如何组织 async.waterfall 调用的循环

How to organize a loop of async.waterfall calls

本文关键字：循环 调用 waterfall 何组织 async      更新时间：2023-09-26

如何组织async.waterfall调用的循环，以便每次迭代仅在前一个完全完成后启动。

到目前为止，我有以下代码：

var hasMoreData = true;
async.whilst(function () {
  // * has more data?
  return hasMoreData;
},
function processData(callback1) {
  async.waterfall([
    function readDataFromSource(callback2) {
      // * read data
      readData(function readFinished(data, hasMore) {
        // * got data 
        hasMoreData = hasMore;
        callback2(null, data);
      });
    }, 
    function writeDataToDest(data, callback2) {
      // * write data
      writeData(data, function writeFinished() {
        callback2();
      });
    },
  ], function (err) {
    callback1(err);
  });
},
function finished(err) {
});

它的工作原理是这样的：

  • 有更多数据 ？(1(
  • 读取数据 (1(
  • 有更多数据 ？(二(
  • 读取数据 (2(
  • 获取数据 (1(
  • 写入数据 (1(
  • 。

我理解为什么它按这个顺序工作，在某些情况下这可能是完全正确的。但是在我的特定情况下，我只需要在前一个迭代完成后才开始新的迭代：

  • 有更多数据 ？(1(
  • 读取数据 (1(
  • 获取数据 (1(
  • 写入数据 (1(
  • 有更多数据 ？(二(
  • 读取数据 (2(
  • 。

PS：例如，我无法将所有数据读取到数组中，然后使用 async.each 处理数组。我也无法预测外部源有多少数据。

我不使用

async ，但只看代码和描述以及 API 文档，似乎这符合您的要求，根本不使用waterfall：

var hasMoreData = true;
async.whilst(function () {
  // * has more data?
  return hasMoreData;
},
function processData(callback1) {
  // * read data
  // blocking i/o operation
  readData(function readFinished(data, hasMore) {
    // * got data
    hasMoreData = hasMore;
    // * write data
    // blocking i/o operation
    writeData(data, function writeFinished() {
      callback1();
    });
  });
},
function finished(err) {
});

实时示例(使用填充程序进行readData和writeData(：

// Shims
var datacount = 0;
function readData(callback) {
  // since you said "blocking", busy-wait for a quarter second
  var done = Date.now() + 250;
  while (done > Date.now()) {
  }
  // but calls like this are almost always async, so we'll complete async
  ++datacount;
  setTimeout(function() {
    callback(datacount, datacount < 3);
  }, 0);
}
function writeData(data, callback) {
  // since you said "blocking", busy-wait for a quarter second
  var done = Date.now() + 250;
  while (done > Date.now()) {
  }
  // but calls like this are almost always async, so we'll complete async
  setTimeout(function() {
    callback();
  }, 0);
}
// The code
var hasMoreData = true;
async.whilst(function() {
  // * has more data?
  snippet.log("has more data?");
  return hasMoreData;
},
function processData(callback1) {
  // * read data
  snippet.log("read data");
  // blocking i/o operation
  readData(function readFinished(data, hasMore) {
    // * got data
    snippet.log("got data: " + data + " (more? " + hasMore + ")");
    hasMoreData = hasMore;
    // * write data
    snippet.log("write data");
    // blocking i/o operation
    writeData(data, function writeFinished() {
      callback1();
    });
  });
},
function finished(err) {
  snippet.log("finished");
});
<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="//tjcrowder.github.io/simple-snippets-console/snippet.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/async/1.5.0/async.min.js"></script>

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