'readable' 事件

当有可从流中读取的数据时，则会触发 'readable' 事件。 在某些情况下，为 'readable' 事件绑定监听器会导致一些数据被读入内部缓冲区。

const readable = getReadableStreamSomehow();
readable.on('readable', function() {
  // 现在有一些数据要读取。
  let data;

  while (data = this.read()) {
    console.log(data);
  }
});

一旦到达流数据的末尾但在 'end' 事件被触发之前，'readable' 事件也将被触发。

实际上，'readable' 事件表明流有新的信息：新的数据可用或已到达流末尾。 在前一种情况下，stream.read() 将返回可用的数据。 在后一种情况下，stream.read() 将返回 null。 例如，在以下示例中，foo.txt 是一个空文件：

const fs = require('fs');
const rr = fs.createReadStream('foo.txt');
rr.on('readable', () => {
  console.log(`readable: ${rr.read()}`);
});
rr.on('end', () => {
  console.log('end');
});

运行此脚本的输出是：

$ node test.js
readable: null
end

一般来说，readable.pipe() 和 'data' 事件机制比 'readable' 事件更容易理解。 但是，处理 'readable' 可能会导致吞吐量增加。

如果同时使用 'readable' 和 'data'，则 'readable' 优先控制流，即只有在调用 stream.read() 时才会触发 'data'。 readableFlowing 属性将变为 false。 如果在移除 'readable' 时有 'data' 个监听器，则流将开始流动，即 'data' 事件将在不调用 .resume() 的情况下触发。

The 'readable' event is emitted when there is data available to be read from the stream. In some cases, attaching a listener for the 'readable' event will cause some amount of data to be read into an internal buffer.

const readable = getReadableStreamSomehow();
readable.on('readable', function() {
  // There is some data to read now.
  let data;

  while (data = this.read()) {
    console.log(data);
  }
});

The 'readable' event will also be emitted once the end of the stream data has been reached but before the 'end' event is emitted.

Effectively, the 'readable' event indicates that the stream has new information: either new data is available or the end of the stream has been reached. In the former case, stream.read() will return the available data. In the latter case, stream.read() will return null. For instance, in the following example, foo.txt is an empty file:

const fs = require('fs');
const rr = fs.createReadStream('foo.txt');
rr.on('readable', () => {
  console.log(`readable: ${rr.read()}`);
});
rr.on('end', () => {
  console.log('end');
});

The output of running this script is:

$ node test.js
readable: null
end

In general, the readable.pipe() and 'data' event mechanisms are easier to understand than the 'readable' event. However, handling 'readable' might result in increased throughput.

If both 'readable' and 'data' are used at the same time, 'readable' takes precedence in controlling the flow, i.e. 'data' will be emitted only when stream.read() is called. The readableFlowing property would become false. If there are 'data' listeners when 'readable' is removed, the stream will start flowing, i.e. 'data' events will be emitted without calling .resume().