dojo/aspect模塊是dojo框架中對(duì)于AOP的實(shí)現(xiàn)。關(guān)于AOP的詳細(xì)解釋請(qǐng)讀者另行查看其它資料，這里簡(jiǎn)單復(fù)習(xí)一下AOP中的基本概念：

切面（Aspect）：其實(shí)就是共有功能的實(shí)現(xiàn)。如日志切面、權(quán)限切面、事務(wù)切面等。

通知（Advice）：是切面的具體實(shí)現(xiàn)。以目標(biāo)方法為參照點(diǎn)，根據(jù)放置的地方不同，可分為前置通知（Before）、后置通知（After）與環(huán)繞通知（Around）。

連接點(diǎn)（Joinpoint）：就是程序在運(yùn)行過(guò)程中能夠插入切面的地點(diǎn)。

目標(biāo)對(duì)象（Target）：就是那些即將切入切面的對(duì)象，也就是那些被通知的對(duì)象。這些對(duì)象中已經(jīng)只剩下干干凈凈的核心業(yè)務(wù)邏輯代碼了，所有的共有功能代碼等待AOP容器的切入。

代理對(duì)象（Proxy）：將通知應(yīng)用到目標(biāo)對(duì)象之后被動(dòng)態(tài)創(chuàng)建的對(duì)象。可以簡(jiǎn)單地理解為，代理對(duì)象的功能等于目標(biāo)對(duì)象的核心業(yè)務(wù)邏輯功能加上共有功能。代理對(duì)象對(duì)于使用者而言是透明的，是程序運(yùn)行過(guò)程中的產(chǎn)物。

織入（Weaving）：將切面應(yīng)用到目標(biāo)對(duì)象從而創(chuàng)建一個(gè)新的代理對(duì)象的過(guò)程。這個(gè)過(guò)程可以發(fā)生在編譯期、類裝載期及運(yùn)行期，當(dāng)然不同的發(fā)生點(diǎn)有著不同的前提條件。譬如發(fā)生在編譯期的話，就要求有一個(gè)支持這種AOP實(shí)現(xiàn)的特殊編譯器；發(fā)生在類裝載期，就要求有一個(gè)支持AOP實(shí)現(xiàn)的特殊類裝載器；只有發(fā)生在運(yùn)行期，則可直接通過(guò)Java語(yǔ)言的反射機(jī)制與動(dòng)態(tài)代理機(jī)制來(lái)動(dòng)態(tài)實(shí)現(xiàn)。

　　生成代理對(duì)象的過(guò)程可以按照下圖理解：

　　

　　

　　dojo/aspect模塊代碼主要分為兩部分：

• advise方法，通過(guò)使用閉包跟鏈?zhǔn)侥Ｐ蛠?lái)構(gòu)造“通知”鏈。 "use strict";var undefined, nextId = 0;function advise(dispatcher, type, advice, receiveArguments){var previous = dispatcher[type];var around = type == "around";var signal;if(around){var advised = advice(function(){return previous.advice(this, arguments);});signal = {remove: function(){if(advised){advised = dispatcher = advice = null;}},advice: function(target, args){return advised ?advised.apply(target, args) : // called the advised functionprevious.advice(target, args); // cancelled, skip to next one }};}else{// create the remove handlersignal = {remove: function(){if(signal.advice){var previous = signal.previous;var next = signal.next;if(!next && !previous){delete dispatcher[type];}else{if(previous){previous.next = next;}else{dispatcher[type] = next;}if(next){next.previous = previous;}}// remove the advice to signal that this signal has been removeddispatcher = advice = signal.advice = null;}},id: nextId++,advice: advice,receiveArguments: receiveArguments};}if(previous && !around){if(type == "after"){// add the listener to the end of the list// note that we had to change this loop a little bit to workaround a bizarre IE10 JIT bugwhile(previous.next && (previous = previous.next)){}previous.next = signal;signal.previous = previous;}else if(type == "before"){// add to beginningdispatcher[type] = signal;signal.next = previous;previous.previous = signal;}}else{// around or first one just replacesdispatcher[type] = signal;}return signal;} View Code
• aspect方法，這個(gè)函數(shù)返回一個(gè)閉包。閉包的作用是將“通知”方法織入到目標(biāo)函數(shù)中，java中運(yùn)行時(shí)通過(guò)反射的方式來(lái)織入，而js中通過(guò)動(dòng)態(tài)更改目標(biāo)函數(shù)來(lái)實(shí)現(xiàn)織入過(guò)程，這時(shí)調(diào)用該方法可以使切面函數(shù)與業(yè)務(wù)邏輯同時(shí)進(jìn)行。 function aspect(type){return function(target, methodName, advice, receiveArguments){var existing = target[methodName], dispatcher;if(!existing || existing.target != target){// no dispatcher in placetarget[methodName] = dispatcher = function(){var executionId = nextId;// before advicevar args = arguments;var before = dispatcher.before;while(before){args = before.advice.apply(this, args) || args;before = before.next;}// around adviceif(dispatcher.around){var results = dispatcher.around.advice(this, args);}// after advicevar after = dispatcher.after;while(after && after.id < executionId){if(after.receiveArguments){var newResults = after.advice.apply(this, args);// change the return value only if a new value was returnedresults = newResults === undefined ? results : newResults;}else{results = after.advice.call(this, results, args);}after = after.next;}return results;};if(existing){dispatcher.around = {advice: function(target, args){return existing.apply(target, args);}};}dispatcher.target = target;}var results = advise((dispatcher || existing), type, advice, receiveArguments);advice = null;return results;};} View Code

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　　注意：dojo的處理過(guò)程中并不生成代理對(duì)象，而是直接更改原有的對(duì)象的方法。

　　關(guān)于aspect.after方法（before方法與其類似）的解釋請(qǐng)看這篇文章：Javascript事件機(jī)制兼容性解決方案；aspect.around的由來(lái)在這篇文章Javascript aop(面向切面編程)之a(chǎn)round(環(huán)繞)里有其一步步的演化過(guò)程。

　　本文給出aspect模塊調(diào)用后的示意圖：

　　before與after函數(shù)：

　　

　　around函數(shù)：

　　

var advised = advice(function(){return previous.advice(this, arguments);});signal = {remove: function(){if(advised){advised = dispatcher = advice = null;}},advice: function(target, args){return advised ? //一旦調(diào)用remove，adviced變?yōu)榭?#xff0c;便會(huì)跳過(guò)本次環(huán)繞通知，進(jìn)入上一層的advice方法。advised.apply(target, args) : // called the advised functionprevious.advice(target, args); // cancelled, skip to next one }};

　　可以看到around函數(shù)中借用閉包形成環(huán)繞函數(shù)鏈。這里調(diào)用remove方法后并沒(méi)有像before跟after中將通知方法徹底移除，注冊(cè)過(guò)的環(huán)繞方法仍然會(huì)存在內(nèi)存中，所以這個(gè)方法無(wú)法移除環(huán)繞通知，僅僅是避免了在函數(shù)鏈中執(zhí)行它而已。內(nèi)存無(wú)法釋放，不建議使用太多。

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