GDBehavior

An addon for writing behavior trees in GDScript. Written in Godot 3.1.

Influences

• https://github.com/godot-addons/godot-behavior-tree-plugin
• Behavior Trees in Robotics and AI

Philosophy

• Simple
• Extendable
• Good for:
  • learning
  • rapid prototyping
  • experimentation

Installation

Download from the Godot Asset Store or clone and place into the 'addons' folder in your project.

# Not enabled
const GDB = preload("res://addons/GDBehavior/GDBehavior.gd")
var seq = GDB.Composites.Sequencer.new(children)

# Enabled from Plugins menu
var seq = GDBehavior.Composites.Sequencer.new(children)

Usage

Examples

Result constants are defined in the BTNode class (SUCCESS, FAILURE, RUNNING).

Base node types (Action, Conditional, Composite, Decorator) are in GDBehavior/Base folder.

The Tick

  var tick = Tick.new()
    var result = root.exe(tick)

The tick passes through each node in the tree in order of execution. By default, it holds a history of all nodes currently in a RUNNING state but it has other uses such as holding a blackboard or logging tree history for debugging.

Leaf Nodes

Action

class Print:
    extends "res://addons/GDBehavior/Base/BTAction.gd"
    var msg

    func _init(msg).("print"):
        self.msg = msg

  func _exe(tick):
      print(msg)
      return SUCCESS
    

Override the _exe function to apply an action's effects and be sure to return a result on completion.

_open, _closed, _enter, _exit functions are also inherited from BTNode via BTAction.

Condition

class AlwaysCondition:
    extends "res://addons/GDBehavior/Base/BTCondition.gd"
    var value:bool

    func _init(value:bool).("always-condition"):
        self.value = value

    func _validate(tick):
        return value	

Identical to BTAction except _validate is overriden (and returns a bool) instead of _exe. This class is optional and will operate the same as an action return SUCCESS/FAILURE in place of true/false.

Composites

# execute all 3 actions in order while result==SUCCESS
var wander = SequencerMem.new([
  GotoRandom.new(),
  Wait.new(2.0),
  LookAround()
])

var stay_safe = Selector.new([
  IsSafe.new(),
  # nesting is cool
  SequencerMem.new([
    EvadeThreat.new(),
    LookAround()
  ])
])

Composites expect children as an array of BTNodes.

If you are going to use composites with memory (i.e. SequencerMem) then:

• Call Tick.exit_tree(root, result) after each pass of the tree
• Or use TreeRunner.exe(tick) which will do so automatically

Decorators

# SUCCESS = FAILURE and vice versa
var is_not_speaking = Invert.new(IsSpeaking.new())

MultiCondition.new(
  # if all conditions return SUCCESS
  [is_not_speaking, ActorInRange.new(80.0)],	
  # do this. (can be any type of node.)
  SayRandom.new())

Have a single child. Some example uses are guarding execution, modifying a result from the child and repeating execution of children.

Saving and Loading

const SaveLoad = preload("res://addons/GDBehavior/SaveLoad.gd")
var file_path =  "user://gdbehavior/example/bt_save_test.bt"

var saved = SaveLoad.save_tree(root_node, file_path)

# returns root of loaded tree
var root_loaded = SaveLoad.load_tree(file_path)

Saving and Loading Example

The save and load implementation is in early stages and will likely change in the near future.