javascript - 如何延迟流读取调用

Question

我仍在尝试通过一般的streams 来理解我的方式。我已经能够使用 multiparty 流式传输大文件来自 form.on('part')。但我需要推迟调用并在读取之前解析流。我已经尝试过 PassThrough、through。 through2，但得到了不同的结果，主要是挂起，我不知道该做什么，也不知道调试步骤。我对所有选择持开放态度。感谢所有见解。

import multiparty from 'multiparty'
import {
  PassThrough
} from 'stream';
import through from 'through'
import through2 from 'through2'

export function promisedMultiparty(req) {
  return new Promise((resolve, reject) => {

    const form = new multiparty.Form()
    const form_files = []
    let q_str = ''

    form.on('field', (fieldname, value) => {
      if (value) q_str = appendQStr(fieldname, value, q_str)
    })

    form.on('part', async (part) => {
      if (part.filename) {

        const pass1 = new PassThrough() // this hangs at 10% 

        const pass2 = through(function write(data) { // this hangs from the beginning
            this.queue(data)
          },
          function end() {
            this.queue(null)
          })

        const pass3 = through2() // this hangs at 10%

        /* 
            // This way works for large files, but I want to defer 
            // invocation

            const form_data = new FormData()
            form_data.append(savepath, part, {
              filename,
            })

            const r = request.post(url, {
              headers: {
                'transfer-encoding': 'chunked'
              }
            }, responseCallback(resolve))
            r._form = form

        */

        form_files.push({
          part: part.pipe(pass1),
          // part: part.pipe(pass2),
          // part: part.pipe(pass3),
        })

      } else {
        part.resume()
      }
    })

    form.on('close', () => {
      resolve({
        fields: qs.parse(q_str),
        forms: form_files,
      })
    })

    form.parse(req)
  })
}

附注如果有人可以使用正确的术语，请肯定标题会更好。谢谢。

Answer 1

我相信这是因为您没有正确使用 through2 - 即，一旦缓冲区已满，实际上并没有清空缓冲区(这就是为什么它在较大的文件上挂起 10%，但在较小的文件上有效)。

我相信像这样的实现应该做到这一点:

const pass2 = through2(function(chunk, encoding, next) {

   // do something with the data


   // Use this only if you want to send the data further to another stream reader 
   // Note - From your implementation you don't seem to need it
   // this.push(data)

   // This is what tells through2 it's ready to empty the 
   //  buffer and read more data
   next();
})