Call 同步依次执行函数，只要有一个返回err,直接返回 通过此方法可以减少if err != nil { return }的判断.

IgnoreErrorCaller return new function that will ignore error.

shutdown 关闭服务.

WaitClose is a helper function that waits for the channel to close and invoke the callback.

ChanWatcher implements the Watcher interface with channel.

Watcher.