Nodejs在上一个功能完成之前触发功能 [英] Nodejs triggers function before previous function finish

问题描述

我需要做的是

1. 检查 fileFullPath 是否存在


2. 如果没有，在成功下载文件后，调用 saveInfo 。

1. to check if fileFullPath exist
2. if not, in the end of successfull file download, to call saveInfo.

当我执行应用程序时，观察到的是，在完成文件写入操作之前，它会调用 saveInfo 。我收到错误消息：

When I execute the application, what I observe is, it calls saveInfo before finishing file write operation. And I get error message:

(node:20224) UnhandledPromiseRejectionWarning: Error: BatchCluster has ended, cannot enqueue -charset

我在做什么错了？

async function dl(url, path, data = null) {

    await request.get({
        url: url,
    })
        .on("error", async function (error) {
            console.log(error);
            return false;
        })
        .on('response', async function (res) {
            var fileExt = res.headers['content-type'].split('/')[1];
            var fileFullPath = `${path}.${fileExt}`;
            await res.pipe(fs.createWriteStream(fileFullPath));
            console.log("file downloaded");
            if (data) {
                await saveInfo(fileFullPath, data);
            }
        });
    return true;

}

async function saveInfo(filePath, data) {
    await exiftool.write(filePath, {
        Keywords: data.keywords,
        Copyright: data.copyright,
    });
    console.log("Tags are saved");
    exiftool.end();
}

推荐答案

好的，我找到了一种方法去做这个。流到管道对诺言不是很友好，所以我最终做了一些手动的诺言操纵。我认为对node.js的流支持更好，因为我们已经有了一些异步迭代器。无论如何，这是一种通过监视流中正确的事件来使事情正常运行的方法：

OK, I found a way to do this. piping to streams is not very friendly to promises so I ended up doing some manual promise manipulations. I think better promise support for streams is coming to node.js as we already have some async iterators. Anyway, here's a way to make things work by watching for the right events on your streams:

function dl(url, path, data = null) {

    return new Promise((resolve, reject) => {
        request.get({
            url: url,
        }).on("error", function (error) {
                console.log(error);
                reject(error);
        }).on('response', function (res) {
            let fileExt = res.headers['content-type'].split('/')[1];
            let fileFullPath = `${path}.${fileExt}`;
            let writeStream = fs.createWriteStream(fileFullPath);

            // set up event handlers to monitor the writeStream for error or completion
            writeStream.on('error', reject).on('close', async () => {
                if (data) {
                    try {
                        await saveInfo(fileFullPath, data);
                    } catch(e) {
                        reject(e);
                        return;
                    }
                }
                console.log("file downloaded");
                resolve(true);
            });

            // send the response stream to our file
            res.pipe(writeStream).on('error', reject);
        });
    });
}

async function saveInfo(filePath, data) {
    await exiftool.write(filePath, {
        Keywords: data.keywords,
        Copyright: data.copyright,
    });
    console.log("Tags are saved");
    exiftool.end();
}

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