C# 代理应用：Cachable

放心，这次不是说设计模式中的代理模式，说的是C#的RealProxy的用法，主要用于：通过给class 贴标签，让class做更多的工作，比如判断是否存在缓存，有则直接返回缓存object，没有则保存为缓 存，等待下次请求是可以更快的获取数据（当然这只是其中一种常用用途，MVC的Action就是采用这种 方式）

下面是序列图：

.Net Object Generation interceptor属于.NET自身行为，不需要额外写代码。

Code Consumer指想调用RealObject来进行调用的对象，比如控制台程序，或者WEB程序。

ProxyAttribute里定义了具体代理类是哪个，这个代理类是自己 继承RealProxy写的一个代理类， 这个类中需要加入前置、后置、环绕等方法（具体根据需求）

下面我们来看具体如何在.Net中实现：

public class FollowAction
    {
        public bool StopExecute { get; set; }             //用于在前置方法中，指示是否停止执行真正的方法体，比如已经找到了cache value，因此不需要继续运行方法体的情况
        public object Result { get; set; }                //保存cache value的变量
    }

public abstract class CachableRealProxy : RealProxy
    {
        private MarshalByRefObject target;

        public MyAOPRealProxy(Type objType, MarshalByRefObject obj)
            : base(objType)
        {
            target = obj;
        }

        public override IMessage Invoke(IMessage msg)
        {
            IMessage retMsg = null;
            IMethodCallMessage methodCall = (IMethodCallMessage)msg;
            IMethodReturnMessage methodReturn = null;
            object[] copiedArgs = Array.CreateInstance(typeof(object), methodCall.Args.Length) as object[];
            methodCall.Args.CopyTo(copiedArgs, 0);
            if (msg is IConstructionCallMessage)
            {

                IConstructionCallMessage ccm = (IConstructionCallMessage)msg;
                RemotingServices.GetRealProxy(target).InitializeServerObject(ccm);
                ObjRef oRef = RemotingServices.Marshal(target);
                RemotingServices.Unmarshal(oRef);
                retMsg = EnterpriseServicesHelper.CreateConstructionReturnMessage(ccm, (MarshalByRefObject)this.GetTransparentProxy());

            }
            else
            {
                bool aopAttributeExists = false;
                object[] attrs = methodCall.MethodBase.GetCustomAttributes(false);
                if (attrs != null && attrs.Count() > 0)
                {
                    foreach(object o in attrs)
                    {
                        CachableAttribute attr = o as CachableAttribute;
                        if (attr != null)
                        {
                            aopAttributeExists = true;
                            break;
                        }
                    }
                }
                FollowAction follow=null;
                if (aopAttributeExists)
                    follow = this.PreProcess(msg);

                try
                {
                    object returnValue = null;
                    if (follow != null && follow.StopExecute)
                        returnValue = follow.Result;
                    else
                        returnValue = methodCall.MethodBase.Invoke(this.target, copiedArgs);
                    methodReturn = new ReturnMessage(returnValue, copiedArgs, copiedArgs.Length, methodCall.LogicalCallContext, methodCall);

                    if (follow == null || !follow.StopExecute)
                        if (aopAttributeExists)
                            this.PostProcess(msg, methodReturn);
                }
                catch (Exception ex)
                {
                    if (null != ex.InnerException)
                    {
                        methodReturn = new ReturnMessage(ex.InnerException, methodCall);
                    }
                    else
                    {
                        methodReturn = new ReturnMessage(ex, methodCall);
                    }
                }
                retMsg = methodReturn;

            }
            return retMsg;
        }

       public override FollowAction PreProcess(System.Runtime.Remoting.Messaging.IMessage requestMsg)  //处理前置方法
        {
            bool cacheDefinationTagExists = true;
            CachableAttribute cacheDefine = CheckCacheDefinationTag(requestMsg, ref cacheDefinationTagExists);
            if (!cacheDefinationTagExists)
                return null;

            string cacheKey = cacheDefine.GenerateCacheKey();
            object o=CacheManager.Instance().GetCacheCore().Get(cacheDefine.Location, cacheKey);
            if (o != null)
            {
                FollowAction follow = new FollowAction();
                follow.Result = o;
                follow.StopExecute = true;
                return follow;
            }

            return null;
        }

        public override void PostProcess(System.Runtime.Remoting.Messaging.IMessage requestMsg, System.Runtime.Remoting.Messaging.IMessage responseMsg)//处理后置方法
        {
            bool cacheDefinationTagExists = true;
            CachableAttribute cacheDefine = CheckCacheDefinationTag(requestMsg, ref cacheDefinationTagExists);
            if (!cacheDefinationTagExists)
                return;

            ReturnMessage returnMsg = (ReturnMessage)responseMsg;

            string cacheKey = cacheDefine.GenerateCacheKey();
            CacheManager.Instance().GetCacheCore().Set(cacheDefine.Location, cacheKey, returnMsg.ReturnValue);
        }
        private static CachableAttribute CheckCacheDefinationTag(System.Runtime.Remoting.Messaging.IMessage requestMsg, ref bool cacheDefinationTagExists)//Help函数
        {
            IMethodCallMessage methodCall = (IMethodCallMessage)requestMsg;
            object[] attrs = methodCall.MethodBase.GetCustomAttributes(typeof(CachableAttribute), false);
            if (attrs == null || attrs.Count() <= 0)
                cacheDefinationTagExists = false;
            CachableAttribute cacheDefine = attrs[0] as CachableAttribute;
            if (cacheDefine == null)
                cacheDefinationTagExists = false;
            return cacheDefine;
        }
    }

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