behavior

behavior

behavior <type-name>
  script_variables ... end_script_variables
  on_init          ... end_on_init
  on_message       ... end_on_message
  precondition     ... end_precondition
  on_new_execute   ... end_on_new_execute
  on_new_fail      ... end_on_new_fail
  execute          ... end_execute
  <finite state machine inputs>
  <zero or more nodes>
end_behavior

Overview

A behavior (also called a behavior node or a leaf node) is one of the two basic node types used by a behavior_tree. A behavior is defined globally at the simulation level. Every behavior defined in a scenario must be given a unique name. Once a behavior is defined, any behavior tree can use it, by referencing it by name.

Behavior Mode of Operation

Behavior nodes themselves can have child nodes of any type; child nodes are executed after the behavior performs its execute script. Behavior nodes can also have a finite state machine (FSM) on them, one per behavior node. To implement the FSM, define states with transition rules inside the behavior block. The transition rules are defined using the WSF scripting language. The FSM is evaluated after the child nodes of a behavior are executed.

In summary, a behavior node processes itself in this order:

• The behavior node’s on_init script block is run when the owning processor initializes in the simulation

• The behavior_tree has to traverse to the node when it updates (the node’s position in the tree determines if this happens)

• The node’s precondition script block is run

• If the precondition returns false, and the node was ran last time, then:

  • The node’s on_new_fail script is run

• If the precondition returns successful, then:

  • If the node was not ran last time, it’s on_new_execute script is run

  • The node’s execute script block is run (if exists)

  • The node’s children nodes are run (if exist)

  • The node’s finite state machine is run (if exists), current state evaluated and zero or one state transitions made

Behavior Node Components

behavior <type-name> … end_behavior

The following script blocks comprised the guts of a behavior:

on_init … end_on_init

Behaviors have an on_init script block to help them initialize any necessary member variables or perform any preprocessing.

on_message … end_on_message

Behaviors can received and process messages. Be sure the comm is linked to the processor that the behavior is on. See the script processor on_message section for more information on using this feature.

behavior test_node
  on_message
    default
      script
        writeln("received message ", MESSAGE.Type(), " from ", MESSAGE.Originator());
      end_script
  end_on_message
end_behavior

precondition … end_precondition

Every behavior must have a precondition script block that returns either a true/false Boolean value or a real number. Some parent nodes will check a behavior’s precondition and use its return value as a real number and some will use its return value as a Boolean number. If a precondition returns true/false and it’s used a real number, then “true” equates to 1.0 and “false” equates to 0.0. If a precondition returns a real number and it’s used as a Boolean than 0.0 equates to “false” and everything else equates to “true.” Any tree or node that owns this behavior uses the precondition to determine whether or not that behavior will execute, and subsequently how the whole tree will be traversed and processed on each update. The node types that use real numbers are priority_selector and weighted_random connector nodes.

on_new_execute … end_on_new_execute

If a behavior node is run and was not ran last update, then this script block is run.

on_new_fail … end_on_new_fail

If a behavior node is not run and was ran last update, then this script block is run.

execute … end_execute

If a behavior node is traversed to by the tree, and its precondition returns a value that the parent node uses to select it for execution, then the behavior’s execute script is performed. This script block should be significant, it is what the behavior exists for.

show_state_evaluations

Indicates that information about state evaluations should be written to standard output. This essentially shows the true or false status of the evaluation of each next_state block.

show_state_transitions

Indicates that information about state transitions should be written to standard output.

state <state-name>

Defines a state in a state machine with the name <state-name>. Notice that states can have their own behavior tree within them; it is updated when the state is evaluated.

state <state-name>
  on_entry
     … Script Body …
  end_on_entry
  on_exit
     … Script Body …
  end_on_exit
  next_state <next-state-name-1>
     … Script Body …
  end_next_state
  next_state <next-state-name-n>
     … Script Body …
  end_next_state
  behavior_tree
     ... :command:`Behavior Tree Commands <behavior_tree>` ...
  end_behavior_tree
end_state

Military Specific

Example Behavior:

# this behavior checks to make sure the platform has a target and a weapon
# it also checks that this weapon has not already been used
# if the precondition passes, it tries to fire once at the target

behavior check_and_fire

  script_variables
    WsfWeapon myWeapon;
  end_script_variables

  on_init
    myWeapon = PLATFORM.WeaponEntry(0);  #just grabs the first defined weapon
  end_on_init

  precondition
    WsfTrack myTarget = PLATFORM.CurrentTargetTrack();
    if (myWeapon.IsValid() && myTarget.IsValid())
    {
      if (myWeapon.TotalQuantityUsed() < 1.0)
      {
        return true;
      }
    }
    return false;
  end_precondition

  execute
    WsfTrack myTarget = PLATFORM.CurrentTargetTrack();  #should be valid, if precondition passed
    myWeapon.Fire(myTarget);
  end_execute

end_behavior