NodeJS 如何使用ES 8异步/等待流?

nxowjjhe  于 2022-12-29  发布在  Node.js
关注(0)|答案(7)|浏览(236)

https://stackoverflow.com/a/18658613/779159中是一个如何使用内置加密库和流计算文件的md5的示例。

var fs = require('fs');
var crypto = require('crypto');

// the file you want to get the hash    
var fd = fs.createReadStream('/some/file/name.txt');
var hash = crypto.createHash('sha1');
hash.setEncoding('hex');

fd.on('end', function() {
    hash.end();
    console.log(hash.read()); // the desired sha1sum
});

// read all file and pipe it (write it) to the hash object
fd.pipe(hash);

但是,有没有可能将其转换为使用ES 8 async/await,而不是使用上面看到的回调,同时仍然保持使用流的效率?

disbfnqx

disbfnqx1#

async/await只适用于promise,不适用于streams。有一些想法可以创建一个额外的类似stream-like的数据类型,它将拥有自己的语法,但这些都是高度实验性的,我不会详细介绍。
无论如何,回调只是等待流的结束,这非常适合于承诺。您只需要 Package 流:

var fd = fs.createReadStream('/some/file/name.txt');
var hash = crypto.createHash('sha1');
hash.setEncoding('hex');
// read all file and pipe it (write it) to the hash object
fd.pipe(hash);

var end = new Promise(function(resolve, reject) {
    hash.on('end', () => resolve(hash.read()));
    fd.on('error', reject); // or something like that. might need to close `hash`
});

现在你可以等待承诺了

(async function() {
    let sha1sum = await end;
    console.log(sha1sum);
}());
ckocjqey

ckocjqey2#

如果您使用的节点版本〉= v10.0.0,那么您可以使用stream.pipeline和util.promisify。

const fs = require('fs');
const crypto = require('crypto');
const util = require('util');
const stream = require('stream');

const pipeline = util.promisify(stream.pipeline);

const hash = crypto.createHash('sha1');
hash.setEncoding('hex');

async function run() {
  await pipeline(
    fs.createReadStream('/some/file/name.txt'),
    hash
  );
  console.log('Pipeline succeeded');
}

run().catch(console.error);
5f0d552i

5f0d552i3#

节点V15现在在stream/promises中有promisfiy管道,这是最干净最正式的方式。

const { pipeline } = require('stream/promises');

async function run() {
  await pipeline(
    fs.createReadStream('archive.tar'),
    zlib.createGzip(),
    fs.createWriteStream('archive.tar.gz')
  );
  console.log('Pipeline succeeded.');
}

run().catch(console.error);

我们都应该感谢它在这里做了多少工作:

  • 捕获所有流中的错误。
  • 引发错误时销毁未完成的流。
  • 仅当最后一个可写流完成时返回。

这个管道是Node.JS最强大的特性之一。让它完全异步并不容易。现在我们有了它。

rur96b6h

rur96b6h4#

类似这样的方法是可行的:

for (var res of fetchResponses){ //node-fetch package responses
    const dest = fs.createWriteStream(filePath,{flags:'a'});
    totalBytes += Number(res.headers.get('content-length'));
    await new Promise((resolve, reject) => {
        res.body.pipe(dest);
        res.body.on("error", (err) => {
            reject(err);
        });
        dest.on("finish", function() {
            resolve();
        });
    });         
}
zzoitvuj

zzoitvuj5#

2021年更新:
节点文档中的新示例:

async function print(readable) {
  readable.setEncoding('utf8');
  let data = '';
  for await (const chunk of readable) {
    data += chunk;
  }
  console.log(data);
}

see https://nodejs.org/api/stream.html#stream_readable_symbol_asynciterator

fwzugrvs

fwzugrvs6#

我想评论一下,但是我没有足够的声誉。
提醒一句:如果你有一个应用程序正在传递流并且执行async/await,在你等待之前要非常小心地连接所有的管道。你可能最终得到的流并不包含你认为它们所做的事情。

const { PassThrough } = require('stream');

async function main() {
    const initialStream = new PassThrough();

    const otherStream = new PassThrough();
    const data = [];
    otherStream.on('data', dat => data.push(dat));
    const resultOtherStreamPromise = new Promise(resolve => otherStream.on('end', () => { resolve(Buffer.concat(data)) }));

    const yetAnotherStream = new PassThrough();
    const data2 = [];
    yetAnotherStream.on('data', dat => data2.push(dat));
    const resultYetAnotherStreamPromise = new Promise(resolve => yetAnotherStream.on('end', () => { resolve(Buffer.concat(data2)) }));

    initialStream.pipe(otherStream);
    initialStream.write('some ');

    await Promise.resolve(); // Completely unrelated await

    initialStream.pipe(yetAnotherStream);
    initialStream.end('data');
    const [resultOtherStream, resultYetAnotherStream] = await Promise.all([
        resultOtherStreamPromise,
        resultYetAnotherStreamPromise,
    ]);

    console.log('other stream:', resultOtherStream.toString()); // other stream: some data
    console.log('yet another stream:', resultYetAnotherStream.toString()); // yet another stream: data
}
main();
bakd9h0s

bakd9h0s7#

我相信它会对某人有所帮助:

async function readFile(filename) {
    let records = []
    return new Promise(resolve => {
        fs.createReadStream(filename)
            .on("data", (data) => {
                records.push(data);
            })
            .on("end", () => {
                resolve(records)
            });
    })
}

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