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | :ref:`BehaviorTree<class_BehaviorTree>` | :ref:`clone<class_BehaviorTree_method_clone>`\ (\ ) |const|                                                                                                                                                 |
-   +-----------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+   +-----------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | |void|                                  | :ref:`copy_other<class_BehaviorTree_method_copy_other>`\ (\ other\: :ref:`BehaviorTree<class_BehaviorTree>`\ )                                                                                              |
-   +-----------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+   +-----------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | :ref:`BTTask<class_BTTask>`             | :ref:`get_root_task<class_BehaviorTree_method_get_root_task>`\ (\ ) |const|                                                                                                                                 |
-   +-----------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+   +-----------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
-   | :ref:`BTInstance<class_BTInstance>`     | :ref:`instantiate<class_BehaviorTree_method_instantiate>`\ (\ agent\: ``Node``, blackboard\: :ref:`Blackboard<class_Blackboard>`, instance_owner\: ``Node``\ ) |const| |
+   | :ref:`BTInstance<class_BTInstance>`     | :ref:`instantiate<class_BehaviorTree_method_instantiate>`\ (\ agent\: ``Node``, blackboard\: :ref:`Blackboard<class_Blackboard>`, instance_owner\: ``Node``, custom_scene_root\: ``Node`` = null\ ) |const| |
-   +-----------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+   +-----------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | |void|                                  | :ref:`set_root_task<class_BehaviorTree_method_set_root_task>`\ (\ task\: :ref:`BTTask<class_BTTask>`\ )                                                                                                     |
-   +-----------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+   +-----------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
 
 .. rst-class:: classref-section-separator
 
@@ -172,10 +172,12 @@ Returns the root task of the BehaviorTree resource.
 
 .. rst-class:: classref-method
 
-:ref:`BTInstance<class_BTInstance>` **instantiate**\ (\ agent\: ``Node``, blackboard\: :ref:`Blackboard<class_Blackboard>`, instance_owner\: ``Node``\ ) |const| :ref:`🔗<class_BehaviorTree_method_instantiate>`
+:ref:`BTInstance<class_BTInstance>` **instantiate**\ (\ agent\: ``Node``, blackboard\: :ref:`Blackboard<class_Blackboard>`, instance_owner\: ``Node``, custom_scene_root\: ``Node`` = null\ ) |const| :ref:`🔗<class_BehaviorTree_method_instantiate>`
 
 Instantiates the behavior tree and returns :ref:`BTInstance<class_BTInstance>`. ``instance_owner`` should be the scene node that will own the behavior tree instance. This is typically a :ref:`BTPlayer<class_BTPlayer>`, :ref:`BTState<class_BTState>`, or a custom player node that controls the behavior tree execution. Make sure to pass a :ref:`Blackboard<class_Blackboard>` with values populated from :ref:`blackboard_plan<class_BehaviorTree_property_blackboard_plan>`. See also ``BlackboardPlan.populate_blackboard`` & ``BlackboardPlan.create_blackboard``.
 
+If ``custom_scene_root`` is not ``null``, it will be used as the scene root for the newly instantiated behavior tree; otherwise, the scene root will be set to ``instance_owner.owner``. Scene root is essential for :ref:`BBNode<class_BBNode>` instances to work properly.
+
 .. rst-class:: classref-item-separator
 
 ----

doc/source/classes/class_btplayer.rst

@@ -62,6 +62,8 @@ Methods
    +-------------------------------------+------------------------------------------------------------------------------------------------------------------------+
    | |void|                              | :ref:`set_bt_instance<class_BTPlayer_method_set_bt_instance>`\ (\ bt_instance\: :ref:`BTInstance<class_BTInstance>`\ ) |
    +-------------------------------------+------------------------------------------------------------------------------------------------------------------------+
+   | |void|                              | :ref:`set_scene_root_hint<class_BTPlayer_method_set_scene_root_hint>`\ (\ scene_root\: ``Node``\ )                     |
+   +-------------------------------------+------------------------------------------------------------------------------------------------------------------------+
    | |void|                              | :ref:`update<class_BTPlayer_method_update>`\ (\ delta\: ``float``\ )                                                   |
    +-------------------------------------+------------------------------------------------------------------------------------------------------------------------+
 
@@ -303,6 +305,18 @@ Sets the :ref:`BTInstance<class_BTInstance>` to play. This method is useful when
 
 ----
 
+.. _class_BTPlayer_method_set_scene_root_hint:
+
+.. rst-class:: classref-method
+
+|void| **set_scene_root_hint**\ (\ scene_root\: ``Node``\ ) :ref:`🔗<class_BTPlayer_method_set_scene_root_hint>`
+
+Sets the ``Node`` that will be used as the scene root for the newly instantiated behavior tree. Should be called before the **BTPlayer** is added to the scene tree (before ``NOTIFICATION_READY``). This is typically useful when creating **BTPlayer** nodes dynamically from code.
+
+.. rst-class:: classref-item-separator
+
+----
+
 .. _class_BTPlayer_method_update:
 
 .. rst-class:: classref-method

doc_classes/BTPlayer.xml

@@ -29,6 +29,13 @@
 				Sets the [BTInstance] to play. This method is useful when you want to switch to a different behavior tree instance at runtime. See also [member BehaviorTree.instantiate].
 			</description>
 		</method>
+		<method name="set_scene_root_hint">
+			<return type="void" />
+			<param index="0" name="scene_root" type="Node" />
+			<description>
+				Sets the [Node] that will be used as the scene root for the newly instantiated behavior tree. Should be called before the [BTPlayer] is added to the scene tree (before [code]NOTIFICATION_READY[/code]). This is typically useful when creating [BTPlayer] nodes dynamically from code.
+			</description>
+		</method>
 		<method name="update">
 			<return type="void" />
 			<param index="0" name="delta" type="float" />

doc_classes/BehaviorTree.xml

@@ -39,8 +39,10 @@
 			<param index="0" name="agent" type="Node" />
 			<param index="1" name="blackboard" type="Blackboard" />
 			<param index="2" name="instance_owner" type="Node" />
+			<param index="3" name="custom_scene_root" type="Node" default="null" />
 			<description>
 				Instantiates the behavior tree and returns [BTInstance]. [param instance_owner] should be the scene node that will own the behavior tree instance. This is typically a [BTPlayer], [BTState], or a custom player node that controls the behavior tree execution. Make sure to pass a [Blackboard] with values populated from [member blackboard_plan]. See also [BlackboardPlan.populate_blackboard] &amp; [BlackboardPlan.create_blackboard].
+				If [param custom_scene_root] is not [code]null[/code], it will be used as the scene root for the newly instantiated behavior tree; otherwise, the scene root will be set to [code]instance_owner.owner[/code]. Scene root is essential for [BBNode] instances to work properly.
 			</description>
 		</method>
 		<method name="set_root_task">