淺談Node.js之異步流控制

更新時間：2017年10月25日 08:54:27 作者：馬在路上

本篇文章主要介紹了淺談Node.js之異步流控制，小編覺得挺不錯的，現(xiàn)在分享給大家，也給大家做個參考。一起跟隨小編過來看看吧

前言

在沒有深度使用函數(shù)回調(diào)的經(jīng)驗的時候，去看這些內(nèi)容還是有一點吃力的。由于Node.js獨特的異步特性，才出現(xiàn)了“回調(diào)地獄”的問題，這篇文章中，我比較詳細(xì)的記錄了如何解決異步流問題。

文章會很長，而且這篇是對異步流模式的解釋。文中會使用一個簡單的網(wǎng)絡(luò)蜘蛛的例子，它的作用是抓取指定URL的網(wǎng)頁內(nèi)容并保存在項目中，在文章的最后，可以找到整篇文章中的源碼demo。

1.原生JavaScript模式

本篇不針對初學(xué)者，因此會省略掉大部分的基礎(chǔ)內(nèi)容的講解：

(spider_v1.js)

const request = require("request");
const fs = require("fs");
const mkdirp = require("mkdirp");
const path = require("path");
const utilities = require("./utilities");

function spider(url, callback) {
  const filename = utilities.urlToFilename(url);
  console.log(`filename: ${filename}`);

  fs.exists(filename, exists => {
    if (!exists) {
      console.log(`Downloading ${url}`);

      request(url, (err, response, body) => {
        if (err) {
          callback(err);
        } else {
          mkdirp(path.dirname(filename), err => {
            if (err) {
              callback(err);
            } else {
              fs.writeFile(filename, body, err => {
                if (err) {
                  callback(err);
                } else {
                  callback(null, filename, true);
                }
              });
            }
          });
        }
      });
    } else {
      callback(null, filename, false);
    }
  });
}

spider(process.argv[2], (err, filename, downloaded) => {
  if (err) {
    console.log(err);
  } else if (downloaded) {
    console.log(`Completed the download of ${filename}`);
  } else {
    console.log(`${filename} was already downloaded`);
  }
});

上邊的代碼的流程大概是這樣的：

把url轉(zhuǎn)換成filename
判斷該文件名是否存在，若存在直接返回，否則進(jìn)入下一步
發(fā)請求，獲取body
把body寫入到文件中

這是一個非常簡單版本的蜘蛛，他只能抓取一個url的內(nèi)容，看到上邊的回調(diào)多么令人頭疼。那么我們開始進(jìn)行優(yōu)化。

首先，if else 這種方式可以進(jìn)行優(yōu)化，這個很簡單，不用多說，放一個對比效果：

/// before
if (err) {
  callback(err);
} else {
  callback(null, filename, true);
}

/// after
if (err) {
  return callback(err);
}
callback(null, filename, true);

代碼這么寫，嵌套就會少一層，但經(jīng)驗豐富的程序員會認(rèn)為，這樣寫過重強(qiáng)調(diào)了error，我們編程的重點應(yīng)該放在處理正確的數(shù)據(jù)上，在可讀性上也存在這樣的要求。

另一個優(yōu)化是函數(shù)拆分，上邊代碼中的spider函數(shù)中，可以把下載文件和保存文件拆分出去。

(spider_v2.js)

const request = require("request");
const fs = require("fs");
const mkdirp = require("mkdirp");
const path = require("path");
const utilities = require("./utilities");

function saveFile(filename, contents, callback) {
  mkdirp(path.dirname(filename), err => {
    if (err) {
      return callback(err);
    }
    fs.writeFile(filename, contents, callback);
  });
}

function download(url, filename, callback) {
  console.log(`Downloading ${url}`);

  request(url, (err, response, body) => {
    if (err) {
      return callback(err);
    }
    saveFile(filename, body, err => {
      if (err) {
        return callback(err);
      }
      console.log(`Downloaded and saved: ${url}`);
      callback(null, body);
    });
  })
}

function spider(url, callback) {
  const filename = utilities.urlToFilename(url);
  console.log(`filename: ${filename}`);

  fs.exists(filename, exists => {
    if (exists) {
      return callback(null, filename, false);
    }
    download(url, filename, err => {
      if (err) {
        return callback(err);
      }
      callback(null, filename, true);
    })
  });
}

spider(process.argv[2], (err, filename, downloaded) => {
  if (err) {
    console.log(err);
  } else if (downloaded) {
    console.log(`Completed the download of ${filename}`);
  } else {
    console.log(`${filename} was already downloaded`);
  }
});

上邊的代碼基本上是采用原生優(yōu)化后的結(jié)果，但這個蜘蛛的功能太過簡單，我們現(xiàn)在需要抓取某個網(wǎng)頁中的所有url，這樣才會引申出串行和并行的問題。

(spider_v3.js)

const request = require("request");
const fs = require("fs");
const mkdirp = require("mkdirp");
const path = require("path");
const utilities = require("./utilities");

function saveFile(filename, contents, callback) {
  mkdirp(path.dirname(filename), err => {
    if (err) {
      return callback(err);
    }
    fs.writeFile(filename, contents, callback);
  });
}

function download(url, filename, callback) {
  console.log(`Downloading ${url}`);

  request(url, (err, response, body) => {
    if (err) {
      return callback(err);
    }
    saveFile(filename, body, err => {
      if (err) {
        return callback(err);
      }
      console.log(`Downloaded and saved: ${url}`);
      callback(null, body);
    });
  })
}

/// 最大的啟發(fā)是實現(xiàn)了如何異步循環(huán)遍歷數(shù)組
function spiderLinks(currentUrl, body, nesting, callback) {
  if (nesting === 0) {
    return process.nextTick(callback);
  }

  const links = utilities.getPageLinks(currentUrl, body);

  function iterate(index) {
    if (index === links.length) {
      return callback();
    }
    spider(links[index], nesting - 1, err => {
      if (err) {
        return callback(err);
      }
      iterate((index + 1));
    })
  }

  iterate(0);
}

function spider(url, nesting, callback) {
  const filename = utilities.urlToFilename(url);

  fs.readFile(filename, "utf8", (err, body) => {
    if (err) {
      if (err.code !== 'ENOENT') {
        return callback(err);
      }
      return download(url, filename, (err, body) => {
        if (err) {
          return callback(err);
        }
        spiderLinks(url, body, nesting, callback);
      });
    }

    spiderLinks(url, body, nesting, callback);
  });
}

spider(process.argv[2], 2, (err, filename, downloaded) => {
  if (err) {
    console.log(err);
  } else if (downloaded) {
    console.log(`Completed the download of ${filename}`);
  } else {
    console.log(`${filename} was already downloaded`);
  }
});

上邊的代碼相比之前的代碼多了兩個核心功能，首先是通過輔助類獲取到了某個body中的links：

const links = utilities.getPageLinks(currentUrl, body);

內(nèi)部實現(xiàn)就不解釋了，另一個核心代碼就是：

/// 最大的啟發(fā)是實現(xiàn)了如何異步循環(huán)遍歷數(shù)組
function spiderLinks(currentUrl, body, nesting, callback) {
  if (nesting === 0) {
    return process.nextTick(callback);
  }

  const links = utilities.getPageLinks(currentUrl, body);

  function iterate(index) {
    if (index === links.length) {
      return callback();
    }
    spider(links[index], nesting - 1, err => {
      if (err) {
        return callback(err);
      }
      iterate((index + 1));
    })
  }

  iterate(0);
}

可以說上邊這一小段代碼，就是采用原生實現(xiàn)異步串行的pattern了。除了這些之外，還引入了nesting的概念，通過這是這個屬性，可以控制抓取層次。

到這里我們就完整的實現(xiàn)了串行的功能，考慮到性能，我們要開發(fā)并行抓取的功能。

(spider_v4.js)

const request = require("request");
const fs = require("fs");
const mkdirp = require("mkdirp");
const path = require("path");
const utilities = require("./utilities");

function saveFile(filename, contents, callback) {
  mkdirp(path.dirname(filename), err => {
    if (err) {
      return callback(err);
    }
    fs.writeFile(filename, contents, callback);
  });
}

function download(url, filename, callback) {
  console.log(`Downloading ${url}`);

  request(url, (err, response, body) => {
    if (err) {
      return callback(err);
    }
    saveFile(filename, body, err => {
      if (err) {
        return callback(err);
      }
      console.log(`Downloaded and saved: ${url}`);
      callback(null, body);
    });
  })
}

/// 最大的啟發(fā)是實現(xiàn)了如何異步循環(huán)遍歷數(shù)組
function spiderLinks(currentUrl, body, nesting, callback) {
  if (nesting === 0) {
    return process.nextTick(callback);
  }

  const links = utilities.getPageLinks(currentUrl, body);
  if (links.length === 0) {
    return process.nextTick(callback);
  }

  let completed = 0, hasErrors = false;

  function done(err) {
    if (err) {
      hasErrors = true;
      return callback(err);
    }

    if (++completed === links.length && !hasErrors) {
      return callback();
    }
  }

  links.forEach(link => {
    spider(link, nesting - 1, done);
  });
}

const spidering = new Map();

function spider(url, nesting, callback) {
  if (spidering.has(url)) {
    return process.nextTick(callback);
  }

  spidering.set(url, true);

  const filename = utilities.urlToFilename(url);

  /// In this pattern, there will be some issues.
  /// Possible problems to download the same url again and again。
  fs.readFile(filename, "utf8", (err, body) => {
    if (err) {
      if (err.code !== 'ENOENT') {
        return callback(err);
      }
      return download(url, filename, (err, body) => {
        if (err) {
          return callback(err);
        }
        spiderLinks(url, body, nesting, callback);
      });
    }

    spiderLinks(url, body, nesting, callback);
  });
}

spider(process.argv[2], 2, (err, filename, downloaded) => {
  if (err) {
    console.log(err);
  } else if (downloaded) {
    console.log(`Completed the download of ${filename}`);
  } else {
    console.log(`${filename} was already downloaded`);
  }
});

這段代碼同樣很簡單，也有兩個核心內(nèi)容。一個是如何實現(xiàn)并發(fā)：

/// 最大的啟發(fā)是實現(xiàn)了如何異步循環(huán)遍歷數(shù)組
function spiderLinks(currentUrl, body, nesting, callback) {
  if (nesting === 0) {
    return process.nextTick(callback);
  }

  const links = utilities.getPageLinks(currentUrl, body);
  if (links.length === 0) {
    return process.nextTick(callback);
  }

  let completed = 0, hasErrors = false;

  function done(err) {
    if (err) {
      hasErrors = true;
      return callback(err);
    }

    if (++completed === links.length && !hasErrors) {
      return callback();
    }
  }

  links.forEach(link => {
    spider(link, nesting - 1, done);
  });
}

上邊的代碼可以說是實現(xiàn)并發(fā)的一個pattern。利用循環(huán)遍歷來實現(xiàn)。另一個核心是，既然是并發(fā)的，那么利用 fs.exists 就會存在問題，可能會重復(fù)下載同一文件，這里的解決方案是：

使用Map緩存某一url，url應(yīng)該作為key

現(xiàn)在我們又有了新的需求，要求限制同時并發(fā)的最大數(shù)，那么在這里就引進(jìn)了一個我認(rèn)為最重要的概念：隊列。

(task-Queue.js)

class TaskQueue {
  constructor(concurrency) {
    this.concurrency = concurrency;
    this.running = 0;
    this.queue = [];
  }

  pushTask(task) {
    this.queue.push(task);
    this.next();
  }

  next() {
    while (this.running < this.concurrency && this.queue.length) {
      const task = this.queue.shift();
      task(() => {
        this.running--;
        this.next();
      });
      this.running++;
    }
  }
}

module.exports = TaskQueue;

上邊的代碼就是隊列的實現(xiàn)代碼，核心是 next() 方法，可以看出，當(dāng)task加入隊列中后，會立刻執(zhí)行，這不是說這個任務(wù)一定馬上執(zhí)行，而是指的是next會立刻調(diào)用。

(spider_v5.js)

const request = require("request");
const fs = require("fs");
const mkdirp = require("mkdirp");
const path = require("path");
const utilities = require("./utilities");
const TaskQueue = require("./task-Queue");
const downloadQueue = new TaskQueue(2);

function saveFile(filename, contents, callback) {
  mkdirp(path.dirname(filename), err => {
    if (err) {
      return callback(err);
    }
    fs.writeFile(filename, contents, callback);
  });
}

function download(url, filename, callback) {
  console.log(`Downloading ${url}`);

  request(url, (err, response, body) => {
    if (err) {
      return callback(err);
    }
    saveFile(filename, body, err => {
      if (err) {
        return callback(err);
      }
      console.log(`Downloaded and saved: ${url}`);
      callback(null, body);
    });
  })
}

/// 最大的啟發(fā)是實現(xiàn)了如何異步循環(huán)遍歷數(shù)組
function spiderLinks(currentUrl, body, nesting, callback) {
  if (nesting === 0) {
    return process.nextTick(callback);
  }

  const links = utilities.getPageLinks(currentUrl, body);
  if (links.length === 0) {
    return process.nextTick(callback);
  }

  let completed = 0, hasErrors = false;

  links.forEach(link => {
    /// 給隊列出傳遞一個任務(wù)，這個任務(wù)首先是一個函數(shù)，其次該函數(shù)接受一個參數(shù)
    /// 當(dāng)調(diào)用任務(wù)時，觸發(fā)該函數(shù)，然后給函數(shù)傳遞一個參數(shù)，告訴該函數(shù)在任務(wù)結(jié)束時干什么
    downloadQueue.pushTask(done => {
      spider(link, nesting - 1, err => {
        /// 這里表示，只要發(fā)生錯誤，隊列就會退出
        if (err) {
          hasErrors = true;
          return callback(err);
        }
        if (++completed === links.length && !hasErrors) {
          callback();
        }

        done();
      });
    });

  });
}

const spidering = new Map();

function spider(url, nesting, callback) {
  if (spidering.has(url)) {
    return process.nextTick(callback);
  }

  spidering.set(url, true);

  const filename = utilities.urlToFilename(url);

  /// In this pattern, there will be some issues.
  /// Possible problems to download the same url again and again。
  fs.readFile(filename, "utf8", (err, body) => {
    if (err) {
      if (err.code !== 'ENOENT') {
        return callback(err);
      }
      return download(url, filename, (err, body) => {
        if (err) {
          return callback(err);
        }
        spiderLinks(url, body, nesting, callback);
      });
    }

    spiderLinks(url, body, nesting, callback);
  });
}

spider(process.argv[2], 2, (err, filename, downloaded) => {
  if (err) {
    console.log(`error: ${err}`);
  } else if (downloaded) {
    console.log(`Completed the download of ${filename}`);
  } else {
    console.log(`${filename} was already downloaded`);
  }
});

因此，為了限制并發(fā)的個數(shù)，只需在 spiderLinks 方法中，把task遍歷放入隊列就可以了。這相對來說很簡單。

到這里為止，我們使用原生JavaScript實現(xiàn)了一個有相對完整功能的網(wǎng)絡(luò)蜘蛛，既能串行，也能并發(fā)，還可以控制并發(fā)個數(shù)。

2.使用async庫

把不同的功能放到不同的函數(shù)中，會給我們帶來巨大的好處，async庫十分流行，它的性能也不錯，它內(nèi)部基于callback。

(spider_v6.js)

const request = require("request");
const fs = require("fs");
const mkdirp = require("mkdirp");
const path = require("path");
const utilities = require("./utilities");
const series = require("async/series");
const eachSeries = require("async/eachSeries");

function download(url, filename, callback) {
  console.log(`Downloading ${url}`);

  let body;

  series([
    callback => {
      request(url, (err, response, resBody) => {
        if (err) {
          return callback(err);
        }
        body = resBody;
        callback();
      });
    },
    mkdirp.bind(null, path.dirname(filename)),
    callback => {
      fs.writeFile(filename, body, callback);
    }
  ], err => {
    if (err) {
      return callback(err);
    }
    console.log(`Downloaded and saved: ${url}`);
    callback(null, body);
  });
}

/// 最大的啟發(fā)是實現(xiàn)了如何異步循環(huán)遍歷數(shù)組
function spiderLinks(currentUrl, body, nesting, callback) {
  if (nesting === 0) {
    return process.nextTick(callback);
  }

  const links = utilities.getPageLinks(currentUrl, body);
  if (links.length === 0) {
    return process.nextTick(callback);
  }

  eachSeries(links, (link, cb) => {
    "use strict";
    spider(link, nesting - 1, cb);
  }, callback);
}

const spidering = new Map();

function spider(url, nesting, callback) {
  if (spidering.has(url)) {
    return process.nextTick(callback);
  }

  spidering.set(url, true);

  const filename = utilities.urlToFilename(url);

  fs.readFile(filename, "utf8", (err, body) => {
    if (err) {
      if (err.code !== 'ENOENT') {
        return callback(err);
      }
      return download(url, filename, (err, body) => {
        if (err) {
          return callback(err);
        }
        spiderLinks(url, body, nesting, callback);
      });
    }

    spiderLinks(url, body, nesting, callback);
  });
}

spider(process.argv[2], 1, (err, filename, downloaded) => {
  if (err) {
    console.log(err);
  } else if (downloaded) {
    console.log(`Completed the download of ${filename}`);
  } else {
    console.log(`${filename} was already downloaded`);
  }
});

在上邊的代碼中，我們只使用了async的三個功能：

const series = require("async/series"); // 串行
const eachSeries = require("async/eachSeries"); // 并行
const queue = require("async/queue"); // 隊列

由于比較簡單，就不做解釋了。async中的隊列的代碼在(spider_v7.js)中，和上邊我們自定義的隊列很相似，也不做更多解釋了。

3.Promise

Promise是一個協(xié)議，有很多庫實現(xiàn)了這個協(xié)議，我們用的是ES6的實現(xiàn)。簡單來說promise就是一個約定，如果完成了，就調(diào)用它的resolve方法，失敗了就調(diào)用它的reject方法。它內(nèi)有實現(xiàn)了then方法，then返回promise本身，這樣就形成了調(diào)用鏈。

其實Promise的內(nèi)容有很多，在實際應(yīng)用中是如何把普通的函數(shù)promise化。這方面的內(nèi)容在這里也不講了，我自己也不夠格

(spider_v8.js)

const utilities = require("./utilities");
const request = utilities.promisify(require("request"));
const fs = require("fs");
const readFile = utilities.promisify(fs.readFile);
const writeFile = utilities.promisify(fs.writeFile);
const mkdirp = utilities.promisify(require("mkdirp"));
const path = require("path");


function saveFile(filename, contents, callback) {
  mkdirp(path.dirname(filename), err => {
    if (err) {
      return callback(err);
    }
    fs.writeFile(filename, contents, callback);
  });
}

function download(url, filename) {
  console.log(`Downloading ${url}`);

  let body;

  return request(url)
    .then(response => {
      "use strict";
      body = response.body;
      return mkdirp(path.dirname(filename));
    })
    .then(() => writeFile(filename, body))
    .then(() => {
      "use strict";
      console.log(`Downloaded adn saved: ${url}`);
      return body;
    });
}

/// promise編程的本質(zhì)就是為了解決在函數(shù)中設(shè)置回調(diào)函數(shù)的問題
/// 通過中間層promise來實現(xiàn)異步函數(shù)同步化
function spiderLinks(currentUrl, body, nesting) {
  let promise = Promise.resolve();
  if (nesting === 0) {
    return promise;
  }

  const links = utilities.getPageLinks(currentUrl, body);

  links.forEach(link => {
    "use strict";
    promise = promise.then(() => spider(link, nesting - 1));
  });

  return promise;
}

function spider(url, nesting) {
  const filename = utilities.urlToFilename(url);

  return readFile(filename, "utf8")
    .then(
      body => spiderLinks(url, body, nesting),
      err => {
        "use strict";
        if (err.code !== 'ENOENT') {
          /// 拋出錯誤，這個方便與在整個異步鏈的最后通過呢catch來捕獲這個鏈中的錯誤
          throw err;
        }
        return download(url, filename)
          .then(body => spiderLinks(url, body, nesting));
      }
    );
}

spider(process.argv[2], 1)
  .then(() => {
    "use strict";
    console.log('Download complete');
  })
  .catch(err => {
    "use strict";
    console.log(err);
  });

可以看到上邊的代碼中的函數(shù)都是沒有callback的，只需要在最后catch就可以了。

在設(shè)計api的時候，應(yīng)該支持兩種方式，及支持callback，又支持promise

function asyncDivision(dividend, divisor, cb) {
  return new Promise((resolve, reject) => {
    "use strict";
    process.nextTick(() => {
      const result = dividend / divisor;
      if (isNaN(result) || !Number.isFinite(result)) {
        const error = new Error("Invalid operands");
        if (cb) {
          cb(error);
        }
        return reject(error);
      }

      if (cb) {
        cb(null, result);
      }
      resolve(result);
    });
  });
}

asyncDivision(10, 2, (err, result) => {
  "use strict";
  if (err) {
    return console.log(err);
  }
  console.log(result);
});

asyncDivision(22, 11)
  .then((result) => console.log(result))
  .catch((err) => console.log(err));

4.Generator

Generator很有意思，他可以讓暫停函數(shù)和恢復(fù)函數(shù)，利用thunkify和co這兩個庫，我們下邊的代碼實現(xiàn)起來非?？帷?/p>

(spider_v9.js)

const thunkify = require("thunkify");
const co = require("co");
const path = require("path");
const utilities = require("./utilities");

const request = thunkify(require("request"));
const fs = require("fs");
const mkdirp = thunkify(require("mkdirp"));
const readFile = thunkify(fs.readFile);
const writeFile = thunkify(fs.writeFile);
const nextTick = thunkify(process.nextTick);

function* download(url, filename) {
  console.log(`Downloading ${url}`);

  const response = yield request(url);
  console.log(response);

  const body = response[1];
  yield mkdirp(path.dirname(filename));

  yield writeFile(filename, body);

  console.log(`Downloaded and saved ${url}`);
  return body;
}

function* spider(url, nesting) {
  const filename = utilities.urlToFilename(url);

  let body;

  try {
    body = yield readFile(filename, "utf8");
  } catch (err) {
    if (err.code !== 'ENOENT') {
      throw err;
    }
    body = yield download(url, filename);
  }

  yield spiderLinks(url, body, nesting);
}

function* spiderLinks(currentUrl, body, nesting) {
  if (nesting === 0) {
    return nextTick();
  }

  const links = utilities.getPageLinks(currentUrl, body);

  for (let i = 0; i < links.length; i++) {
    yield spider(links[i], nesting - 1);
  }
}

/// 通過co就自動處理了回調(diào)函數(shù)，直接返回了回調(diào)函數(shù)中的參數(shù)，把這些參數(shù)放到一個數(shù)組中，但是去掉了err信息
co(function* () {
  try {
    yield spider(process.argv[2], 1);
    console.log('Download complete');
  } catch (err) {
    console.log(err);
  }
});

總結(jié)

我并沒有寫promise和generator并發(fā)的代碼。以上這些內(nèi)容來自于這本書nodejs-design-patterns 。

demo下載

以上就是本文的全部內(nèi)容，希望對大家的學(xué)習(xí)有所幫助，也希望大家多多支持腳本之家。

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