在node.js中使用promises与流

Question

我重构了一个使用promises的简单实用程序.它从Web获取pdf并将其保存到磁盘.然后它应该保存到磁盘后在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预览应用程序,但会弹出一个对话框,抱怨该文件为空.

将文件流写入磁盘后,执行shell函数的最佳方法是什么？

// 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);
  })
}

Answer 1

在这一行

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

你正在resolve立即执行,并且调用的结果resolve(将是未定义的,因为那是resolve返回的)被用作参数stream.on- 而不是你想要的,对吧.

.on第二个参数需要是一个函数,而不是调用函数的结果

因此,代码需要

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

或者,如果你是老学校:

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

另一种老派的方式就像是

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

不,不要那样做:p看评论

Answer 2

在最新的 nodejs 中，特别是流 v3，你可以这样做：

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

const rs = fs.createReadStream('archive.tar');

async function run() {
  await finished(rs);
  console.log('Stream is done reading.');
}

run().catch(console.error);
rs.resume(); // Drain the stream.

https://nodejs.org/api/stream.html#stream_event_finish

Answer 3

经过一堆尝试，我找到了一个始终有效的解决方案。有关更多信息，请参见JSDoc注释。

/**
 * Streams input to output and resolves only after stream has successfully ended.
 * Closes the output stream in success and error cases.
 * @param input {stream.Readable} Read from
 * @param output {stream.Writable} Write to
 * @return Promise Resolves only after the output stream is "end"ed or "finish"ed.
 */
function promisifiedPipe(input, output) {
    let ended = false;
    function end() {
        if (!ended) {
            ended = true;
            output.close && output.close();
            input.close && input.close();
            return true;
        }
    }

    return new Promise((resolve, reject) => {
        input.pipe(output);
        input.on('error', errorEnding);

        function niceEnding() {
            if (end()) resolve();
        }

        function errorEnding(error) {
            if (end()) reject(error);
        }

        output.on('finish', niceEnding);
        output.on('end', niceEnding);
        output.on('error', errorEnding);
    });
};

用法示例：

function downloadFile(req, res, next) {
  promisifiedPipe(fs.createReadStream(req.params.file), res).catch(next);
}