javascript - 如何进行递归 Promise 调用以作为 Javascript 中减少的返回？

Question

我正在尝试在 JS中创建一个递归Promise调用。reduce我的系统在这里的目标是对数组中的每个项目进行 n 次大调用reduce，然后，如果在其中reduce那个大调用决定它需要 n 个较小的调用才能返回到大的调用，那么reduce应该' t 跳到下一个项目，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;
}

Base_Ajax_Requester处理 AJAX 请求并在完成时返回一个帮助程序类在哪里，Promise以便可以围绕它编写代码。

我fake_data的是：

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

如您所见fake_data.steps_to_import.reduce(..，将遍历这 3 个值，对于它们中的每一个 call download_demo_file，等待它完成，然后继续下一个。我们可以说我想在之间elementor-hf和之间post进行 n 次较小的通话。

最初的调用download_demo_file是在那里看到的，这里总是会从后端返回：

{
    '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.
}

重试调用再次download_demo_file看起来像：

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

...依此类推，直到后端发送remaining_number_of_files : 0，然后一切都停止了，因为没有更多文件要下载，它可以跳到下一个大调用。

我怎样才能做到这一点？

标签： javascriptes6-promise

Answer 1

在每个reduce回调中，我会创建一个调用download_demo_file（使用更改file_counter）的函数，然后在 Promise 解决之后，递归返回对自身的调用 if remaining_number_of_files > 0。这将意味着getProm()它将不断地调用自己，直到remaining_number_of_files > 0不再满足条件，并且只有在那之后，该特定reduce迭代的整个 Promise 才会解决。

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/代码可能会更容易阅读和理解：asyncawait

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?

捕获异步函数的使用者。