Javascript 如何在流中使用 ES8 async/await？
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How to use ES8 async/await with streams?
提问by user779159
In https://stackoverflow.com/a/18658613/779159is an example of how to calculate the md5 of a file using the built-in crypto library and streams.
在https://stackoverflow.com/a/18658613/779159 中是如何使用内置加密库和流计算文件 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);
But is it possible to convert this to using ES8 async/await instead of using the callback as seen above, but while still keeping the efficiency of using streams?
但是是否可以将其转换为使用 ES8 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？
回答by Bergi
async/awaitonly works with promises, not with streams. There are ideas to make an extra stream-like data type that would get its own syntax, but those are highly experimental if at all and I won't go into details.
async/await仅适用于承诺，不适用于流。有一些想法可以创建一个额外的类似流的数据类型，以获得自己的语法，但如果有的话，这些都是高度实验性的，我不会详细介绍。
Anyway, your callback is only waiting for the end of the stream, which is a perfect fit for a promise. You'd just have to wrap the stream:
无论如何，您的回调只是在等待流结束，这非常适合承诺。你只需要包装流：
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`
});
Now you can await that promise:
现在你可以等待那个承诺：
(async function() {
let sha1sum = await end;
console.log(sha1sum);
}());
回答by shusson
If you are using node version >= v10.0.0 then you can use stream.pipelineand util.promisify.
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);
回答by Ronnie Royston
Something like this works:
像这样的工作：
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();
});
});
}
