A test case from my bevy HTN crate

htn.rs

#[test]
fn test_travel_htn() {
    {
        // Don't need app for test, just want to set up the logger.
        let mut app = App::new();
        app.add_plugins(bevy::log::LogPlugin::default());
    }

    // DEFINE OPERATORS (which are behaviour trees)

    // the default behaviour of operators is to be emitted as triggers,
    // ie. Behave::trigger(WalkOperator)
    // but you can also make them components to be spawned, or implement HtnOperator
    // yourself to provide a custom behaviour tree.
    #[derive(Reflect, Default, Clone, Debug, PartialEq, Eq, HtnOperator)]
    #[reflect(Default, HtnOperator)]
    struct WalkOperator;

    #[derive(Reflect, Default, Clone, Debug, PartialEq, Eq, HtnOperator)]
    #[reflect(Default, HtnOperator)]
    struct TaxiOperator;

    // an operator that returns a custom behaviour tree.
    #[derive(Reflect, Default, Clone, Debug, PartialEq, Eq)]
    #[reflect(Default, HtnOperator)]
    struct RideTaxiOperator;
    impl HtnOperator for RideTaxiOperator {
        fn to_tree(&self) -> Option<Tree<Behave>> {
            Some(behave! { Behave::Wait(3.0) })
        }
    }

    // this one would get spawned into an entity using Behave::spawn_named.
    #[derive(Reflect, Default, Clone, Debug, PartialEq, Eq, HtnOperator, Component)]
    #[reflect(Default, HtnOperator)]
    #[spawn_named = "Paying the taxi!"]
    struct PayTaxiOperator;

    // DEFINE PLANNER STATE

    #[derive(Reflect, Default, Clone, Debug, PartialEq, Eq)]
    #[reflect(Default)]
    enum Location {
        #[default]
        Home,
        Other,
        Park,
    }

    #[derive(Reflect, Resource, Clone, Debug, Default)]
    #[reflect(Default, Resource)]
    struct TravelState {
        cash: i32,
        distance_to_park: i32,
        happy: bool,
        my_location: Location,
        taxi_location: Location,
    }

    // DEFINE HTN (can be loaded from an .htn file by asset loader too)

    // for an initial state with distance > 4 this will cause the planner to try the first
    // TravelToPark method, then backtrack when the precondition for Walk is not met,
    // then try the second method, which succeeds (get a taxi).
    let src = r#"
    schema {
        version: 0.1.0
    }

    compound_task "TravelToPark" {
        method {
            subtasks: [ Walk ]
        }
        method {
            subtasks: [ Taxi ]
        }
    }
            
    primitive_task "Walk" {
        operator: WalkOperator
        preconditions: [distance_to_park <= 4, my_location != Location::Park, happy == false]
        effects: [
            my_location = Location::Park,
            happy = true,
        ]
    }

    compound_task "Taxi" {
        method {
            subtasks: [CallTaxi, RideTaxi, PayTaxi]
        }
    }

    primitive_task "CallTaxi" {
        operator: TaxiOperator
        preconditions: [taxi_location != Location::Park, cash >= 1]
        effects: [taxi_location = Location::Home]
    }

    primitive_task "RideTaxi" {
        operator: RideTaxiOperator
        preconditions: [taxi_location == Location::Home, cash >= 1]
        effects: [taxi_location = Location::Park, my_location = Location::Park, happy = true]
    }

    primitive_task "PayTaxi" {
        operator: PayTaxiOperator
        preconditions: [taxi_location == Location::Park, cash >= 1]
        effects: [cash -= 1]
    }
    "#;

    // REGISTER TYPES USED IN HTN

    // normally you'd use app.register_type or Res<AppTypeRegistry>
    let atr = AppTypeRegistry::default();
    {
        let mut atr = atr.write();
        atr.register::<TravelState>();
        atr.register::<Location>();
        atr.register::<WalkOperator>();
        atr.register::<TaxiOperator>();
        atr.register::<RideTaxiOperator>();
        atr.register::<PayTaxiOperator>();
    }
    let htn = parse_htn::<TravelState>(src);

    // verify via reflection that any types used in the htn are registered:
    match htn.verify_all(&TravelState::default(), &atr) {
        Ok(_) => {}
        Err(e) => panic!("HTN type verification failed: {e:?}"),
    }

    let mut planner = HtnPlanner::new(&htn, &atr);

    // Run the planner with alternative starting states to see different outcomes:
    {
        warn!("Testing walking state");
        let initial_state = TravelState {
            cash: 10,
            distance_to_park: 1,
            my_location: Location::Home,
            taxi_location: Location::Other,
            happy: false,
        };
        let plan = planner.plan(&initial_state);
        assert_eq!(plan.task_names(), vec!["Walk"]);
    }

    {
        warn!("Testing taxi state");
        let initial_state = TravelState {
            cash: 10,
            distance_to_park: 5,
            my_location: Location::Home,
            taxi_location: Location::Other,
            happy: false,
        };
        let plan = planner.plan(&initial_state);
        assert_eq!(plan.task_names(), vec!["CallTaxi", "RideTaxi", "PayTaxi"]);
    }
}