深承诺.all

Question

我有一个以保存文件列表结尾的承诺链。我想等到所有文件都保存或失败，然后继续我的链。问题是，当我在具有自己的子链的 Promise 列表中使用 Promise.all 时，Promise.all 上的 thens 在 Promise.all 内的 Promise 上的 thens 之前开始解决.请参阅下面的注释示例。

const fs = require('fs');
const util = require('util');
// Make a promise out of the file write function
const promiseWriteFile = (file, data, options) => {
    return util.promisify(fs.writeFile).call(fs, file, data, options);
};

...

console.log('Received files');
console.group();
// Start long promise chain
somePromise(...)
    .then((result) => {
        console.log('validating blah blah');
    })
    .then((result) => {
        ...
    })
    .then((result) => {
        // Now I need to save the files to the disk
        let path = './uploaded_files/';
        // Here I want to resolve all the file save promises, or find one that fails
        return Promise.all(
            request.files.map((file) => {
                let filename = path + Date.now() + '_' + file.originalname;
                // Purposely change `path` so that the next file write will fail
                path = './garbage/';
                console.log('Trying to save file:', filename);
                return promiseWriteFile(filename, file.buffer, 'binary')
                    .then(() => {
                        console.log('Saving file:', filename);
                    })
                    .catch((error) => {
                        console.log('Could not save file:', filename);
                        throw error;
                    });
            }),
        );
    })
    .then(() => { // <======= I don't want this to happen until the promises in the `Promise.all` above have fully, DEEPLY resolved
        // set success message
        message = 'Part successfully created.';
        console.groupEnd();
        console.log('Part created successfully.');
    })
    .catch((exception) => {
        message = exception.message;
        console.groupEnd();
        console.log('Part invalid.');
    });

输出如下：

Received files
  validating blah blah
  Trying to save file: ./uploaded_files/A.txt
  Trying to save file: ./garbage/B.txt
Part invalid.
Could not save file: ./garbage/B.txt
Saving file: ./uploaded_files/A.txt

如您所见，打印“无法保存文件”和“保存文件”的行在大承诺链的 then/catch 之后执行。你可以知道，因为console.groupEnd()首先被调用，然后有进一步的输出。

如何确保在所有子 Promise 都完成之前，大 Promise 链上的 then/catch 不会发生？

Answer 1

您在 map 的 catch 块中重新抛出错误

.catch((error) => {
  console.log('Could not save file:', filename);
  throw error;
});

这会破坏您的Promise.all()，并导致它在完成所有承诺之前陷入困境。相反，您应该只是转发该错误而不是使错误链崩溃。您需要重新考虑如何确认所有写入以及以下步骤。

稍后处理退货的示例：

    .then((result) => {
        let path = './uploaded_files/';
        return Promise.all(
            request.files.map((file) => {
                let filename = path + Date.now() + '_' + file.originalname;
                path = './garbage/';
                console.log('Trying to save file:', filename);
                return promiseWriteFile(filename, file.buffer, 'binary')
                    .then(() => {
                        return  { filename };
                    })
                    .catch((error) => {
                        return { error, filename };
                    });
            }),
        );
    })
    .then((results) => {
        const successes = results.filter(({ error }) => !error);
        const failures = results.filter(({ error }) => error);
        // process them here
    })

现在您可以根据需要处理它们，并且每个对象在返回中都有一个文件名，让您可以识别它们，删除它们，随心所欲地使用它们。

Answer 2

当您在 promise.all 上使用 .catch 时，它将为您添加的每个子 Promise 添加 catch。没有“catchFinally”方法。

要阻止其他承诺发生，请查看以下示例：

Promise.config({ cancellation: true }); // <-- enables this non-standard feature

const promise1 = new Promise((resolve, reject) => {
    setTimeout(resolve, 1000, 'resolve1');
}).then(a => { console.log('then1'); return a; });

const promise2 = new Promise((resolve, reject) => {
    setTimeout(reject, 2000, 'reject2');
}).then(a => { console.log('then2'); return a; });

const promise3 = new Promise((resolve, reject) => {
    setTimeout(resolve, 3000, 'resolve3');
}).then(a => { console.log('then3'); return a; });

const promises = [promise1, promise2, promise3];

Promise.all(promises)
    .then(values => { 
        console.log('then', values); 
    })
    .catch(err => { 
        console.log('catch', err); 
        promises.forEach(p => p.cancel()); // <--- Does not work with standard promises
    });

请注意，即使 promise3 被取消，它的 setTimeout 回调仍然会被调用。但它不会触发 then 或 catch 回调。就好像这个承诺永远不会成为一个决议......永远。