处理流时进行异步操作

Question

I need some help,

I need to read a file as a stream and on the first chunk I need to create a database table which is an async operation. So what I'd like to do is creating a duplex/transformer stream which would create the table when we are processing the start of the file but otherwise work as a normal stream.

As a pseudocode I thought something like this

fileLoaderService
  .loadfileAsStream()
  .pipe(parser)
  .pipe(
    if !headerProcessed
      createTable
    passThrough 
  )
  .pipe(insertToTable)

Is this possible?

Answer 1

You can use start function of ReadableStream controller to perform tasks before processing stream at pull

let stream = new ReadableStream({
  start(controller) {
     // create table
  },
  pull(controller) {
     // enqueue next chunk
  }
})