promise/sequence.js

/**
 * 这段代码来自 深入浅出 Node.js 91~92 页
 */

var Deferred = function () {
  this.promise = new Promise()
}

// 完成态
Deferred.prototype.resolve = function(obj) {
  var promise = this.promise
  var handler
  while ((handler = promise.queue.shift())) {
    if (handler && handler.fulfilled) {
      var ret = handler.fulfilled(obj)
      if (ret && ret.isPromise) {
        ret.queue = promise.queue
        this.promise = ret
        return
      }
    }
  }
}

// 失败态
Deferred.prototype.reject = function(err) {
  var promise = this.promise
  var handler
  while ((handler = promise.queue.shift())) {
    if (handler && handler.error) {
      var ret = handler.error(err)
      if (ret && ret.isPromise) {
        ret.queue = promise.queue
        this.promise = ret
        return
      }
    }
  }
}

// 生成回调函数
Deferred.prototype.callback = function() {
  var that = this
  return function (err, file) {
    if (err) {
      return that.reject(err)
    }
    that.resolve(file)
  }
}

var Promise = function () {
  this.queue = []   // 队列用于存储待执行的回调函数
  this.isPromise = true
}

Promise.prototype.then = function (fulfilledHandler, errorHandler, progressHandler) {
  var handler = {}
  if (typeof fulfilledHandler === 'function') {
    handler.fulfilled = fulfilledHandler
  }
  if (typeof errorHandler === 'function') {
    handler.error = errorHandler
  }
  this.queue.push(handler)
  return this
}


var fs = require('fs')

var readFile1 = function(file, encoding) {
  var deferred = new Deferred()
  fs.readFile(file, encoding, deferred.callback())
  return deferred.promise
}

var readFile2 = function(file, encoding) {
  var deferred = new Deferred()
  fs.readFile(file, encoding, deferred.callback())
  return deferred.promise
}

readFile1('./file1.txt', 'utf8').then(function(file1) {
  return readFile2(file1, 'utf8')
}).then(function(file2) {
  console.log(file2)
})