seajs源码分析

seajs主要做了2件事

1.定义什么是模块，模块的接口:id、deps、exports　　

2.解决了如何加载模块：将模块的id与模块路径用规则关联上，能够透明的加载模块和顺序执行

　　核心的代码是util-path.js和module.js

对外接口

seajs.use　　　　执行的入口方法

define　　　　　  模块定义的初始化方法

核心类：Module

静态方法

Module.use(使用模块load后exec)、Module.preload(获取data.preload参数后执行Module.use)、Module.get(拿到模块,若无则创建)

模块use的执行过程

load　　　　　加载此模块的js

fetch　　　　 获取此模块的依赖模块，并关联依赖模块load后回调

load　　　　　在fetch后，request全部依赖模块，onRequest时回调load判断是否remain==0,调用onload

onload　　　　执行callback函数，Module.use中callback会执行deps.exec

exec　　　　　执行模块factory(即define(fn)的fn)，且将require关键字映射exec上，达到链式递归执行所有依赖模块

/*
    创建seajs对象，标识版本号。创建data数据对象，data挂在seajs为外部访问
*/
var seajs = global.seajs = {
  // The current version of Sea.js being used
  version: "@VERSION"
}

var data = seajs.data = {}

/*
    seajs自己的事件触发，on绑定事件，off移除，emit触发
*/
/**
 * util-events.js - The minimal events support
 */

var events = data.events = {}

// Bind event
seajs.on = function(name, callback) {
  var list = events[name] || (events[name] = [])
  list.push(callback)
  return seajs
}

// Remove event. If `callback` is undefined, remove all callbacks for the
// event. If `event` and `callback` are both undefined, remove all callbacks
// for all events
seajs.off = function(name, callback) {
  // Remove *all* events
  if (!(name || callback)) {
    events = data.events = {}
    return seajs
  }

  var list = events[name]
  if (list) {
    if (callback) {
      for (var i = list.length - 1; i >= 0; i--) {
        if (list[i] === callback) {
          list.splice(i, 1)
        }
      }
    }
    else {
      delete events[name]
    }
  }

  return seajs
}

// Emit event, firing all bound callbacks. Callbacks are passed the same
// arguments as `emit` is, apart from the event name
var emit = seajs.emit = function(name, data) {
  var list = events[name], fn

  if (list) {
    // Copy callback lists to prevent modification
    list = list.slice()

    // Execute event callbacks
    while ((fn = list.shift())) {
      fn(data)
    }
  }

  return seajs
}

/*
    类型判断和计数函数
*/
/**
 * util-lang.js - The minimal language enhancement
 */

function isType(type) {
  return function(obj) {
    return Object.prototype.toString.call(obj) === "[object " + type + "]"
  }
}

var isObject = isType("Object")
var isString = isType("String")
var isArray = Array.isArray || isType("Array")
var isFunction = isType("Function")

var _cid = 0
function cid() {
  return _cid++
}

/*
    解析路径的处理方法，包括：
    dirname        获取路径的目录
    realpath    过滤目录中的./ ../
    normalize    补全文件名.js后缀
    parseAlias    解析配置别名
    parsePaths    解析配置中的path
    parseVars    解析配置中{}
    parseMap    加载路径根据配置匹配规则映射
    addBase        通过id，路由规则生成完整绝对路径
    
    id2Uri        根据id获取uri，完整的绝对路径
    
    loaderDir    seajs文件路径的获取：根据id=seajsnode查找seajs的dom，获取src后匹配目录
*/
/**
 * util-path.js - The utilities for operating path such as id, uri
 */

var DIRNAME_RE = /[^?#]*\//

var DOT_RE = /\/\.\//g
var DOUBLE_DOT_RE = /\/[^/]+\/\.\.\//

// Extract the directory portion of a path
// dirname("a/b/c.js?t=123#xx/zz") ==> "a/b/"
// ref: http://jsperf.com/regex-vs-split/2
function dirname(path) {
  return path.match(DIRNAME_RE)[0]
}

// Canonicalize a path
// realpath("http://test.com/a//./b/../c") ==> "http://test.com/a/c"
function realpath(path) {
  // /a/b/./c/./d ==> /a/b/c/d
  path = path.replace(DOT_RE, "/")

  // a/b/c/../../d  ==>  a/b/../d  ==>  a/d
  while (path.match(DOUBLE_DOT_RE)) {
    path = path.replace(DOUBLE_DOT_RE, "/")
  }

  return path
}

// Normalize an id
// normalize("path/to/a") ==> "path/to/a.js"
// NOTICE: substring is faster than negative slice and RegExp
function normalize(path) {
  var last = path.length - 1

  // If the uri ends with `#`, just return it without '#'
  if (path.charAt(last) === "#") {
    return path.substring(0, last)
  }

  return  (path.substring(last - 2) === ".js" ||
      path.indexOf("?") > 0 ||
      path.substring(last - 3) === ".css") ? path : path + ".js"
}


var PATHS_RE = /^([^/:]+)(\/.+)$/
var VARS_RE = /{([^{]+)}/g

function parseAlias(id) {
  var alias = data.alias
  return alias && isString(alias[id]) ? alias[id] : id
}

function parsePaths(id) {
  var paths = data.paths
  var m

  if (paths && (m = id.match(PATHS_RE)) && isString(paths[m[1]])) {
    id = paths[m[1]] + m[2]
  }

  return id
}

function parseVars(id) {
  var vars = data.vars

  if (vars && id.indexOf("{") > -1) {
    id = id.replace(VARS_RE, function(m, key) {
      return isString(vars[key]) ? vars[key] : m
    })
  }

  return id
}

function parseMap(uri) {
  var map = data.map
  var ret = uri

  if (map) {
    for (var i = 0, len = map.length; i < len; i++) {
      var rule = map[i]

      ret = isFunction(rule) ?
          (rule(uri) || uri) :
          uri.replace(rule[0], rule[1])

      // Only apply the first matched rule
      if (ret !== uri) break
    }
  }

  return ret
}


var ABSOLUTE_RE = /^\/\/.|:\//
var ROOT_DIR_RE = /^.*?\/\/.*?\//

function addBase(id, refUri) {
  var ret
  var first = id.charAt(0)

  // Absolute
  if (ABSOLUTE_RE.test(id)) {
    ret = id
  }
  // Relative
  else if (first === ".") {
    ret = realpath((refUri ? dirname(refUri) : data.cwd) + id)
  }
  // Root
  else if (first === "/") {
    var m = data.cwd.match(ROOT_DIR_RE)
    ret = m ? m[0] + id.substring(1) : id
  }
  // Top-level
  else {
    ret = data.base + id
  }

  return ret
}

function id2Uri(id, refUri) {
  if (!id) return ""

  id = parseAlias(id)
  id = parsePaths(id)
  id = parseVars(id)
  id = normalize(id)

  var uri = addBase(id, refUri)
  uri = parseMap(uri)

  return uri
}


var doc = document
var loc = location
var cwd = dirname(loc.href)
var scripts = doc.getElementsByTagName("script")

// Recommend to add `seajsnode` id for the `sea.js` script element
var loaderScript = doc.getElementById("seajsnode") ||
    scripts[scripts.length - 1]

// When `sea.js` is inline, set loaderDir to current working directory
var loaderDir = dirname(getScriptAbsoluteSrc(loaderScript) || cwd)

function getScriptAbsoluteSrc(node) {
  return node.hasAttribute ? // non-IE6/7
      node.src :
    // see http://msdn.microsoft.com/en-us/library/ms536429(VS.85).aspx
      node.getAttribute("src", 4)
}

/*
    request     动态加载css或者script到head
                创建dom原生link、script标签
    addOnload    定义浏览器兼容的异步请求回调
    getCurrentScript    获取当前资源加载状态为进行中interactive的资源
*/
/**
 * util-request.js - The utilities for requesting script and style files
 * ref: tests/research/load-js-css/test.html
 */

var head = doc.getElementsByTagName("head")[0] || doc.documentElement
var baseElement = head.getElementsByTagName("base")[0]

var IS_CSS_RE = /\.css(?:\?|$)/i
var READY_STATE_RE = /^(?:loaded|complete|undefined)$/

var currentlyAddingScript
var interactiveScript

// `onload` event is supported in WebKit < 535.23 and Firefox < 9.0
// ref:
//  - https://bugs.webkit.org/show_activity.cgi?id=38995
//  - https://bugzilla.mozilla.org/show_bug.cgi?id=185236
//  - https://developer.mozilla.org/en/HTML/Element/link#Stylesheet_load_events
var isOldWebKit = (navigator.userAgent
    .replace(/.*AppleWebKit\/(\d+)\..*/, "$1")) * 1 < 536


function request(url, callback, charset) {
  var isCSS = IS_CSS_RE.test(url)
  var node = doc.createElement(isCSS ? "link" : "script")

  if (charset) {
    var cs = isFunction(charset) ? charset(url) : charset
    if (cs) {
      node.charset = cs
    }
  }

  addOnload(node, callback, isCSS)

  if (isCSS) {
    node.rel = "stylesheet"
    node.href = url
  }
  else {
    node.async = true
    node.src = url
  }

  // For some cache cases in IE 6-8, the script executes IMMEDIATELY after
  // the end of the insert execution, so use `currentlyAddingScript` to
  // hold current node, for deriving url in `define` call
  currentlyAddingScript = node

  // ref: #185 & http://dev.jquery.com/ticket/2709
  baseElement ?
      head.insertBefore(node, baseElement) :
      head.appendChild(node)

  currentlyAddingScript = null
}

function addOnload(node, callback, isCSS) {
  var missingOnload = isCSS && (isOldWebKit || !("onload" in node))

  // for Old WebKit and Old Firefox
  if (missingOnload) {
    setTimeout(function() {
      pollCss(node, callback)
    }, 1) // Begin after node insertion
    return
  }

  node.onload = node.onerror = node.onreadystatechange = function() {
    if (READY_STATE_RE.test(node.readyState)) {

      // Ensure only run once and handle memory leak in IE
      node.onload = node.onerror = node.onreadystatechange = null

      // Remove the script to reduce memory leak
      if (!isCSS && !data.debug) {
        head.removeChild(node)
      }

      // Dereference the node
      node = null

      callback()
    }
  }
}

function pollCss(node, callback) {
  var sheet = node.sheet
  var isLoaded

  // for WebKit < 536
  if (isOldWebKit) {
    if (sheet) {
      isLoaded = true
    }
  }
  // for Firefox < 9.0
  else if (sheet) {
    try {
      if (sheet.cssRules) {
        isLoaded = true
      }
    } catch (ex) {
      // The value of `ex.name` is changed from "NS_ERROR_DOM_SECURITY_ERR"
      // to "SecurityError" since Firefox 13.0. But Firefox is less than 9.0
      // in here, So it is ok to just rely on "NS_ERROR_DOM_SECURITY_ERR"
      if (ex.name === "NS_ERROR_DOM_SECURITY_ERR") {
        isLoaded = true
      }
    }
  }

  setTimeout(function() {
    if (isLoaded) {
      // Place callback here to give time for style rendering
      callback()
    }
    else {
      pollCss(node, callback)
    }
  }, 20)
}

function getCurrentScript() {
  if (currentlyAddingScript) {
    return currentlyAddingScript
  }

  // For IE6-9 browsers, the script onload event may not fire right
  // after the the script is evaluated. Kris Zyp found that it
  // could query the script nodes and the one that is in "interactive"
  // mode indicates the current script
  // ref: http://goo.gl/JHfFW
  if (interactiveScript && interactiveScript.readyState === "interactive") {
    return interactiveScript
  }

  var scripts = head.getElementsByTagName("script")

  for (var i = scripts.length - 1; i >= 0; i--) {
    var script = scripts[i]
    if (script.readyState === "interactive") {
      interactiveScript = script
      return interactiveScript
    }
  }
}

/**
 * module.js - The core of module loader
 */
//系统全局对象，用来保存全局数据和状态
var cachedMods = seajs.cache = {}
var anonymousMeta

var fetchingList = {}
var fetchedList = {}
var callbackList = {}

var STATUS = Module.STATUS = {
  // 1 - The `module.uri` is being fetched
  FETCHING: 1,
  // 2 - The meta data has been saved to cachedMods
  SAVED: 2,
  // 3 - The `module.dependencies` are being loaded
  LOADING: 3,
  // 4 - The module are ready to execute
  LOADED: 4,
  // 5 - The module is being executed
  EXECUTING: 5,
  // 6 - The `module.exports` is available
  EXECUTED: 6
}

//模块类   id、deps、exports 构造函数
function Module(uri, deps) {
  this.uri = uri
  this.dependencies = deps || []
  this.exports = null
  this.status = 0//模块状态
  // The number of unloaded dependencies
  this._remain = 0
  // Who depend on me
  this._waitings = {}
}

//Module静态方法，Module.use使用模块，Module.preload调用data.preload后回调Module.use
// Load preload modules before all other modules
Module.preload = function(callback) {
  var preloadMods = data.preload
  var len = preloadMods.length

  if (len) {
    Module.use(preloadMods, function() {
      // Remove the loaded preload modules
      preloadMods.splice(0, len)

      // Allow preload modules to add new preload modules
      Module.preload(callback)
    }, data.cwd + "_preload_" + cid())
  }
  else {
    callback()
  }
}

//入口模块，ids--依赖模块，uri模块id：获取uri模块
// Use function is equal to load a anonymous module
Module.use = function (ids, callback, uri) {
  var mod = Module.get(uri, isArray(ids) ? ids : [ids])
  mod.callback = function() {
    var exports = []
    var uris = mod.resolve()
    for (var i = 0, len = uris.length; i < len; i++) {
      exports[i] = cachedMods[uris[i]].exec()
    }
    if (callback) {
      callback.apply(global, exports)
    }
    delete mod.callback
  }
  mod.load()
}

//获取uri模块，若cachedMods中无则创建个新模块
Module.get = function(uri, deps) {
  return cachedMods[uri] || (cachedMods[uri] = new Module(uri, deps))
}

// seajs.use 入口方法
// 执行过程：A依赖B,B依赖C，seajs.use(A):
/*
fetch            获取A模块的依赖模块(B),注册B.load的回调A.load(每次依赖的文件onRquest，要跟新自己的状态)
load            发送A模块的所有依赖模块request，直到remain==0，触发onload
onload            当A模块所有依赖都已加载，触发A.callback
exec            seajs.use 将执行模块的callback设置为exec，调用A.exec,且A模块中的require映射了exec方法，链式的执行下去。。
*/
seajs.use = function(ids, callback) {
  Module.preload(function() {
    Module.use(ids, callback, data.cwd + "_use_" + cid())
  })
  return seajs
}

//将依赖的模块通过fetch方法注册，最终执行load请求js，onRequest回调onload
// Load module.dependencies and fire onload when all done
Module.prototype.load = function() {
  var mod = this
  // If the module is being loaded, just wait it onload call
  if (mod.status >= STATUS.LOADING) {
    return
  }
  mod.status = STATUS.LOADING

  // Emit `load` event for plugins such as combo plugin
  var uris = mod.resolve()//获取依赖uris
  emit("load", uris)

  var len = mod._remain = uris.length
  var m
  // Initialize modules and register waitings 拿到依赖模块注册依赖
  for (var i = 0; i < len; i++) {
    m = Module.get(uris[i])//获取依赖module
    if (m.status < STATUS.LOADED) {//模块未加载
      // Maybe duplicate
      m._waitings[mod.uri] = (m._waitings[mod.uri] || 0) + 1//记录下有几个模块依赖自己
    }
    else {
      mod._remain--//加载后--
    }
  }

   //若所有依赖都加载后，执行onload
  if (mod._remain === 0) {
    mod.onload()
    return
  }

  // Begin parallel loading
  var requestCache = {}

  for (i = 0; i < len; i++) {
    m = cachedMods[uris[i]]

    if (m.status < STATUS.FETCHING) {
      m.fetch(requestCache)
    }
    else if (m.status === STATUS.SAVED) {
      m.load()
    }
  }

  // Send all requests at last to avoid cache bug in IE6-9. Issues#808
  for (var requestUri in requestCache) {
    if (requestCache.hasOwnProperty(requestUri)) {
      requestCache[requestUri]()
    }
  }
}

//将模块的所有依赖的uri都注册到此模块的requestCache列表中
// Fetch a module
Module.prototype.fetch = function(requestCache) {
  var mod = this
  var uri = mod.uri

  mod.status = STATUS.FETCHING

  // Emit `fetch` event for plugins such as combo plugin
  var emitData = { uri: uri }
  emit("fetch", emitData)
  var requestUri = emitData.requestUri || uri

  // Empty uri or a non-CMD module
  if (!requestUri || fetchedList[requestUri]) {
    mod.load()
    return
  }

  if (fetchingList[requestUri]) {
    callbackList[requestUri].push(mod)
    return
  }
  fetchingList[requestUri] = true
  callbackList[requestUri] = [mod]

  // Emit `request` event for plugins such as text plugin
  emit("request", emitData = {
    uri: uri,
    requestUri: requestUri,
    onRequest: onRequest,
    charset: data.charset
  })

  if (!emitData.requested) {
    requestCache ?
        requestCache[emitData.requestUri] = sendRequest :
        sendRequest()
  }

  function sendRequest() {
    request(emitData.requestUri, emitData.onRequest, emitData.charset)
  }

  function onRequest() {
    delete fetchingList[requestUri]
    fetchedList[requestUri] = true

    // Save meta data of anonymous module
    if (anonymousMeta) {
      save(uri, anonymousMeta)
      anonymousMeta = null
    }

    // Call callbacks
    var m, mods = callbackList[requestUri]
    delete callbackList[requestUri]
    while ((m = mods.shift())) m.load()
  }
}

//若模块的依赖加载全部加载触发onload回调mod.callback，同时通知所有依赖他的模块
// Call this method when module is loaded
Module.prototype.onload = function() {
  var mod = this
  mod.status = STATUS.LOADED

  if (mod.callback) {
    mod.callback()
  }

  // Notify waiting modules to fire onload
  var waitings = mod._waitings
  var uri, m

  for (uri in waitings) {
    if (waitings.hasOwnProperty(uri)) {
      m = cachedMods[uri]
      m._remain -= waitings[uri]
      if (m._remain === 0) {
        m.onload()
      }
    }
  }

  // Reduce memory taken
  delete mod._waitings
  delete mod._remain
}

//解析模块依赖的uris 获取绝对uris
// Resolve module.dependencies
Module.prototype.resolve = function() {
  var mod = this
  var ids = mod.dependencies
  var uris = []

  for (var i = 0, len = ids.length; i < len; i++) {
    uris[i] = resolve(ids[i], mod.uri)
  }
  return uris
}

//触发resolve事件，返回id2Uri
// Helpers
function resolve(id, refUri) {
  // Emit `resolve` event for plugins such as text plugin
  var emitData = { id: id, refUri: refUri }
  emit("resolve", emitData)
  return emitData.uri || id2Uri(emitData.id, refUri)
}

//加载完所有依赖的模块后，执行自身模块，同时通过关键字require将依赖的模块顺序执行
// Execute a module
Module.prototype.exec = function () {
  var mod = this
  // When module is executed, DO NOT execute it again. When module
  // is being executed, just return `module.exports` too, for avoiding
  // circularly calling
  if (mod.status >= STATUS.EXECUTING) {
    return mod.exports//模块已执行
  }
  mod.status = STATUS.EXECUTING
  // Create require
  var uri = mod.uri

  //执行require的id
  function require(id) {
    return cachedMods[require.resolve(id)].exec()
  }
    
  //获得id模块的绝对路径  
  require.resolve = function(id) {
    return resolve(id, uri)
  }

  //异步请求uri
  require.async = function(ids, callback) {
    Module.use(ids, callback, uri + "_async_" + cid())
    return require
  }

  // Exec factory
  var factory = mod.factory

  var exports = isFunction(factory) ?
      factory(require, mod.exports = {}, mod) :
      factory

  if (exports === undefined) {
    exports = mod.exports
  }

  // Emit `error` event
  if (exports === null && !IS_CSS_RE.test(uri)) {
    emit("error", mod)
  }

  // Reduce memory leak
  delete mod.factory

  mod.exports = exports
  mod.status = STATUS.EXECUTED

  // Emit `exec` event
  emit("exec", mod)

  return exports
}

//对外的接口 define(function(){...})用于定义模块，初始化模块的基础属性(并不执行模块factory方法)
Module.define.cmd = {}
global.define = Module.define
// Define a module
Module.define = function (id, deps, factory) {
  var argsLen = arguments.length

  // define(factory)
  if (argsLen === 1) {
    factory = id
    id = undefined
  }
  else if (argsLen === 2) {
    factory = deps

    // define(deps, factory)
    if (isArray(id)) {
      deps = id
      id = undefined
    }
    // define(id, factory)
    else {
      deps = undefined
    }
  }

  // Parse dependencies according to the module factory code
  if (!isArray(deps) && isFunction(factory)) {
    deps = parseDependencies(factory.toString())
  }

  var meta = {
    id: id,
    uri: resolve(id),
    deps: deps,
    factory: factory
  }

  // Try to derive uri in IE6-9 for anonymous modules
  if (!meta.uri && doc.attachEvent) {
    var script = getCurrentScript()

    if (script) {
      meta.uri = script.src
    }

    // NOTE: If the id-deriving methods above is failed, then falls back
    // to use onload event to get the uri
  }

  // Emit `define` event, used in nocache plugin, seajs node version etc
  emit("define", meta)

  meta.uri ? save(meta.uri, meta) :
      // Save information for "saving" work in the script onload event
      anonymousMeta = meta
}

function save(uri, meta) {
  var mod = Module.get(uri)

  // Do NOT override already saved modules
  if (mod.status < STATUS.SAVED) {
    mod.id = meta.id || uri
    mod.dependencies = meta.deps || []
    mod.factory = meta.factory
    mod.status = STATUS.SAVED
  }
}

// For Developers 调试接口
seajs.Module = Module
data.fetchedList = fetchedList
data.cid = cid
//获取模块id
seajs.resolve = id2Uri
//获取模块exports接口
seajs.require = function(id) {
  return (cachedMods[resolve(id)] || {}).exports
}