Events and debugging

Events as std::variant

The tree is ticked with an event, not only a clock. Make the event type a std::variant and let each node pick out what it cares about with bt::overloaded:

struct Tick {};
struct BatteryUpdate { int level; };
struct ObjectSpotted { float distance; };
using Event = std::variant<Tick, BatteryUpdate, ObjectSpotted>;

struct CheckBattery : NodeBase {
    using EventType = Event;
    using ContextType = Context;

    Status process(const Event& e, Context& ctx) {
        std::visit(overloaded{
            [&](const BatteryUpdate& b) { ctx.battery = b.level; },
            [](const auto&) {}
        }, e);
        return ctx.battery > 20 ? Status::Success : Status::Failure;
    }
};

Sensor callbacks push their message as an event, and a timer pushes Tick. The ROS 2 example works this way.

Printing a tree

<behavior_tree_lite/debug.hpp> prints the tree's shape from its type:

#include <behavior_tree_lite/debug.hpp>

auto tree = (A{} && B{}) || C{};
bt::print_tree(tree);

Selector
  Sequence
    A
    B
  C

Decorators print their parameter, such as Retry (3x) or Timeout (10 ticks). Leaf names come from the compiler's type name, so they read best for non-template structs. print_tree takes an optional std::ostream&, which defaults to std::cout.

Graphviz

to_dot writes the same tree as a Graphviz digraph, one node per tree node and one edge from each parent:

bt::to_dot(tree, file);

dot -Tsvg tree.dot -o tree.svg