我有一个 for 循环的 Promise 数组,所以我使用 Promise.all 来遍历它们,然后调用 then 。
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 和promise3 是一些http 请求,如果其中一个是应用程序/pdf,那么我将其写入流并解析其中的文本。但这段代码在解析 pdf 中的测试之前运行下一次迭代。有没有办法让代码等到流的管道和提取完成后再继续下一次迭代?
如果没有 async/await,那是相当令人讨厌的。使用 async/await,只需执行以下操作:
Promise.all(promises).then(async (responses) => {
for (...) {
await new Promise(resolve => stream.on("finish", resolve));
//extract the text out of the PDF
}
})
像下面这样的东西也可以工作。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();
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()
将
await
与 stream.pipeline()
一起使用,而不是 stream.pipe()
:
import * as StreamPromises from "stream/promises";
...
await StreamPromises.pipeline(sourceStream, destinationStream);
您可以将 else 部分写在自调用函数中。这样流的处理就会并行发生
(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)
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。
到目前为止,您正在创建 Promise 并将其添加到数组中,而不是将 Promise.then 添加到数组中(这也是一个 Promise)。然后在处理程序内部进行流处理。
我相信在将回调附加到
finish
事件后,您可以简单地为 Node.js 15+ 执行此操作...
const { finished } = require('node:stream/promises');
await finished(stream);
https://nodejs.org/api/stream.html#streamfinishedstream-options