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Composition of Behaviors with Subtree

We can build large scale behavior composing together smaller and reusable behaviors into larger ones.

In other words, we want to create hierarchical behavior trees.

This can be achieved easily defining multiple trees in the XML including one into the other.

CrossDoor behavior

This example is inspired by a popular article about behavior trees.

It is also the first practical example that uses Decorators and Fallback.

<root main_tree_to_execute = "MainTree">

    <BehaviorTree ID="DoorClosed">
        <Sequence name="door_closed_sequence">
            <RetryUntilSuccesful num_attempts="4">

    <BehaviorTree ID="MainTree">
        <Fallback name="root_Fallback">
            <Sequence name="door_open_sequence">
            <SubTree ID="DoorClosed"/>


It may be noticed that we incapsulated a quite complex branch of the tree, the one to execute when the door is closed, into a separate tree called DoorClosed.

The desired behavior is:

  • If the door is open, PassThroughDoor.
  • If the door is closed, try up to 4 times to OpenDoor and, then, PassThroughDoor.
  • If it was not possible to open the closed door, PassThroughWindow.


On the C++ side we don't need to do anything to build reusable subtrees.

Therefore we take this opportunity to introduce another neat feature of BehaviorTree.CPP : Loggers.

A Logger is a mechanism to display, record and/or publish any state change in the tree.

int main()
    using namespace BT;
    BehaviorTreeFactory factory;

    // register all the actions into the factory
    // We don't show how these actions are implemented, since most of the 
    // times they just print a message on screen and return SUCCESS.
    // See the code on Github for more details.
    factory.registerSimpleCondition("IsDoorOpen", std::bind(IsDoorOpen));
    factory.registerSimpleAction("PassThroughDoor", std::bind(PassThroughDoor));
    factory.registerSimpleAction("PassThroughWindow", std::bind(PassThroughWindow));
    factory.registerSimpleAction("OpenDoor", std::bind(OpenDoor));
    factory.registerSimpleAction("CloseDoor", std::bind(CloseDoor));
    factory.registerSimpleCondition("IsDoorLocked", std::bind(IsDoorLocked));
    factory.registerSimpleAction("UnlockDoor", std::bind(UnlockDoor));

    // Load from text or file...
    auto tree = factory.createTreeFromText(xml_text);

    // This logger prints state changes on console
    StdCoutLogger logger_cout(tree.root_node);

    // This logger saves state changes on file
    FileLogger logger_file(tree.root_node, "bt_trace.fbl");

    // This logger stores the execution time of each node
    MinitraceLogger logger_minitrace(tree.root_node, "bt_trace.json");


    //while (1)
        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();
            CrossDoor::SleepMS(1);   // optional sleep to avoid "busy loops"
    return 0;