在 node.js 中对流使用 promise [英] Using promises with streams in node.js

问题描述

我重构了一个简单的实用程序来使用 promise.它从网上获取 pdf 并将其保存到磁盘.一旦保存到磁盘，它应该然后在 pdf 查看器中打开文件.该文件出现在磁盘上并且有效，shell 命令打开 OSX Preview 应用程序，但弹出一个对话框，抱怨该文件为空.

I've refactored a simple utility to use promises. It fetches a pdf from the web and saves it to disk. It should then open the file in a pdf viewer once saved to disk. The file appears on disk and is valid, the shell command opens the OSX Preview application, but a dialog pops up complaining that the file is empty.

将文件流写入磁盘后，执行 shell 函数的最佳方法是什么?

What's the best way to execute the shell function once the filestream has been written to disk?

// download a pdf and save to disk
// open pdf in osx preview for example
download_pdf()
  .then(function(path) {
    shell.exec('open ' + path).code !== 0);
  });

function download_pdf() {
  const path = '/local/some.pdf';
  const url = 'http://somewebsite/some.pdf';
  const stream = request(url);
  const write = stream.pipe(fs.createWriteStream(path))
  return streamToPromise(stream);
}

function streamToPromise(stream) {
  return new Promise(function(resolve, reject) {
    // resolve with location of saved file
    stream.on("end", resolve(stream.dests[0].path));
    stream.on("error", reject);
  })
}

推荐答案

在这一行

stream.on("end", resolve(stream.dests[0].path));

您正在立即执行 resolve，以及调用 resolve 的结果(这将是未定义的，因为这就是 resolve 返回)用作 stream.on 的参数 - 根本不是你想要的，对吧.

you are executing resolve immediately, and the result of calling resolve (which will be undefined, because that's what resolve returns) is used as the argument to stream.on - not what you want at all, right.

.on 的第二个参数需要是一个函数，而不是调用函数的结果

.on's second argument needs to be a function, rather than the result of calling a function

因此，代码需要

stream.on("end", () => resolve(stream.dests[0].path));

或者，如果你是老派:

stream.on("end", function () { resolve(stream.dests[0].path); });

另一种老派的方式就像

another old school way would be something like

stream.on("end", resolve.bind(null, stream.dests[0].path));

不，不要那样做 :p 看评论

No, don't do that :p see comments

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