在试用React Native遇到了这个问题Unable to download JS bundle

E/unknown:React: Unable to download JS bundle
E/unknown:React: java.net.ConnectException: failed to connect to localhost/127.0.0.1 (port 8081) after 5000ms: isConnected failed: ECONNREFUSED (Connection refused)
E/unknown:React:     at libcore.io.IoBridge.isConnected(IoBridge.java:223)
E/unknown:React:     at libcore.io.IoBridge.connectErrno(IoBridge.java:161)
E/unknown:React:     at libcore.io.IoBridge.connect(IoBridge.java:112)
...
E/unknown:React:  Caused by: libcore.io.ErrnoException: isConnected failed: ECONNREFUSED (Connection refused)
E/unknown:React:     at libcore.io.IoBridge.isConnected(IoBridge.java:208)
E/unknown:React:     at libcore.io.IoBridge.connectErrno(IoBridge.java:161) 
E/unknown:React:     at libcore.io.IoBridge.connect(IoBridge.java:112) 
E/unknown:React:     at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:192) 
E/unknown:React:     at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:460) 
E/unknown:React:     at java.net.Socket.connect(Socket.java:833) 
...

后来发现是少了一步 -> 因为机器是Android 5.0

  adb reverse tcp:8081 tcp:8081