Blueshell

Blueshell is a Behavior Tree implementation written in Typescript.

It is synchronous for performance - asynchronous operations can be handled by returning async operations to your framework for evaluation, these can then generate new events to drive the behavior tree.

Node Types

Actions (aka Tasks or Execution Nodes)

Action

• Handles the event normally and must return a result

Predicate

• Executes a provided synchronous function ((state, event) => boolean) and returns SUCCESS or FAILURE based upon the boolean result of such function.

SideEffect

• Executes a provided synchronous function ((state, event) => void) and always returns SUCCESS.

Success

• Immediately returns SUCCESS when activated.

Composites (aka Control Flow Nodes)

Selector

(aka Priority or Fallback)

• Sends an event to each child until one of them returns SUCCESS or RUNNING, then returns that value.
• If we exhaust all the children, return FAILURE.

LatchedSelector

• Sends an event to each child until one of them returns SUCCESS or RUNNING, then returns that value.
• If we exhaust all the children, return FAILURE.
• If a child returns RUNNING, subsequent events start at that child.

Sequence

• Sends an event to each child until one of the returns FAILURE, or RUNNING, then returns that value.
• If all children return SUCCESS, return SUCCESS.

LatchedSequence

• Sends an event to each child until one of the returns FAILURE, or RUNNING, then returns that value.
• If all children return SUCCESS, return SUCCESS.
• If a child returns RUNNING, subsequent events start at that child.

IfElse

• Accepts a conditional function a consequent node, and an optional alternative node.
• If conditional(state, event) returns true, will return the result of activating the consequent node.
• If conditional(state, event) returns false, will return the result of activating the alternative node, if one is provided.
• If conditional(state, event) returns false, will return FAILURE if no alternative node is provided.

Decorators

Decorators intercept and can modify the event sent to or the result from the child.

Not

• Returns 'FAILURE' when the child returns 'SUCCESS', and vice-versa

RepeatWhen

• Repeats the child when an evaluation function returns true.

RepeatOnResult

• Repeats the child if it returns the specified status.

ResultSwap

• Allows you to swap one result of a child node for another.
• For example, you can use this to mask FAILURE as SUCCESS.

Base Classes

Base

• The base of all nodes.

Composite

• The base class for all nodes which have children.

Decorator

• The base class for decorators.
• Can only have one child.

Publishing

• The publisher can be registered with the tree which will log each transition.

Inspiration and Further Reading

The following are sources used when designing this library

• Handling Node Traversal
• Difference between a Decision Tree and a Behavior Tree
• Behavior Trees for AI
• Wikipedia Article
• What is a Behavior Tree?
• Halo 2's AI
  • Halo 2 was the first game to popularize Behavior Trees.

Name

Blueshell is named for a Skroderider from the novel A Fire Upon the Deep by Vernor Vinge. Skroderiders are intelligent plants (trees) that use mechanical constructs to give them locomotion and short-term memory.

In other words, Blueshell is an intelligent tree. Or a tree with behavior. Get it?