function Module(uri, deps) {
 this.uri = uri
 this.dependencies = deps || []
 this.exports = null
 this.status = 0

 // Who depends on me
 this._waitings = {}

 // The number of unloaded dependencies
 this._remain = 0
}

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
}

// 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
 // 如果deps为非数组，则序列化工厂函数获取入参。
 if (!isArray(deps) && isFunction(factory)) {
 deps = parseDependencies(factory.toString())
 }

 var meta = {
 id: id,
 uri: Module.resolve(id), // 绝对url
 deps: deps,
 factory: factory
 }

 // Try to derive uri in IE6-9 for anonymous modules
 // 导出匿名模块的uri
 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 ? Module.save(meta.uri, meta) :
 // Save information for "saving" work in the script onload event
 anonymousMeta = meta
}

var REQUIRE_RE = /"(?:\\"|[^"])*"|'(?:\\'|[^'])*'|\/\*[\S\s]*?\*\/|\/(?:\\\/|[^\/\r\n])+\/(?=[^\/])|\/\/.*|\.\s*require|(?:^|[^$])\brequire\s*\(\s*(["'])(.+?)\1\s*\)/g
var SLASH_RE = /\\\\/g

function parseDependencies(code) {
 var ret = []
 // 此处使用函数序列化（传入的factory）进行字符串匹配，寻找require（“...”）的关键字
 code.replace(SLASH_RE, "")
 .replace(REQUIRE_RE, function(m, m1, m2) {
 if (m2) {
  ret.push(m2)
 }
 })

 return ret
}

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
 }
 }

 // 添加 onload 函数。
 addOnload(node, callback, isCSS, url)

 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, url) {
 var supportOnload = "onload" in node

 // for Old WebKit and Old Firefox
 if (isCSS && (isOldWebKit || !supportOnload)) {
 setTimeout(function() {
 pollCss(node, callback)
 }, 1) // Begin after node insertion
 return
 }

 if (supportOnload) {
 node.onload = onload
 node.onerror = function() {
 emit("error", { uri: url, node: node })
 onload()
 }
 }
 else {
 node.onreadystatechange = function() {
 if (/loaded|complete/.test(node.readyState)) {
 onload()
 }
 }
 }

 function onload() {
 // 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()
 }
}
// 针对 旧webkit和不支持onload的CSS节点判断加载完毕的方法
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)
}

// Fetch a module
// 加载该模块，fetch函数中调用了seajs.request函数
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() {
 seajs.request(emitData.requestUri, emitData.onRequest, emitData.charset)
 }
 // 回调函数
 function onRequest() {
 delete fetchingList[requestUri]
 fetchedList[requestUri] = true

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

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