'drain' 事件

如果对 stream.write(chunk) 的调用返回 false，则 'drain' 事件将在适合继续将数据写入流时触发。

// 将数据写入提供的可写流一百万次。
// 注意背压。
function writeOneMillionTimes(writer, data, encoding, callback) {
  let i = 1000000;
  write();
  function write() {
    let ok = true;
    do {
      i--;
      if (i === 0) {
        // 最后一次！
        writer.write(data, encoding, callback);
      } else {
        // 看看是应该继续，还是等待。
        // 不要传入回调，因为还没有完成。
        ok = writer.write(data, encoding);
      }
    } while (i > 0 && ok);
    if (i > 0) {
      // 必须早点停下来！
      // 等它排空时再写一些。
      writer.once('drain', write);
    }
  }
}

If a call to stream.write(chunk) returns false, the 'drain' event will be emitted when it is appropriate to resume writing data to the stream.

// Write the data to the supplied writable stream one million times.
// Be attentive to back-pressure.
function writeOneMillionTimes(writer, data, encoding, callback) {
  let i = 1000000;
  write();
  function write() {
    let ok = true;
    do {
      i--;
      if (i === 0) {
        // Last time!
        writer.write(data, encoding, callback);
      } else {
        // See if we should continue, or wait.
        // Don't pass the callback, because we're not done yet.
        ok = writer.write(data, encoding);
      }
    } while (i > 0 && ok);
    if (i > 0) {
      // Had to stop early!
      // Write some more once it drains.
      writer.once('drain', write);
    }
  }
}