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Remapping ports between Trees and SubTrees

In the CrossDoor example we saw that a SubTree looks like a single leaf Node from the point of view of its parent (MainTree in the example).

Furthermore, to avoid name clashing in very large trees, any tree and subtree use a different instance of the Blackboard.

For this reason, we need to explicitly connect the ports of a tree to those of its subtrees.

Once again, you won't need to modify your C++ implementation since this remapping is done entirely in the XML definition.


Let's consider this Beahavior Tree.

<root main_tree_to_execute = "MainTree">

    <BehaviorTree ID="MainTree">

        <Sequence name="main_sequence">
            <SetBlackboard output_key="move_goal" value="1;2;3" />
            <SubTree ID="MoveRobot" target="move_goal" output="move_result" />
            <SaySomething message="{move_result}"/>


    <BehaviorTree ID="MoveRobot">
        <Fallback name="move_robot_main">
                <MoveBase       goal="{target}"/>
                <SetBlackboard output_key="output" value="mission accomplished" />
                <SetBlackboard output_key="output" value="mission failed" />


You may notice that:

  • We have a MainTree that includes a subtree called MoveRobot.
  • We want to "connect" (i.e. "remap") ports inside the MoveRobot subtree with other ports in the MainTree.
  • This is done using the XMl tag , where the words internal/external refer respectively to a subtree and its parent.

The following image shows remapping between these two different trees.

Note that this diagram represents the dataflow and the entries in the respective blackboard, not the relationship in terms of Behavior Trees.

ports remapping

In terms of C++, we don't need to do much. For debugging purpose, we may show some information about the current state of a blackboard with the method debugMessage().

int main()
    BT::BehaviorTreeFactory factory;


    auto tree = factory.createTreeFromText(xml_text);

    NodeStatus status = NodeStatus::RUNNING;
    // Keep on ticking until you get either a SUCCESS or FAILURE state
    while( status == NodeStatus::RUNNING)
        status = tree.root_node->executeTick();
        SleepMS(1);   // optional sleep to avoid "busy loops"

    // let's visualize some information about the current state of the blackboards.
    std::cout << "--------------" << std::endl;
    std::cout << "--------------" << std::endl;
    std::cout << "--------------" << std::endl;

    return 0;

/* Expected output:

    [ MoveBase: STARTED ]. goal: x=1 y=2.0 theta=3.00
    [ MoveBase: FINISHED ]
    Robot says: mission accomplished
    move_result (std::string) -> full
    move_goal (Pose2D) -> full
    output (std::string) -> remapped to parent [move_result]
    target (Pose2D) -> remapped to parent [move_goal]