datetime:2023/05/11 15:12

author:nzb

BT3：库中基本类型Tree和TreeNode

本文重点讲述BehaviorTree.CPP中的几个重要的基本的类定义，尤其是类所包含的数据。

Tree

定义在BehaviorTree.CPP/include/behaviortree_cpp_v3/bt_factory.h。有3个重要的public成员变量。

public:
    // 保存树的所有node
    std::vector<TreeNode::Ptr> nodes;
    // 保存所有blackboard
    std::vector<Blackboard::Ptr> blackboard_stack;
    // 保存所有node注册信息
    std::unordered_map<std::string, TreeNodeManifest> manifests;

以BehaviorTree.CPP/examples/t06_subtree_port_remapping.cpp中的行为树为例，打印出以上3个容器的元素数量。

<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>

共有10个节点，2个blackboard（因为有2棵树），31个注册节点信息（可以理解为该tree认识31个节点，却只创建和包含了10个节点） 。

tree nodes count = 10
tree blackboard_stack count = 2
tree manifests count = 31

将节点的名称和注册节点信息的名称打印如下，可见一棵子树也会作为一个节点对待。而31个manifests中，就包含了BehaviorTree.CPP库提供的所有ControlNodes和DecoratorNodes， 以及示例用的SaySomething和MoveBase。

tree root node = main_sequence

nodes[1] = main_sequence
nodes[2] = SetBlackboard
nodes[3] = MoveRobot
nodes[4] = move_robot_main
nodes[5] = SequenceStar
nodes[6] = MoveBase
nodes[7] = SetBlackboard
nodes[8] = ForceFailure
nodes[9] = SetBlackboard
nodes[10] = SaySomething

manifests[1] = SaySomething
manifests[2] = Switch4
manifests[3] = Switch6
manifests[4] = BlackboardCheckDouble
manifests[5] = BlackboardCheckInt
manifests[6] = SubTree
manifests[7] = KeepRunningUntilFailure
manifests[8] = Switch5
manifests[9] = ReactiveSequence
manifests[10] = Parallel
manifests[11] = Delay
manifests[12] = SetBlackboard
manifests[13] = SequenceStar
manifests[14] = Fallback
manifests[15] = AlwaysSuccess
manifests[16] = ReactiveFallback
manifests[17] = Sequence
manifests[18] = Switch3
manifests[19] = Switch2
manifests[20] = AlwaysFailure
manifests[21] = IfThenElse
manifests[22] = WhileDoElse
manifests[23] = SubTreePlus
manifests[24] = ForceSuccess
manifests[25] = Inverter
manifests[26] = BlackboardCheckString
manifests[27] = RetryUntilSuccesful
manifests[28] = ForceFailure
manifests[29] = MoveBase
manifests[30] = Repeat
manifests[31] = Timeout

TreeNode

定义在BehaviorTree.CPP/include/behaviortree_cpp_v3/tree_node.h。注意区分name_和registration_ID_的区别。

private:
    const std::string name_;  // 从xml获得的node名称，可没有，可重复
    NodeStatus status_;       // node的返回结果，即执行状态
    std::condition_variable state_condition_variable_;
    mutable std::mutex state_mutex_;
    StatusChangeSignal state_change_signal_;  // 订阅的信号
    const uint16_t uid_;      // 唯一ID
    NodeConfiguration config_;
    std::string registration_ID_; // 类型名称，一定与class name相同

以BehaviorTree.CPP/examples/t06_subtree_port_remapping.cpp中的行为树为例，打印出所有node的name和注册ID对比。 可见registration_ID_ 和node的class name相同（非强制，下篇讲解），而name可以随意指定，如nodes[4]，当不设置时默认是registration_ID_，如其中的nodes[2]和nodes[5]。

nodes[1].name=main_sequence, reg_id=Sequence
nodes[2].name=SetBlackboard, reg_id=SetBlackboard
nodes[3].name=MoveRobot, reg_id=SubTree
nodes[4].name=move_robot_main, reg_id=Fallback
nodes[5].name=SequenceStar, reg_id=SequenceStar
nodes[6].name=MoveBase, reg_id=MoveBase
nodes[7].name=SetBlackboard, reg_id=SetBlackboard
nodes[8].name=ForceFailure, reg_id=ForceFailure
nodes[9].name=SetBlackboard, reg_id=SetBlackboard
nodes[10].name=SaySomething, reg_id=SaySomething

NodeConfiguration中包含了blackboard的指针，和输入输出ports的映射信息。

typedef std::unordered_map<std::string, std::string> PortsRemapping;

struct NodeConfiguration {
    Blackboard::Ptr blackboard;
    PortsRemapping input_ports;   // 输入port的映射关系
    PortsRemapping output_ports;  // 输出port的映射关系
};

打印SaySomething的input_ports如下，没有output ports。

input port: message --- {move_result}

打印SetBlackboard的ports如下，该node比较特殊，output_key是个INOUT双向port，以后会单独介绍。

input port: output_key --- move_goal
input port: value --- 1;2;3

output port: output_key --- move_goal

TreeNode类中提供了2个容易迷惑的接口，1个是虚函数executeTick()，1个是纯虚函数tick()，那么开发者应该实现和调用哪一个呢？

public:
  /// The method that should be used to invoke tick() and setStatus();
  virtual BT::NodeStatus executeTick();
protected:
  /// Method to be implemented by the user
  virtual BT::NodeStatus tick() = 0;

executeTick()提供了默认实现，即先调用tick()然后设置返回的状态。而各种ControlNodes和DecoratorNodes， 都是在tick()中调用child_node_->executeTick()，所以开发者只需实现子类的tick()就好了。

NodeStatus TreeNode::executeTick() {
  const NodeStatus status = tick();
  setStatus(status);
  return status;
}

行为树执行时会是这样的逻辑：