新增于: v0.7.7
如果 Node.js 进程是使用 IPC 通道衍生的（详见子进程和集群），则当 IPC 通道关闭时触发'disconnect'事件。
Added in: v0.7.7
If the Node.js process is spawned with an IPC channel (see the Child Process
and Cluster documentation), the 'disconnect' event will be emitted when
the IPC channel is closed.