Interface Watcher

This interface specifies the public interface an event handler class must implement. A ZooKeeper client will get various events from the ZooKeeper server it connects to. An application using such a client handles these events by registering a callback object with the client. The callback object is expected to be an instance of a class that implements Watcher interface. When process(WatchedEvent) is triggered by a watch firing, such as Watcher.Event.EventType.NodeDataChanged, WatchedEvent.getZxid() will return the zxid of the transaction that caused said watch to fire. If -1L is returned then the server must be updated to support this feature.