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TreeExecutionServer

This base class is used to implement a Behavior Tree executor wrapped inside a rclcpp_action::Server.

Users are expected to create a derived class to improve its functionalities, but often it can be used out of the box directly.

Further, the terms "load" will be equivalent to "register into the BT::BehaviorTreeFactory".

The TreeExecutionServer offers the following features:

  • Configurable using ROS parameters (see below).
  • Load Behavior Trees definitions (XML files) from a list of folders.
  • Load BT plugins from a list of folders. These plugins may depend or not on ROS.
  • Invoke the execution of a Tree from an external ROS Node, using rclcpp_action::Client.

Furthermore, the user can customize it to:

  • Register custom BT Nodes directly (static linking).
  • Attach additional loggers. The Groot2 publisher will be attached by default.
  • Use the "global blackboard", a new idiom/pattern explained in this tutorial.
  • Customize the feedback of the rclcpp_action::Server.

Customization points

These are the virtual method of TreeExecutionServer that can be overridden by the user.

void onTreeCreated(BT::Tree& tree)

Callback invoked when a tree is created; this happens after rclcpp_action::Server receive a command from a client.

It can be used, for instance, to initialize a logger or the global blackboard.

void registerNodesIntoFactory(BT::BehaviorTreeFactory& factory)

Called at the beginning, after all the plugins have been loaded.

It can be used to register programmatically more BT.CPP Nodes.

std::optionalBT::NodeStatus onLoopAfterTick(BT::NodeStatus status)

Used to do something after the tree was ticked, in its execution loop.

If this method returns something other than std::nullopt, the tree execution is interrupted an the specified BT::NodeStatus is returned to the rclcpp_action::Client.

void onTreeExecutionCompleted(BT::NodeStatus status, bool was_cancelled)

Callback when the tree execution reaches its end.

This happens if:

  1. Ticking the tree returns SUCCESS/FAILURE
  2. The rclcpp_action::Client cancels the action.
  3. Callback onLoopAfterTickcancels the execution.

Argument was_cancelledis true in the 1st case, false otherwise.

std::optionalstd::string onLoopFeedback()

This callback is invoked after tree.tickOnce and onLoopAfterTick.

If it returns something other than std::nullopt, the provided string will be sent as feedback to the rclcpp_action::Client.

ROS Parameters

Documentation for the parameters used by the TreeExecutionServer can be found here.

If the parameter documentation needs updating you can regenerate it with:

generate_parameter_library_markdown --input_yaml src/bt_executor_parameters.yaml --output_markdown_file bt_executor_parameters.md