dojo/aspect源码解析

　dojo/aspect模块是dojo框架中对于AOP的实现。关于AOP的详细解释请读者另行查看其它资料，这里简单复习一下AOP中的基本概念：

1. 切面（Aspect）：其实就是共有功能的实现。如日志切面、权限切面、事务切面等。
2. 通知（Advice）：是切面的具体实现。以目标方法为参照点，根据放置的地方不同，可分为前置通知（Before）、后置通知（After）与环绕通知（Around）。
3. 连接点（Joinpoint）：就是程序在运行过程中能够插入切面的地点。
4. 目标对象（Target）：就是那些即将切入切面的对象，也就是那些被通知的对象。这些对象中已经只剩下干干净净的核心业务逻辑代码了，所有的共有功能代码等待AOP容器的切入。
5. 代理对象（Proxy）：将通知应用到目标对象之后被动态创建的对象。可以简单地理解为，代理对象的功能等于目标对象的核心业务逻辑功能加上共有功能。代理对象对于使用者而言是透明的，是程序运行过程中的产物。
6. 织入（Weaving）：将切面应用到目标对象从而创建一个新的代理对象的过程。这个过程可以发生在编译期、类装载期及运行期，当然不同的发生点有着不同的前提条件。譬如发生在编译期的话，就要求有一个支持这种AOP实现的特殊编译器；发生在类装载期，就要求有一个支持AOP实现的特殊类装载器；只有发生在运行期，则可直接通过Java语言的反射机制与动态代理机制来动态实现。

　　生成代理对象的过程可以按照下图理解：

　　dojo/aspect模块代码主要分为两部分：

• advise方法，通过使用闭包跟链式模型来构造“通知”链。 

"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 function
                        previous.advice(target, args); // cancelled, skip to next one
                }
            };
        }else{
            // create the remove handler
            signal = {
                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 removed
                        dispatcher = 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 bug
                while(previous.next && (previous = previous.next)){}
                previous.next = signal;
                signal.previous = previous;
            }else if(type == "before"){
                // add to beginning
                dispatcher[type] = signal;
                signal.next = previous;
                previous.previous = signal;
            }
        }else{
            // around or first one just replaces
            dispatcher[type] = signal;
        }
        return signal;
    }

• aspect方法，这个函数返回一个闭包。闭包的作用是将“通知”方法织入到目标函数中，java中运行时通过反射的方式来织入，而js中通过动态更改目标函数来实现织入过程，这时调用该方法可以使切面函数与业务逻辑同时进行。 

function aspect(type){
        return function(target, methodName, advice, receiveArguments){
            var existing = target[methodName], dispatcher;
            if(!existing || existing.target != target){
                // no dispatcher in place
                target[methodName] = dispatcher = function(){
                    var executionId = nextId;
                    // before advice
                    var args = arguments;
                    var before = dispatcher.before;
                    while(before){
                        args = before.advice.apply(this, args) || args;
                        before = before.next;
                    }
                    // around advice
                    if(dispatcher.around){
                        var results = dispatcher.around.advice(this, args);
                    }
                    // after advice
                    var 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 returned
                            results = 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;
        };
    }

　注意：dojo的处理过程中并不生成代理对象，而是直接更改原有的对象的方法。

　　关于aspect.after方法（before方法与其类似）的解释请看这篇文章：Javascript事件机制兼容性解决方案；aspect.around的由来在这篇文章Javascript aop(面向切面编程)之around(环绕)里有其一步步的演化过程。

　　本文给出aspect模块调用后的示意图：

　　before与after函数：

　　around函数：

var advised = advice(function(){
                return previous.advice(this, arguments);
            });
            signal = {
                remove: function(){
                    if(advised){
                        advised = dispatcher = advice = null;
                    }
                },
                advice: function(target, args){
                    return advised ? //一旦调用remove，adviced变为空，便会跳过本次环绕通知，进入上一层的advice方法。
                        advised.apply(target, args) :  // called the advised function
                        previous.advice(target, args); // cancelled, skip to next one
                }
            };