Fluent Behavior Tree

This is a Typescript/Javascript implementation of https://github.com/codecapers/Fluent-Behaviour-Tree

JS/TS behaviour tree library with a fluent API.

For a background and walk-through please see the accompanying article.

Understanding Behaviour Trees

Here are some resources to help you understand behaviour trees:

• Behaviour tree (Wikipedia)
• Behavior trees for AI: How they work
• Understanding Behaviour Trees
• Introduction and implementation of Behaviour Trees

Installation

Install with npm:

npm install -s fluent-behavior-tree

Usage

A behavior tree is created through BehaviorTreeBuilder. The tree is returned when the build function is called.

import {BehaviorTreeBuilder, BehaviorTreeStatus, TimeData} from "fluent-behavior-tree";

// ...

const builder = new BehaviorTreeBuilder();
this.tree = builder
    .sequence("my-sequence")
        .do("action1", async (t) => {
            // Action 1.

            return BehaviorTreeStatus.Success;
        })
        .do("action2", async (t) => {
            //Action 2.

            return BehaviorTreeStatus.Failure;
        })
    .end()
    .build();

Then, Tick the behavior tree on each update of your loop

public async update(deltaTime: number): Promise<void> {
    await this.tree.tick(new TimeData(deltaTime));
}

Behavior Tree Status

Behavior tree nodes must return the following status codes:

• BehaviorTreeStatus.Success: The node has finished what it was doing and succeeded.
• BehaviorTreeStatus.Failure: The node has finished, but failed.
• BehaviorTreeStatus.Running: The node is still working on something.

Node Types

Action / Leaf-Node

Call the do function to create an action node at the leaves of the behavior tree.

.do("do-something", async (t) => {
    // ... Do something ...

    return BehaviorTreeStatus.Success;
});

The return value defines the status of the node. Return one of the statuses from above.

Sequence

Runs each child node in sequence. Fails for the first child node that fails. Moves to the next child when the current running child succeeds. Stays on the current child node while it returns running. Succeeds when all child nodes have succeeded.

.sequence("my-sequence")
    .do("action1", async (t) => { // Run this.
        // Action 1.

        return BehaviorTreeStatus.Success;
    })
    .do("action2", async (t) => { // Then run this.
        //Action 2.

        return BehaviorTreeStatus.Failure;
    })
.end()

Parallel

Runs all child nodes in parallel. Continues to run until a required number of child nodes have either failed or succeeded.

let numRequiredToFail: number = 2;
let numRequiredToSuccess: number = 2;

.parallel("my-parallel"m numRequiredtoFail, numRequiredToSucceed)
    .do("action1", async (t) => { // Run this at the same time as action2
        // Parallel action 1

        return BehaviorTreeStatus.Running;
    })
    .do("action12, async (t) => { // Run this at the same time as action1
        // Parallel action 2

        return BehaviorTreeStatus.Running;
    })
.end();

Selector

Runs child nodes in sequence until it finds one that succeeds. Succeeds when it finds the first child that succeeds. For child nodes that fail, it moves forward to the next child node. While a child is running it stays on that child node without moving forward.

.selector("my-selector")
    .do("action1", async (t) => {
        // Action 1

        return BehaviorTreeStatus.Failure; // Fail, move onto the next child
    })
    .do("action2", async (t) => {
        // Action 2

        return BehaviorTreeStatus.Success; // Success, stop here.
    })
    .do("action3", async (t) => {
        // Action 3

        return BehaviorTreeStatus.Success; // Doesn't get this far.
    })
.end();

Condition

The condition function is syntatic sugar for the do function. It allows the return of a boolean value that is then converted to success or failure. It is intended to be used with Selector.

.selector("my-selector")
    .Condition("condition1", async (t) => this.someBooleanConditional()) // Predicate that returns *true* or *false*
    .do("action1", async (t) => this.someAction()) // Action to run if the predicate evaluates to *true*
.end()

Inverter

Inverts the success or failure of the child node. Continues running while the child node is running.

.inverter("inverter1")
    .do("action1", async (t) => BehaviourTreeStatus.Success) // *Success* will be inverted to *failure*.
.end()


.inverter("inverter1")
    .do("action1", async (t) => BehaviourTreeStatus.Failure) // *Failure* will be inverted to *success*.
.end()

Nesting Behaviour Trees

Behaviour trees can be nested to any depth, for example:

.selector("parent")
    .sequence("child-1")
        ...
        .parallel("grand-child")
            ...
        .end()
        ...
    .end()
    .sequence("child-2")
        ...
    .end()
.end()

Splicing a Sub-tree

Separately created sub-trees can be spliced into parent trees. This makes it easy to build behaviour trees from reusable components.

private createSubTree(): BehaviorTreeNodeInterface
{
    return new BehaviourTreeBuilder()
        .sequence("my-sub-tree")
            .do("action1", async (t) => {
                // Action 1.

                return BehaviourTreeStatus.Success;
            })
            .Do("action2", async (t) => {
                // Action 2.

                return BehaviourTreeStatus.Success;
            });
        .end()
        .build();
}

public startup(): void
{
    this.tree = new BehaviourTreeBuilder()
        .sequence("my-parent-sequence")
            .Splice(this.createSubTree()) // Splice the child tree in.
            .Splice(this.createSubTree()) // Splice again.
        .end()
        .build();
}