如何等待流完成管道？ （的NodeJS）

I have a for loop array of promises, so I used Promise.all to go through them and called then afterwards.

我有一个for循环数组，所以我使用Promise.all来完成它们然后调用。

let promises = [];
promises.push(promise1);
promises.push(promise2);
promises.push(promise3);

Promise.all(promises).then((responses) => {
  for (let i = 0; i < promises.length; i++) {
    if (promise.property === something) {
      //do something
    } else {
      let file = fs.createWriteStream('./hello.pdf');
      let stream = responses[i].pipe(file);
      /*
         I WANT THE PIPING AND THE FOLLOWING CODE 
         TO RUN BEFORE NEXT ITERATION OF FOR LOOP
      */
      stream.on('finish', () => {
        //extract the text out of the pdf
        extract(filePath, {splitPages: false}, (err, text) => {
        if (err) {
          console.log(err);
        } else {
          arrayOfDocuments[i].text_contents = text;
        }
      });
    });    
  }
}

promise1, promise2, and promise3 are some http requests, and if one of them is an application/pdf, then I write it to a stream and parse the text out of it. But this code runs the next iteration before parsing the test out of the pdf. Is there a way to make the code wait until the piping to the stream and extracting are finished before moving on to the next iteration?

promise1，promise2和promise3是一些http请求，如果其中一个是application / pdf，那么我将它写入一个流并解析它的文本。但是这段代码在解析pdf之外的测试之前会运行下一次迭代。有没有办法让代码等到流到管道和提取的管道完成后再继续下一次迭代？

2 个解决方案

#1

Something like the following would also work. I use this pattern fairly often:

像下面这样的东西也会起作用。我经常使用这种模式：

let promises = [];
promises.push(promise1);
promises.push(promise2);
promises.push(promise3);

function doNext(){
  if(!promises.length) return;
  promises.shift().then((resolved) =>{
    if(resolved.property === something){
      ...
      doNext();
    }else{
      let file = fs.createWriteStream('./hello.pdf');
      let stream = resolved.pipe(file);
      stream.on('finish', () =>{
        ...
        doNext();
      });
    }

  })
}
doNext();

or break up the handler to a controller and Promisified handler:

或者将处理程序分解为控制器和Promisified处理程序：

function streamOrNot(obj){
  return new Promise(resolve, reject){
    if(obj.property === something){
      resolve();
      return;
    }
    let file = fs.createWriteStream...;
    stream.on('finish', () =>{
      ...
      resolve();
    });
  }
}

function doNext(){
  if(!promises.length) return;
  return promises.shift().then(streamOrNot).then(doNext);
}

doNext()

#2

You can write the else part inside a self invoked function. So that the handling of stream will happen in parallel

您可以在自调用函数中编写else部分。这样流的处理将并行发生

(function(i) {
    let file = fs.createWriteStream('./hello.pdf');
    let stream = responses[i].pipe(file);
  /*
     I WANT THE PIPING AND THE FOLLOWING CODE 
     TO RUN BEFORE NEXT ITERATION OF FOR LOOP
  */
    stream.on('finish', () => {
      //extract the text out of the pdf
      extract(filePath, {splitPages: false}, (err, text) => {
      if (err) {
        console.log(err);
      } 
      else {
        arrayOfDocuments[i].text_contents = text;
      }
    });
  });    
})(i) 

Else you can handle the streaming part as part of the original/individual promise itself.

否则，您可以将流媒体部分作为原始/个人承诺本身的一部分来处理。

As of now you are creating the promise and adding it to array, instead of that you add promise.then to the array(which is also a promise). And inside the handler to then you do your streaming stuff.

截至目前，您正在创建承诺并将其添加到数组，而不是将promise.then添加到数组（这也是一个承诺）。然后在处理程序内部进行流式处理。