我如何进行递归承诺调用，使其返回Javascript中的reduce？

Question

我试图在JS中的Promise中创建一个递归的reduce调用。我这里的系统的目标是对reduce播放的数组中的每一项进行n个大调用，然后，如果在reduce内部，那个大调用决定在返回大调用之前需要n个较小的调用，那么reduce不应该跳到下一个条目，只需要等待它完成。

我目前的代码是：

function download_preliminary_files_1() {
    let demo_schema_download = new Base_Ajax_Requester(
        localized_data.ajax_url,
        {
            'ajax_action': 'download_demo_file_schema',
            'ajax_nonce': localized_data.ajax_nonce,
            'backend': {
                'data_handle': 'download_demo_file_schema_data',
                'data':
                    {
                        'demo_handle' : 'demo-2',
                    }
            }
        }
    );

    let import_files_download = demo_schema_download.call().then(function() {
        fake_data.steps_to_import.reduce(function(previous_promise, next_step_identifier) {
            return previous_promise.then(function() {
                let file_download = download_demo_file({
                    'demo_handle' : 'demo-2',
                    'step' : next_step_identifier
                }).call();

                file_download.then(function(response) {
                    /**
                     * Here, if I detect that response.remaining_number_of_files > 0, I should start
                     * a chain that keeps calling `download_demo_file` with new parameters.
                     *
                     * Then, when this chain is done, resume the normal reduce behavior.
                     */
                });
            });
        }, Promise.resolve())
    }).catch(function(error) {
        console.log( 'Got this error:' + error);
    });

    return import_files_download;
}

其中Base_Ajax_Requester是一个帮助类，它处理AJAX请求并在完成时返回一个Promise，因此可以围绕它编写代码。

我的fake_data是：

let fake_data = {
    'demo_handle' : 'demo-2',
    'steps_to_import' : [ 'elementor-hf','post', 'nav_menu' ]
}

正如您所看到的，fake_data.steps_to_import.reduce(..将遍历这3个值，因为每个值都调用download_demo_file，等待它完成，然后继续到下一个值。我们可以说，我希望，在elementor-hf和post之间有一个较小的电话。

对download_demo_file的初始调用出现在上面，下面是从后端返回的内容：

{
    'message' : '...',
    'file_number' : 1, //Which -n.xml file has been downloaded where n is this number.
    'remaining_number_of_files' : 1 //Calculates, based on what file was downloaded how many files are left. The system knows internally how many files it has to download.
}

再次使用download_demo_file进行重试调用，如下所示：

{
    'demo_handle' : 'demo-2',
    'step' : next_step_identifier,
    'file_counter' : 2 //Dynamic. This will signal that it needs to now download file-2.xml.
}

...and是这样的，直到后端发送remaining_number_of_files : 0，然后一切都停止了，因为没有更多的文件要下载，而且它可以跳到下一个大调用。

我怎样才能做到这一点？

Answer 1

在每个reduce回调中，我都会创建一个函数来调用download_demo_file (通过一个不断变化的file_counter)，在这个承诺解决之后，递归地返回一个调用，如果是remaining_number_of_files > 0。这将意味着getProm()将不断地调用自己，直到remaining_number_of_files > 0条件不再满足为止，并且只有在此之后，对特定reduce迭代的整个承诺才会得到解决。

let import_files_download = demo_schema_download.call().then(function() {
  fake_data.steps_to_import.reduce(function(previous_promise, step) {
    let file_counter = 1;
    return previous_promise.then(function() {
      const getProm = () => download_demo_file({
        demo_handle: 'demo-2',
        step,
        file_counter
      }).call()
        .then((response) => {
          file_counter++;
          return response.remaining_number_of_files > 0
            ? getProm()
            : response
        });
      return getProm();
    });
  }, Promise.resolve())
}).catch(function(error) {
  console.log('Got this error:' + error);
});

但是，使用async/await来阅读和理解代码可能要容易得多：

await demo_schema_download.call();
const import_files_download = [];
for (const step of fake_data.steps_to_import) {
  let response;
  let file_counter = 1;
  do {
    response = await download_demo_file({
      demo_handle: 'demo-2',
      step,
      file_counter
    }).call();
    file_counter++;
  } while (response.remaining_number_of_files > 0);
  import_files_download.push(response); // is this needed?
}
return import_files_download; // is this needed?

在异步函数的使用者中捕获。