js - promise

编写异步程序的最高境界就是它看起来是同步的。

Async Call

以下枚举最为经典的用法：

• series(tasks, [callback]) 串行执行
  • waterfall(tasks, [callback]) 带参数传递的串行执行
• parallel(tasks, [callback]) 并行执行
• whilst(test, fn, callback) Repeatedly call fn, while test returns true. Calls callback when stopped, or an error occurs.

Deferred / Promise

// 经典的针对 Promise 用途的考察
doSomething().then(function () {
    return doSomethingElse();
})；
doSomethin().then(function () {
    doSomethingElse();
});
doSomething().then(doSomethingElse());
doSomething().then(doSomethingElse);

• 串行式异步处理：
  • 避免：Callbacks-Hell 与 意大利面条式的代码

// 比较合理的串行 Promise 方法
remotedb.allDocs(...).then(functioin (resultofAllDocs) {
   return localdb.put(...);
}).then(function (resultOfPut) {
   return localdb.get(...);
}).then(function (resultOfGet) {
   return localdb.put(...);
}).catch(function (err) {
   console.log(err);
});

• 并行式异步处理：Promise.all() 与 Deferred.when([...])
• 摒弃 Deferred 的思想
  • 使用 Promise 包装

new Promise(function (resolve, reject) {
  fs.readFile('myfile.txt', function (err, file) {
      if (err) {
          return reject(err);
      }
      resolve(file);
  });
}).then(...)

• catch() 不可忽略
  • .catch is specified for built-in Javascript promises and is “sugar” for .then(null, function(){}).
• 显示调用 return 方便理解代码
• 同步和异步的模型
  • 同步解析：Promise.resolve(someSynchronousValue).then(...);

getUserByName('nolan').then(function (user) {
  if (user.isLoggedOut()) {
    throw new Error('user logged out!'); // throwing a synchronous error!
  }
  if (inMemoryCache[user.id]) {
    return inMemoryCache[user.id];       // returning a synchronous value!
  }
  return getUserAccountById(user.id);    // returning a promise!
}).then(function (userAccount) {
  // I got a user account!
}).catch(function (err) {
  // Boo, I got an error!
});

• 杜绝 Promise 穿透

// Callback Hell
getArticleList(function(articles){
    getArticle(articles[0].id, function(article){
        getAuthor(article.authorId, function(author){
            alert(author.email);
        })
    })
})
function getAuthor(id, callback){
    $.ajax("http://beta.json-generator.com/api/json/get/E105pDLh",{
        author: id
    }).done(function(result){
        callback(result);
    })
}
function getArticle(id, callback){
    $.ajax("http://beta.json-generator.com/api/json/get/EkI02vUn",{
        id: id
    }).done(function(result){
        callback(result);
    })
}
function getArticleList(callback){
    $.ajax(
    "http://beta.json-generator.com/api/json/get/Ey8JqwIh")
    .done(function(result){
        callback(result);
    });
}
// Promise
getArticleList()
    .then(articles => getArticle(articles[0].id))
    .then(article => getAuthor(article.authorId))
    .then(author => console.log(author.email));
function getAuthor(id){
    return new Promise(function(resolve, reject){
        $.ajax("http://beta.json-generator.com/api/json/get/E105pDLh",{
            author: id
        }).done(function(result){
            resolve(result);
        })
    });
}
function getArticle(id){
    return new Promise(function(resolve, reject){
        $.ajax("http://beta.json-generator.com/api/json/get/EkI02vUn",{
            id: id
        }).done(function(result){
            resolve(result);
        })
    });
}
function getArticleList(){
    return new Promise(function(resolve, reject){
       $.ajax(
        "http://beta.json-generator.com/api/json/get/Ey8JqwIh")
        .done(function(result){
            resolve(result);
        }); 
    });
}
// Generator
function* run(){
  var articles = yield getArticleList();
  var article = yield getArticle(articles[0].id);
  var author = yield getAuthor(article.authorId);
  alert(author.email);  
}
function runGenerator(){
    var gen = run();
    function go(result){
        if(result.done) return;
        result.value.then(function(r){
            go(gen.next(r));
        });
    }
    go(gen.next());
}
runGenerator();
// Async
async function run() {
  var articles = await getArticleList();
  var article = await getArticle(articles[0].id);
  var author = await getAuthor(article.authorId);
  alert(author.email);  
}

Async / Await

More Elegant Usage of Async, it is much better than just promise.

async function logInOrder(urls) {
  // fetch all the URLs in parallel
  const textPromises = urls.map(async url => {
    const response = await fetch(url);
    return response.text();
  });

  // log them in sequence
  for (const textPromise of textPromises) {
    console.log(await textPromise);
  }
}

Processing a Mongo DB Cursor

const mongodb = require('mongodb');

test();

async function test() {
  const db = await mongodb.MongoClient.connect('mongodb://localhost:27017/test');

  await db.collection('Movies').drop();
  await db.collection('Movies').insertMany([
    { name: 'Enter the Dragon' },
    { name: 'Ip Man' },
    { name: 'Kickboxer' }
  ]);

  // Don't `await`, instead get a cursor
  const cursor = db.collection('Movies').find();
  // Use `next()` and `await` to exhaust the cursor
  for (let doc = await cursor.next(); doc != null; doc = await cursor.next()) {
    console.log(doc.name);
  }
}

Process Multitasks

const bcrypt = require('bcrypt');

const NUM_SALT_ROUNDS = 8;

test();

async function test() {
  const pws = ['password', 'password1', 'passw0rd'];

  // `promises` is an array of promises, because `bcrypt.hash()` returns a
  // promise if no callback is supplied.
  const promises = pws.map(pw => bcrypt.hash(pw, NUM_SALT_ROUNDS));

  /**
   * Prints hashed passwords, for example:
   * [ '$2a$08$nUmCaLsQ9rUaGHIiQgFpAOkE2QPrn1Pyx02s4s8HC2zlh7E.o9wxC',
   *   '$2a$08$wdktZmCtsGrorU1mFWvJIOx3A0fbT7yJktRsRfNXa9HLGHOZ8GRjS',
   *   '$2a$08$VCdMy8NSwC8r9ip8eKI1QuBd9wSxPnZoZBw8b1QskK77tL2gxrUk.' ]
   */
  console.log(await Promise.all(promises));
}

Ref

• 参考文章 EFE
• Promise 你真正理解了么？
• Promise A+
• Async Lib
• Google Async Function Introduction
• Async / Await pattern in Node.js