State Like Action In Behavior Tree

Question

From what I understand on Behavior Trees, each Behavior should be a short goal-oriented Action that could be done in a few iterations.

So for example, below is an image of a Behavior Tree:

Now let us assume that the Drive To Enemy behavior takes more than a few iterations in the tree. So on each pass Drive To Enemy is called because it is now running.

The problem is I want to call Evade Enemy if an Enemy is nearby. And considering that Drive To Enemy is always called I never get a chance to call Evade Enemy (Should probably be called Avoid Enemy).

• Should I traverse the Tree EACH passes no matter what Action is currently running?
• Am I going about this the right way?
• What is the proper way of handling such a behavior?

Answer 1

While you are in the "Drive To Enemy" behavior, you include some sort of way to run some additional checks to make sure that behavior should still be continued.

You can use the parallel composite: a type of behavior tree node that runs all its children simultaneously.

It would look like this:

• MainSelector:
  • Evade Enemy
  • Locate Enemy
  • Drive-in opposite direction
• Parallel
  • Is Enemy Nearby?
  • Chase Enemy
  • Find the path to the enemy
  • Drive to enemy
  • Fire Weapon
  • Chase Flag
  • Locate flag
  • Find Path
  • Drive to flag

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