事件：'end'

【Event: 'end'】

'end' 事件来自 stream.Readable 类。当所有数据都输出完毕时，会触发 'end' 事件，这发生在 transform._flush() 中的回调被调用之后。如果发生错误，'end' 不应被触发。

【The 'end' event is from the stream.Readable class. The 'end' event is emitted after all data has been output, which occurs after the callback in transform._flush() has been called. In the case of an error, 'end' should not be emitted.】