Node.js:如何在Write Stream'finish'事件上写()

Question

我正在使用Node.js流逐行浏览文本文件,进行一些转换并输出到SVG文件.

我正在尝试</svg>在处理完成后编写最后一段数据(),但是当写入流发出finish事件时,尝试write()将抛出Error: write after end.

有一种优雅的方式可以解决这个问题吗？

注意:输入文件很大(大约1GB),因此pipe()由于其I/O和内存管理而无法绕过该方法.

var fs = require('fs');
var split2 = require('split2');
var through2 = require('through2');

var read_stream = fs.createReadStream('input.txt');
var write_stream = fs.createWriteStream('output.svg');

write_stream.write('<svg>');
write_stream.on('finish', function() {
  this.write('</svg>'); // doesn't work
});

read_stream
  .pipe(split2())
  .pipe(through2.obj(function(line, encoding, next) {
     this.push(line);
     next();
  }))
  .pipe(write_stream);

解决方案

感谢Jordan和pNre帮助我解决这个问题.

解决方案1(通用)

pipe()带有end:false选项的写入流和手动end()流.

var fs = require('fs');
var split2 = require('split2');
var through2 = require('through2');

var read_stream = fs.createReadStream('input.txt');
var write_stream = fs.createWriteStream('output.svg');

write_stream.write('<svg>');

read_stream
  .pipe(split2())
  .pipe(through2.obj(function(line, encoding, next) {
     this.push(line);
     next();
  }))
  .pipe(write_stream, { end: false });

read_stream.on('end', function() {
  write_stream.end('</svg>');
});

解决方案2(特定于through/ through2转换流)

through2 具有可用于写入最终数据的刷新功能.

var fs = require('fs');
var split2 = require('split2');
var through2 = require('through2');

var read_stream = fs.createReadStream('input.txt');
var write_stream = fs.createWriteStream('output.svg');

write_stream.write('<svg>');

read_stream
  .pipe(split2())
  .pipe(through2.obj(function(line, encoding, next) {
    this.push(line);
    next();
  }, function(flush) {
    this.push('</svg>');
    flush();
  }))
  .pipe(write_stream);

Answer 1

看起来管道在完成时会关闭流.

http://nodejs.org/api/stream.html上的文档说明:

默认情况下,当源流发出结束时,在目标上调用end(),以便该目标不再可写.传递{end:false}作为保持目标流打开的选项.

这使作者保持开放状态,以便最后可以写出"再见".

reader.pipe(writer, { end: false });
reader.on('end', function() {
  writer.end('Goodbye\n');
});