行为树
行为树可以看作 Behavior-Based 方法的升级，其逻辑比状态机清晰，功能比 Behavior-Based 丰富。
Node 类型
| Node type | Succeeds | Fails | Running |
|---|---|---|---|
| Selector | If one child succeeds | If all fail | If one child returns running |
| Sequence | If all children succeeds | If one child fails | If one child returns running |
| Parallell | If N children succeeds | If M-N children fail | If all children return running |
| Decorator | Varies | Varies | Varies |
| Action | Upon completion | When impossible to complete | During completion |
| Condition | If ture | If false | Never |
