automata v0.1.0 Automaton.Types.BT.Composite.Parallel

Parallel Node When the execution of a parallel node starts, then the node’s children are executed in succession from left to right without waiting for a return status from any child before ticking the next one. It returns success if a given number of children M ∈ N return success, it returns failure when the children that return running and success are not enough to reach the given number, even if they would all return success. It returns running otherwise. The purpose of the parallel node is to model those tasks separable in independent sub-tasks performing non-conflicting actions (e.g. a multi object tracking can be performed using several cameras).

We could also configure Parallel to have the policy of the Selector task so it returns success when its first child succeeds and failure only when all have failed. We could also use hybrid policies, where it returns success or failure after some specific number or proportion of its children have succeeded or failed.

Using Parallel blocks to make sure that Conditions hold is an important use-case in behavior trees. With it we can get much of the power of a state machine, and in particular the state machine’s ability to switch tasks when important events occur and new opportunities arise. Rather than events triggering transitions between states, we can use sub-trees as states and have them running in parallel with a set of conditions.