A behavior tree is a tree-shaped data structure consisting of nodes, each of which contain a logical method which executes code. The behavor tree is evaluated recursively starting at the root node. Each node has the abilility to execute code which will either run a script, or execute all of its children. Each node will also return one of 3 outputs to its parent: "success", "failure", or "running". There are two main types of nodes: the control-flow (parent nodes) nodes and the leaf nodes.
Selector
The Selector executes its children sequentially from left to right.
If one of its children returns either "success" or "running", it will halt execution of its children and it will return the result of the child it stopped on.
If all of its children return "failure", the Selector will also return "failure".
Sequencer
The Sequencer executes its children sequentially from left to right.
The Sequencer will not halt execution of its children unless one of them returns "failure" or "running", in which case it will also return "failure" or "running".
Multitasker
The Multitasker runs all of its children concurrently, each in a separate thread.
The Multitasker will return "success" only if all of it's children return "success".
Action Nodes
Action nodes send ROS topic messages from the behavior tree to the robot.
Often the type of message sent from an Action node is a cmd_vel message which encodes movement instructions for the robot.
If all children return "success" the Sequencer will return "success"
If any of its children return "running" but none return "failure", the Multitasker will return "running".
If any of its children return "failure", the Multitasker will return "failure".
Update Nodes
Update nodes are designated for updating data in the blackboard.
Often times the types of data updates performed by Update nodes include preprocessing or processing of message data from the robot.
Conditional Nodes
Conditional nodes will return either "success" or "failure", corresponding to the boolean values "true" and "false" respectively
Conditional nodes will access data in the blackboard and return one of the two values listed above based on if a particular condition is met within the data.