Remapping ports between Trees and SubTrees

在CrossDoor例子中，我们看到一颗SubTree树，从其父节点（例子中是MainTree）来看其就像是一个叶子节点。

进一步，为了避免在大的树中的命名冲突，任何树和子树使用不同的黑板实例。

针对这个原因，我们需要去显示将一颗树和其子树进行连接；

不需要修改C++实现，因为在XML中完成即可；

例子：

<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}"/>
        </Sequence>

    </BehaviorTree>

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

</root>

解释：

主树MainTree包含子树MoveRobot；

想要连接（如重映射）子树MoveRobot中的端口与主树MainTree中的端口；

这都是在XML中完成的，internal对应子树，external对应主树；

端口映射：

int main()
{
    BT::BehaviorTreeFactory factory;

    factory.registerNodeType<SaySomething>("SaySomething");
    factory.registerNodeType<MoveBaseAction>("MoveBase");

    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.tickRoot();
        SleepMS(1);   // optional sleep to avoid "busy loops"
    }

    // let's visualize some information about the current state of the blackboards.
    std::cout << "--------------" << std::endl;
    tree.blackboard_stack[0]->debugMessage();
    std::cout << "--------------" << std::endl;
    tree.blackboard_stack[1]->debugMessage();
    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]
    --------------
*/