Question

我有一个fs.ReadStream对象指向一个非常大的文件。现在我想从ReadStream中读取8000个字节，但内部缓冲区只有6000个字节。所以我的方法是读取那6000个字节并等待内部缓冲区再次填满，使用while循环检查内部缓冲区长度是否不再为0。

这样的事情：

BinaryObject.prototype.read = function(length) {
  var value;

  // Check whether we have enough data in the internal buffer
  if (this.stream._readableState.length < length) {
    // Not enough data - read the full internal buffer to
    // force the ReadStream to fill it again.
    value = this.read(this.stream._readableState.length);
    while (this.stream._readableState.length === 0) {
      // Wait...?
    }
    // We should have some more data in the internal buffer
    // here... Read the rest and add it to our `value` buffer
    // ... something like this:
    //
    // value.push(this.stream.read(length - value.length))
    // return value
  } else {
    value = this.stream.read(length);
    this.stream.position += length;
    return value;
  }
};

问题是，缓冲区不再被填充 - 脚本将在while循环中空闲。

这样做的最佳方法是什么？

Answer 1

这很简单。你不需要做任何缓冲：

var fs = require('fs'),
    rs = fs.createReadStream('/path/to/file');

var CHUNK_SIZE = 8192;

rs.on('readable', function () {
  var chunk;
  while (null !== (chunk = rs.read(CHUNK_SIZE))) {
    console.log('got %d bytes of data', chunk.length);
  }
});

rs.on('end', function () {
  console.log('end');
});

如果CHUNK_SIZE大于内部缓冲区，则节点将返回null并缓冲一些，然后再次发出readable。您甚至可以通过传递：

来配置缓冲区的初始大小

var  rs = fs.createReadStream('/path/to/file', {highWatermark: CHUNK_SIZE});

Answer 2

以下是在流中读取文件的示例。

var fs = require('fs'),
readStream = fs.createReadStream(srcPath);

readStream.on('data', function (chunk) {
  console.log('got %d bytes of data', chunk.length);
});

readStream.on('readable', function () {
  var chunk;
  while (null !== (chunk = readStream.read())) {
   console.log('got %d bytes of data', chunk.length);
  }
});

readStream.on('end', function () {
  console.log('got all bytes of data');
});

ReadStream：内部缓冲区不再填满

2 个答案: