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:

  技术分享

  

  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。
    技术分享
    "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;
        }
    View Code
  • 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。
    技术分享
    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;
            };
        }
    View Code

     

  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:

  技术分享

  around函数:

  技术分享

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
                }
            };

  可以看到around函数中借用闭包形成环绕函数链。这里调用remove方法后并没有像before跟after中将通知方法彻底移除,注册过的环绕方法仍然会存在内存中,所以这个方法无法移除环绕通知,仅仅是避免了在函数链中执行它而已。内存无法释放,不建议使用太多。

 

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