如何组织 async.waterfall 调用循环

Question

如何组织 async.waterfall 调用的循环 以便每次迭代只有在前一个完全完成后才开始。

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) {

});

它是这样工作的：

有更多数据吗？ (1)
读取数据(1)
有更多数据吗？ (2)
读取数据(2)
得到数据(1)
写入数据(1)
...

我明白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。但在我的具体情况下，我只需要在前一个迭代完成后才开始新的迭代：

有更多数据吗？ (1)
读取数据(1)
得到数据(1)
写入数据(1)
有更多数据吗？ (2)
读取数据(2)
...

PS：我无法将所有数据读入数组，然后用async.each处理数组。我也无法预测外部来源有多少数据。

Answer 1

我不使用 async，但仅查看代码和说明以及 API 文档，这似乎符合您的要求，根本没有使用 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（对 readData 和 writeData 使用垫片）：

// 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");
});

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<script src="//tjcrowder.github.io/simple-snippets-console/snippet.js"></script>
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