Flow API:Java 中的背压接口

    Flow 的成员类型:[Processor(接口), Publisher(接口), Subscriber(接口), Subscription(接口)]
    Flow 本身的方法个数:1;defaultBufferSize()=256
      Processor:[] extends [Subscriber, Publisher]
      Subscription:[cancel[], request[long]]
      Subscriber:[onComplete[], onError[Throwable], onNext[Object], onSubscribe[Subscription]]
      Publisher:[subscribe[Subscriber]]

    场景 1:订阅者每次只 request(1),处理完一个再要下一个
      [main     ] onSubscribe
      [main     ] 收到 request(1)
      [pub      ] onNext(1)
      [pub      ] 收到 request(1)
      [pub      ] onNext(2)
      [pub      ] 收到 request(1)
      [pub      ] onNext(3)
      [pub      ] 收到 request(1)
      [pub      ] onNext(4)
      [pub      ] 收到 request(1)
      [pub      ] onNext(5)
      [pub      ] 收到 request(1)
      [pub      ] onComplete
    场景 2:request(3) 一次,之后不再要
      [main     ] onSubscribe
      [main     ] 收到 request(3)
      [pub      ] onNext(1)
      [pub      ] onNext(2)
      [pub      ] onNext(3)
      [main2    ] 300 ms 后仍没有 onComplete,取消
      [main2    ] 收到 cancel()
    场景 3:request(0)
      [main     ] onSubscribe
      [main     ] 收到 request(0)
      [main     ] onError java.lang.IllegalArgumentException: non-positive request
    场景 4:request(Long.MAX_VALUE)(无界)
      [main     ] 收到 request(9223372036854775807)
      [pub      ] onNext(1)
      [pub      ] onNext(2)
      [pub      ] onNext(3)
      [pub      ] onNext(4)
      [pub      ] onNext(5)
      [pub      ] onComplete

      处理器收到上游 onSubscribe
      订阅者 onSubscribe,request(2)
      处理器收到下游 request(2) → 转给上游
      订阅者 onNext <10>
      订阅者 onNext <20>
      只 request 了 2,发 4 个,订阅者看到的见上;现在 close()