在 node.js 中使用异步瀑布 [英] Using Async waterfall in node.js

问题描述

我有 2 个异步运行的函数.我想使用瀑布模型编写它们.问题是，我不知道如何..

I have 2 functions that I'm running asynchronously. I'd like to write them using waterfall model. The thing is, I don't know how..

这是我的代码:

var fs = require('fs');
function updateJson(ticker, value) {
  //var stocksJson = JSON.parse(fs.readFileSync("stocktest.json"));
  fs.readFile('stocktest.json', function(error, file) {
    var stocksJson =  JSON.parse(file);

    if (stocksJson[ticker]!=null) {
      console.log(ticker+" price : " + stocksJson[ticker].price);
      console.log("changing the value...")
      stocksJson[ticker].price =  value;

      console.log("Price after the change has been made -- " + stocksJson[ticker].price);
      console.log("printing the the Json.stringify")
      console.log(JSON.stringify(stocksJson, null, 4));
      fs.writeFile('stocktest.json', JSON.stringify(stocksJson, null, 4), function(err) {  
        if(!err) {
          console.log("File successfully written");
        }
        if (err) {
          console.error(err);
        }
      }); //end of writeFile
    } else {
      console.log(ticker + " doesn't exist on the json");
    }
  });
} // end of updateJson 

知道如何使用瀑布编写它，以便我能够控制它吗?请给我写一些例子，因为我是 node.js 的新手

Any idea how can I write it using waterfall, so i'll be able to control this? Please write me some examples because I'm new to node.js

推荐答案

先确定步骤，写成异步函数(带回调参数)

• 读取文件

  First identify the steps and write them as asynchronous functions (taking a callback argument)

  • read the file

    function readFile(readFileCallback) {
        fs.readFile('stocktest.json', function (error, file) {
            if (error) {
                readFileCallback(error);
            } else {
                readFileCallback(null, file);
            }
        });
    }
    

  • 处理文件(我删除了示例中的大部分 console.log)

  • process the file (I removed most of the console.log in the examples)

    function processFile(file, processFileCallback) {
        var stocksJson = JSON.parse(file);
        if (stocksJson[ticker] != null) {
            stocksJson[ticker].price = value;
            fs.writeFile('stocktest.json', JSON.stringify(stocksJson, null, 4), function (error) {
                if (err) {
                    processFileCallback(error);
                } else {
                    console.log("File successfully written");
                    processFileCallback(null);
                }
            });
        }
        else {
            console.log(ticker + " doesn't exist on the json");
            processFileCallback(null); //callback should always be called once (and only one time)
        }
    }
    

  请注意，我在这里没有做具体的错误处理，我将利用 async.waterfall 将错误处理集中在同一个地方.

  Note that I did no specific error handling here, I'll take benefit of async.waterfall to centralize error handling at the same place.

  还要注意，如果您在异步函数中有 (if/else/switch/...) 分支，它总是会调用回调一次(并且只调用一次).

  Also be careful that if you have (if/else/switch/...) branches in an asynchronous function, it always call the callback one (and only one) time.

  async.waterfall([
      readFile,
      processFile
  ], function (error) {
      if (error) {
          //handle readFile error or processFile error here
      }
  });
  

  干净的例子

  之前的代码过于冗长，使解释更清晰.这是一个完整的清理示例:

  Clean example

  The previous code was excessively verbose to make the explanations clearer. Here is a full cleaned example:

  async.waterfall([
      function readFile(readFileCallback) {
          fs.readFile('stocktest.json', readFileCallback);
      },
      function processFile(file, processFileCallback) {
          var stocksJson = JSON.parse(file);
          if (stocksJson[ticker] != null) {
              stocksJson[ticker].price = value;
              fs.writeFile('stocktest.json', JSON.stringify(stocksJson, null, 4), function (error) {
                  if (!err) {
                      console.log("File successfully written");
                  }
                  processFileCallback(err);
              });
          }
          else {
              console.log(ticker + " doesn't exist on the json");
              processFileCallback(null);
          }
      }
  ], function (error) {
      if (error) {
          //handle readFile error or processFile error here
      }
  });
  

  我保留了函数名称，因为它有助于提高可读性并有助于使用 chrome 调试器等工具进行调试.

  I left the function names because it helps readability and helps debugging with tools like chrome debugger.

  如果你使用 underscore (在 npm)，你也可以用 _.partial(fs.readFile, 'stocktest.json')

  If you use underscore (on npm), you can also replace the first function with _.partial(fs.readFile, 'stocktest.json')

  这篇关于在 node.js 中使用异步瀑布的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！