Modifying Behaviors Using Decorators
My previous post on behavior trees made use of composite and action types, this posts serves as an example on how to improve behaviors using decorators. They take their name from the software design pattern. In the context of behavior trees a decorator node is a node with a single child, it modifies the behavior of the branch in some way, i.e. you don't want your NPC to keep kicking the door forever when it is blocked or replay an animation without completing the cycle.
The problem with my previous Robocode tree was that every time we execute it, it searched through the whole tree and as a consequence it kept switching targets, using decorators we can create loops in the tree that way we don't switch targets unless the robot we are going after dies.
Kill All sequence demonstrates two decorators used for looping, since at the beginning of the match we don't have any enemies to fight we scan around until we find a robot to kill, until-success decorator will keep executing its child until it returns success meaning we found at least one robot to fight. Next in the sequence is Attack branch, using until-fail decorator it will pick a target to attack, keep attacking until the enemy is dead then move on to a new target until select-target fails which means all the robots in the arena are dead, having loops in the tree allows us to execute the tree once unlike the previous example which kept executing the tree forever in a while loop.
In order to combat robots that track our movement, two non deterministic composite nodes exists which shuffle their children prior to execution such as the one used in the Strafe branch which gives us some degree of non determinism.
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