Adds onLoopBeforeTick callback to BT::TreeExecutionServer.

behaviortree_ros2/include/behaviortree_ros2/tree_execution_server.hpp

@@ -104,6 +104,21 @@ class TreeExecutionServer
   virtual void registerNodesIntoFactory(BT::BehaviorTreeFactory& factory)
   {}
 
+  /**
+   * @brief onLoopBeforeTick invoked at each loop, before tree.tickOnce().
+   * It can be overridden to avoid ticking the tree without blocking the
+   * rest of the execute loop.
+   *
+   * Consider whether it's better to inject a preTickCallback to the root node
+   * or another node of the behavior tree.
+   *
+   * @return False to skip the tick.
+   */
+  virtual bool onLoopBeforeTick()
+  {
+    return true;
+  }
+
   /**
    * @brief onLoopAfterTick invoked at each loop, after tree.tickOnce().
    * If it returns a valid NodeStatus, the tree will stop and return that status.

behaviortree_ros2/src/tree_execution_server.cpp

@@ -222,21 +222,24 @@ void TreeExecutionServer::execute(
         return;
       }
 
-      // tick the tree once and publish the action feedback
-      status = p_->tree.tickExactlyOnce();
-
-      if(const auto res = onLoopAfterTick(status); res.has_value())
-      {
-        stop_action(res.value(), "Action Server aborted by onLoopAfterTick()");
-        goal_handle->abort(action_result);
-        return;
-      }
-
-      if(const auto res = onLoopFeedback(); res.has_value())
+      if(onLoopBeforeTick())
       {
-        auto feedback = std::make_shared<ExecuteTree::Feedback>();
-        feedback->message = res.value();
-        goal_handle->publish_feedback(feedback);
+        // tick the tree once and publish the action feedback
+        status = p_->tree.tickExactlyOnce();
+
+        if(const auto res = onLoopAfterTick(status); res.has_value())
+        {
+          stop_action(res.value(), "Action Server aborted by onLoopAfterTick()");
+          goal_handle->abort(action_result);
+          return;
+        }
+
+        if(const auto res = onLoopFeedback(); res.has_value())
+        {
+          auto feedback = std::make_shared<ExecuteTree::Feedback>();
+          feedback->message = res.value();
+          goal_handle->publish_feedback(feedback);
+        }
       }
 
       const auto now = std::chrono::steady_clock::now();