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Version: 7

Debugging Behavior Tree Blackboard Variables

Introduction​

The blackboard is a central data store used by the behavior tree to share key-value pairs between nodes during objective execution. It acts as a runtime memory where behaviors can read from and write to without needing direct coupling or global variables. This design makes your behavior trees modular and easier to debug, as all shared data is available in one place.

How to Use the Blackboard​

There are two ways to inspect and work with the blackboard during development and debugging:

1. Blackboard Viewer in the Web UI​

When you run an objective, the MoveIt Pro web interface offers a Blackboard pane. Here you can:

  • Visualize Variables: See the current scopes, keys, variable types (indicated by the __type field), and their corresponding values.
  • Real-Time Updates: Observe how the variables change as different behaviors execute.

This tool is invaluable when you need a high-level overview of the data flow within your behavior tree.

2. Subscribing to the /blackboard_contents Topic​

During execution, the blackboard information is published as a JSON-encoded message on the /blackboard_contents topic. You can inspect these messages using standard ROS 2 tools. For example, to see the published contents in real time, run:

ros2 topic echo /blackboard_contents

This will output a JSON message containing the blackboard contents.

By utilizing the Blackboard Viewer, subscribing to the /blackboard_contents topic, and debugging within your code, you can effectively monitor and troubleshoot the shared state within your behavior trees. This makes it easier to pinpoint issues during development and maintain reliable behavior execution in your MoveIt Pro applications.

Happy debugging!