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05-《AspNetCore》-AOP

接口
2023-03-31 11:04:37 时间

AOP

视频讲解
面向切面编程AOP的对面向对象编程OOP的一个补充，它的特点是将系统逻辑和业务逻辑采取《非侵入式》分离。我们把系统封装成一个一个的切面（单一职责）进行顺意编排组合，插入（织入）到业务逻辑的执行过程（织入点）。

系统逻辑：异常处理，身份认证，授权，mvc，数据校验，事务处理。

业务逻辑：就是我们的业务Service。

切面：用于封装系统逻辑，比如身份认证filter，或者中间件

切入点：就是管道的位置。名词

织入：就是插入到管道的切入点的过程。动词

AOP的特点：

1.非侵入式

2.低耦合

3.代码服用

4.单一职责

5.可插拔

实现方式：

1.管道链，比如aspnetcore的中间件，mvc中的Filter

2.静态代理：思考如何加强一个List，使得在插入时打印日志？

3.动态代理：Emit

4.三种模式都需要通过一些技术进行串联，实现链式调用，构成管道。静态代理通过接口进行串联，动态代理通过反射进行串联。管道通过接口获取委托进行串联。委托本质也是接口。

代理：就是增强，代理对象必须尽量实现目标对象的功能，在此基础上进行加强。比如vpn，你的电脑的网络就是目标对象，vpn就是代理服务，代理服务起码得实现联网功能吧，然后对网络进行加强，访问到一些你的本机网络访问不到的东西。

掌握了AOP技术我们可以实现很多好处，做到非侵入式的增强业务逻辑。

//侵入式方案，把这个看懂。后面就是围绕这个开展，高出非侵入式
public static void A()
{
    Console.WriteLine("A:开始");
    B();//A，B的调用关系强行绑定，有侵入性
    Console.WriteLine("A:结束");
}
public static void B()
{
    Console.WriteLine("B:开始");
    C();
    Console.WriteLine("B:结束");
}
public static void C()
{
    Console.WriteLine("Hello World");
}
public static void Dobasic()
{
    A();
}

静态代理

1.代理就是实现目标对象的标准（接口），在目标方法执行之前和之后进行逻辑织入的过程。代理的目的就是为了加强。代理不负责实现接口，一般通过target来实现接口。即代理除了可以增强之外还能简化接口的实现。

2.静态代理就是在代理之前就已经确定了代理关系。需要自己实现标准并编写代理类。代理类中的逻辑只能代理一些标准（实现多个接口）。无法代理所有标准。

3.静态代理可以实现不改变目标对象的源码的情况下进行加强，完成目标对象的能力，并且在此基础之上进行加强。

4.可以简化实现的成本，不改变业务代码，只需要编写额外的增强逻辑。不需要关系具体的业务实现。

5.代理和代理直接通过接口可以进行互相代理，链式调用，顺意编排组合，实现系统的多样化。

 /// <summary>
/// 定义标准1
/// </summary>
public interface IPhoneService
{ 
    string Mobile { get; set; }
    string Message { get; set; }
    void Send();
}
//实现标准-不是代理模式
public class PhoneService : IPhoneService
{
    public string Mobile { get; set; }
    public string Message { get; set; }

    public PhoneService(string mobile, string message)
    {
        Mobile = mobile;
        Message = message;
    }

    public virtual void Send()
    {
        Console.WriteLine($"已发送短信：{Message}到{Mobile}");
    }
}
//代理模式：
//1.实现目标对象的标准
//2.依赖目标对象（被代理对象）
//3.业务织入
public class PhoneServiceProxy : IPhoneService//实现标准
{
    private readonly IPhoneService _target;

    public PhoneServiceProxy1(IPhoneService target)
    {
        _target = target;
    }

    public string Mobile { get => _target.Mobile; set => _target.Mobile = value; }
    public string Message { get => _target.Message; set => _target.Message = value; }

    /// <summary>
    /// 子类重写父类方法
    /// </summary>
    public void Send()
    {
        Console.WriteLine("Proxy1:已对手机号进行验证");
        _target.Send();
        Console.WriteLine("Proxy1:已确认对方已经收到");
    }
}


/// <summary>
/// 定义标准1
/// </summary>
public interface IPhoneService
{
    string Mobile { get; set; }
    string Message { get; set; }
    void Send();
}
/// <summary>
/// 定义标准2
/// </summary>
public interface IEmailService
{
    string Email { get; set; }
    string Message { get; set; }
    void Send();
}
/// <summary>
/// 业务逻辑1
/// </summary>
public class PhoneService : IPhoneService
{
    public string Mobile { get; set; }
    public string Message { get; set; }

    public PhoneService(string mobile, string message)
    {
        Mobile = mobile;
        Message = message;
    }

    public virtual void Send()
    {
        Console.WriteLine($"已发送短信：{Message}到{Mobile}");
    }
}
/// <summary>
/// 业务逻辑2
/// </summary>
public class EmailService : IEmailService
{
    public string Email { get; set; }
    public string Message { get; set; }

    public EmailService(string email, string message)
    {
        Email = email;
        Message = message;
    }

    public virtual void Send()
    {
        Console.WriteLine($"已发送邮件：{Message}到{Email}");
    }
}

/// <summary>
/// 切面1：校验能力（系统逻辑）
/// taget方式
/// </summary>
public class PhoneServiceProxy1
    : IPhoneService//实现标准1
{
    private readonly IPhoneService _target;

    public PhoneServiceProxy1(IPhoneService target)
    {
        _target = target;
    }

    /// <summary>
    /// 子类重写父类方法
    /// </summary>
    public void Send()
    {
        Console.WriteLine("Proxy1:已对手机号进行验证");
        _target.Send();
        Console.WriteLine("Proxy1:已确认对方已经收到");
    }
}
/// <summary>
/// 切面2：加速能力（系统逻辑）
/// </summary>
public class PhoneServiceProxy2
   : IPhoneService//实现标准1
{
    private readonly IPhoneService _target;

    public PhoneServiceProxy2(IPhoneService target)
    {
        _target = target;
    }

    /// <summary>
    /// 子类重写父类方法
    /// </summary>
    public void Send()
    {
        Console.WriteLine("Proxy2:已开启加速通道");
        _target.Send();
        Console.WriteLine("Proxy2:已关闭加速通道");
    }
}
//test
public static void TestStaticProxy()
{
    //目标对象
    IPhoneService target = new PhoneService("10088", "你好啊！");
    //切面1：验证，对target进行代理
    IPhoneService proxy1 = new PhoneServiceProxy1(target);
    //切面2：加速，对proxy1进行代理
    IPhoneService proxy2 = new PhoneServiceProxy2(proxy1);
    //执行
    proxy2.Send();
    //思考如果要实现IEmailService标准，是不是要重写实现类了？
}

动态代理

Castle.Core

动态代理和静态代理的区别就是，代理类由工具生成，需要在运行时确认代理类已经代理关系。代理类中的逻辑写到拦截器里面，可以进行复用。缺点是性能差。里面涉及到大量反射技术。

Castle.Core：原理就是通过子类继承父类或者实现父类标准，通过Castle.Core自动帮你生成代理类，通过一个叫拦截器的东西编写代理类要执行的业务逻辑。Castle.Core会帮你生成代理类，并将拦截器织入到代理类中。

动态代理通过invocation进行串联，本质是反射。

/// <summary>
/// 定义标准1
/// </summary>
public interface IPhoneService
{
    string Mobile { get; set; }
    string Message { get; set; }
    void Send();
}
/// <summary>
/// 定义标准2
/// </summary>
public interface IEmailService
{
    string Email { get; set; }
    string Message { get; set; }
    void Send();
}
/// <summary>
/// 业务逻辑1
/// </summary>
public class PhoneService : IPhoneService
{
    public string Mobile { get; set; }
    public string Message { get; set; }

    public PhoneService(string mobile, string message)
    {
        Mobile = mobile;
        Message = message;
    }

    public virtual void Send()
    {
        Console.WriteLine($"已发送短信：{Message}到{Mobile}");
    }
}
/// <summary>
/// 业务逻辑2
/// </summary>
public class EmailService : IEmailService
{
    public string Email { get; set; }
    public string Message { get; set; }

    public EmailService(string email, string message)
    {
        Email = email;
        Message = message;
    }

    public virtual void Send()
    {
        Console.WriteLine($"已发送邮件：{Message}到{Email}");
    }
}
/// <summary>
/// 代理1：任意标准
/// </summary>
public class ShareInterceptor1 : IInterceptor
{
    public void Intercept(IInvocation invocation)
    {
        Console.WriteLine("Proxy1:已对接收方进行验证");
        invocation.Proceed();//执行下一个拦截器或者目标方法
        Console.WriteLine("Proxy1:已确认对方已经收到");
    }
}
/// <summary>
/// 代理2：任意标准
/// </summary>
public class ShareInterceptor2 : IInterceptor
{
    public void Intercept(IInvocation invocation)
    {
        Console.WriteLine("Proxy2:已开启加速通道");
        invocation.Proceed();//执行下一个拦截器或者目标方法
        Console.WriteLine("Proxy2:已关闭加速通道");
    }
}
//通过Castel生成代理类
public static void TestDynamicProxy1()
{
    //创建代理生成器
    var generator = new ProxyGenerator();
    var target1 = new PhoneService("10088", "你好啊！");
    var target2 = new EmailService("1123@116.com", "你好啊！");
    var interceptor1 = new ShareInterceptor1();//代理1，拦截器1，不需要去实现指定的标准
    var interceptor2 = new ShareInterceptor2();//代理2，拦截器2，不需要去实现指定的标准
    //使用代理1和代理2去代理手机的标准
    IPhoneService dynamicProxy1 = generator.CreateInterfaceProxyWithTarget<IPhoneService>(target1, interceptor1, interceptor2);
    dynamicProxy1.Send();
    //代理邮件的标准
    IEmailService dynamicProxy2 = generator.CreateInterfaceProxyWithTarget<IEmailService>(target2, interceptor1, interceptor2);
    dynamicProxy2.Send();
}

手写Castle.Core的代理类

思考：

generator创建的是什么类型的实列？显然不可能是已有的类型。因为它把拦截器织入进去了。而且没有修改我们的代码，站在面向对象的角度来看只能是实现了我们的接口，Emit动态实现了下面的代码

多个拦截器和目标对象（被代理者）通过Invocation进行串联。Invocation中的Arguments完成链式调用。

手动通过Invocation进行串联

//假设有三个拦截器
//第一个拦截器invocation1：Proxy=interceptor2,Method=Intercept,argument=invocation2
//第二个拦截器invocation2：Proxy=interceptor3,Method=Intercept,argument=invocation3
//第三个拦截器invocation2：Proxy=target,Method=method,argument=arguments
//手动实现
public IInvocation GetInvocation(Stack<IInterceptor> stack, object target, Method method, objuect arguments)
{
    var invocation1 = new Invocation()
    {
        Proxy = interceptor2,
        Method = typeof(IInterceptor).GetMethod(nameof(IInterceptor.Intercept)),
        Arguments = new object[]
        {
            new Invocation()
            {
                Proxy = interceptor3,
                Method = typeof(IInterceptor).GetMethod(nameof(IInterceptor.Intercept)),
                Arguments = new object[]
                {
                    new Invocation()
                    {
                        Proxy = target,
                        Method = method,
                        Arguments = arguments
                    }  
                }
            }
        } 
    }
}

//递归实现
public IInvocation GetInvocation(Stack<IInterceptor> stack, object target, Method method, objuect arguments)
{
    if(stack.Any())
    {
        var proxy = stack.Pop();
        return new Invocation()
        {
          	Proxy = proxy, 
            Method = typeof(IInterceptor).GetMethod(nameof(IInterceptor.Intercept)),
            Agrumtns = GetInvocation(stack,method,argumtns)
        };
    }
    else 
    {
        return new Invocation()
        {
          	Proxy = target, 
            Method = method,
            Agrumtns = arguments
        };
    }
}


//Castel.Core自动帮我们生成了下面这个类
public class CastelPhoneServiceProxy : IPhoneService
{
    private IPhoneService _taget;
    private IInterceptor[] _interceptors;

    public CastelPhoneServiceProxy(IPhoneService taget, IInterceptor[] interceptors)
    {
        _taget = taget;
        _interceptors = interceptors;
    }

    public string Mobile { get => _taget.Mobile; set => _taget.Mobile = value; }
    public string Message { get => _taget.Message; set => _taget.Message = value; }

    public void Send()
    {
        var stack = new Stack<IInterceptor>(_interceptors.Reverse());
        if (stack.Any())
        {
            var item = stack.Pop();
            var invocation = GetNextInvocation(stack);
            item.Intercept(invocation);
        }
        else
        {
            _taget.Send();
        }
    }
    /// <summary>
    /// 递归获取Invocaltion
    /// </summary>
    /// <param name="stack"></param>
    /// <returns></returns>
    private IInvocation GetNextInvocation(Stack<IInterceptor> stack)
    {
        if (stack.Any())
        {
            var next = stack.Pop();
            return new Invocaltion
            {
                Arguments = new object[]
                {
                    //递归
                    GetNextInvocation(stack)
                },
                Proxy = next,
                Method = typeof(IInterceptor).GetMethod(nameof(IInterceptor.Intercept)) ?? throw new NullReferenceException()
            };
        }
        else
        {
            return new Invocaltion
            {
                Arguments = new object[]
                {

                },
                Proxy = _taget,
                Method = _taget.GetType().GetMethod(nameof(IPhoneService.Send)) ?? throw new NullReferenceException()
            };
        }
    }
}
//实现一些castle.core的接口
public class Invocaltion : IInvocation
{
    public object[] Arguments { get; set; }

    public Type[] GenericArguments { get; set; }

    public object InvocationTarget { get; set; }

    public MethodInfo Method { get; set; }

    public MethodInfo MethodInvocationTarget { get; set; }

    public object Proxy { get; set; }

    public object ReturnValue { get; set; }

    public Type TargetType { get; set; }

    public IInvocationProceedInfo CaptureProceedInfo()
    {
        throw new NotImplementedException();
    }

    public object GetArgumentValue(int index)
    {
        throw new NotImplementedException();
    }

    public MethodInfo GetConcreteMethod()
    {
        throw new NotImplementedException();
    }

    public MethodInfo GetConcreteMethodInvocationTarget()
    {
        throw new NotImplementedException();
    }

    public void Proceed()
    {
        Method.Invoke(Proxy, Arguments);
    }

    public void SetArgumentValue(int index, object value)
    {
        throw new NotImplementedException();
    }
}

EMIT实现

//链路器
public class EmitInvocation
{
    private object? proxy;
    private MethodInfo? method;
    private object[]? arguments;
    public EmitInvocation(object? proxy, MethodInfo? method, object[]? arguments)
    {
        this.proxy = proxy;
        this.method = method;
        this.arguments = arguments;
    }

    public void Proceed()
    {
        method?.Invoke(proxy, arguments);
    }

}
//拦截器
public interface IEmitInteceptor
{
    void Intercept(EmitInvocation invocation);
}
//实现拦截器1
public class EmitInteceptor1 : IEmitInteceptor
{
    public void Intercept(EmitInvocation invocation)
    {
        Console.WriteLine("prox1:start");
        invocation.Proceed();
        Console.WriteLine("prox1:end");
    }
}
//实现拦截器1
public class EmitInteceptor2 : IEmitInteceptor
{
    public void Intercept(EmitInvocation invocation)
    {
        Console.WriteLine("prox2:start");
        invocation.Proceed();
        Console.WriteLine("prox2:end");
    }
}
//该工具类帮助我们少写emit代码
public static class EmitProxyInvoker
{
    public static EmitInvocation GetNextInvocation(Stack<IEmitInteceptor> stack, MethodInfo method, object target, object[] arguments)
    {
        if (stack.Any())
        {
            var next = stack.Pop();
            arguments = new object[]
            { 
                //递归
                GetNextInvocation(stack, method, target, arguments)
            };
            return new EmitInvocation(next, typeof(IEmitInteceptor).GetMethod(nameof(IEmitInteceptor.Intercept)), arguments);
        }
        else
        {
            return new EmitInvocation(target, method, arguments);
        }
    }

    public static void Invoke(IEmitInteceptor[] interceptors, MethodInfo method, object target, object[] arguments)
    {
        var stack = new Stack<IEmitInteceptor>(interceptors.Reverse());
        if (stack.Any())
        {
            var item = stack.Pop();
            var invocation = GetNextInvocation(stack, method, target, arguments);
            item.Intercept(invocation);
        }
        else
        {
            method.Invoke(target, arguments);
        }
    }
}
//业务接口
public interface IEmitService
{
    void Send();
}
//将来要生成的代理类
public class EmitServiceProxy : IEmitService
{
    private object _target;
    private IEmitInteceptor[] _inteceptors;

    public EmitService()
    {

    }

    public void Send()
    {
        var method = _target.GetType().GetMethod(nameof(EmitService.Send));
        var arguments = new object[] { };
        EmitProxyInvoker.Invoke(_inteceptors, method, _target, new object[] { });
    }
}

public static class EmitProxyGenerator
{
    static AssemblyBuilder _assemblyBuilder;
    static ModuleBuilder _moduleBuilder;
    static EmitProxyGenerator()
    {
        //创建一个程序集
        var assemblyName = new AssemblyName("DynamicProxies");
        _assemblyBuilder = AssemblyBuilder
            .DefineDynamicAssembly(assemblyName, AssemblyBuilderAccess.Run);
        //创建一个模块
        _moduleBuilder = _assemblyBuilder.DefineDynamicModule("Proxies");
    }
    public static TInterface Create<TInterface>(object target, params IEmitInteceptor[] inteceptor)
        where TInterface : class
    {
        
        #region 定义类型
        //定义一个class，如果这个类型已定义直接返回，缓存
        var typeName = $"{target.GetType().Name}EmitProxy";
        var typeBuilder = _moduleBuilder.DefineType(
            typeName, 
            TypeAttributes.Public,typeof(object),
            new Type[] 
            { 
                typeof(TInterface) 
            });
        #endregion

        #region 定义字段
        //定义字段
        var targetFieldBuilder = typeBuilder.DefineField("target", typeof(object), FieldAttributes.Private);
        var inteceptorFieldBuilder = typeBuilder.DefineField("inteceptor", typeof(IEmitInteceptor[]), FieldAttributes.Private);
        #endregion

        #region 定义构造器
        //定义构造器
        var constructorBuilder = typeBuilder.DefineConstructor(MethodAttributes.Public, CallingConventions.ExplicitThis, new Type[]
        {
            typeof(object),
            typeof(IEmitInteceptor[])
        });
        //获取IL编辑器
        var generator = constructorBuilder.GetILGenerator();
        generator.Emit(OpCodes.Ldarg_0);//加载this
        generator.Emit(OpCodes.Call, typeof(object).GetConstructor(Type.EmptyTypes) ?? throw new InvalidOperationException());
        generator.Emit(OpCodes.Nop);
        generator.Emit(OpCodes.Nop);
        // this.age = age;
        generator.Emit(OpCodes.Ldarg_0);//加载this
        generator.Emit(OpCodes.Ldarg_1);//加载target参数
        generator.Emit(OpCodes.Stfld, targetFieldBuilder);//加载target字段
        // this.name = name;
        generator.Emit(OpCodes.Ldarg_0);//加载this
        generator.Emit(OpCodes.Ldarg_2);//加载inteceptor参数
        generator.Emit(OpCodes.Stfld, inteceptorFieldBuilder);//加载inteceptor字段
        generator.Emit(OpCodes.Ret);

        #endregion

        #region 实现接口
        var methods = typeof(TInterface).GetMethods();
        foreach (var item in methods)
        {
            var parameterTypes = item.GetParameters().Select(s => s.ParameterType).ToArray();
            var methodBuilder = typeBuilder.DefineMethod(item.Name,
                MethodAttributes.Public| MethodAttributes.Final |MethodAttributes.Virtual | MethodAttributes.NewSlot | MethodAttributes.HideBySig,
                CallingConventions.Standard|CallingConventions.HasThis,
                item.ReturnType,
                parameterTypes);
            var generator1 = methodBuilder.GetILGenerator();
            //init
            var methodInfoLocal = generator1.DeclareLocal(typeof(MethodInfo));
            var argumentLocal = generator1.DeclareLocal(typeof(object[]));
            generator1.Emit(OpCodes.Nop);
            generator1.Emit(OpCodes.Ldarg_0);
            generator1.Emit(OpCodes.Ldfld, targetFieldBuilder);
            generator1.Emit(OpCodes.Callvirt, typeof(Type).GetMethod(nameof(Type.GetType),Type.EmptyTypes));
            generator1.Emit(OpCodes.Ldstr, item.Name);
            generator1.Emit(OpCodes.Callvirt, typeof(Type).GetMethod(nameof(Type.GetMethod), new Type[] { typeof(string) }));
            generator1.Emit(OpCodes.Stloc, methodInfoLocal);
            generator1.Emit(OpCodes.Ldc_I4_0);
            generator1.Emit(OpCodes.Newarr, typeof(object));
            generator1.Emit(OpCodes.Stloc, argumentLocal);
            generator1.Emit(OpCodes.Ldarg_0);
            generator1.Emit(OpCodes.Ldfld, inteceptorFieldBuilder);
            generator1.Emit(OpCodes.Ldloc_0);
            generator1.Emit(OpCodes.Ldarg_0);
            generator1.Emit(OpCodes.Ldfld, targetFieldBuilder);
            generator1.Emit(OpCodes.Ldc_I4_0);
            generator1.Emit(OpCodes.Newarr, typeof(object));
            generator1.Emit(OpCodes.Call, typeof(EmitProxyInvoker).GetMethod(nameof(EmitProxyUtil.Invoke)));
            generator1.Emit(OpCodes.Nop);
            generator1.Emit(OpCodes.Ret);
        }
        #endregion
        //创建:这个type可以用一个线程安全的字典缓存起来，第二次需要这个代理类的时候，就不需要在生成一次emit代码了。
        var type = typeBuilder.CreateType() ?? throw new ArgumentException();
        var instance = Activator.CreateInstance(type, target, inteceptor);
        return (TInterface)instance;
    }
}

容器支持

public class DbContext
{
    
}

public class AService
{
    public DbContext DbContext { get; }
    
    public AService(DbContext context)
    {
        DbContext = context;
    }
}
public static void Test()
{
    var services = new ServiceCollection();
    services.AddScoped<DbContext>();
    var generator = new ProxyGenerator();
    //泛型-不支持动态注入
    services.AddScoped(sp => 
    {
        //通过容器解析依赖
        var target = ActivatorUtilities.CreateInstance<AService>(sp);
        return generator.CreateClassProxyWithTarget(target);
    });
    //反射-可以扫描批量注入
    services.AddScoped(typeof(AService), sp =>
    {
        //通过容器解析依赖
        var target = ActivatorUtilities.CreateInstance(sp, typeof(AService));
        return generator.CreateClassProxyWithTarget(target);
    });
}

管道方式

委托方式

1.通过委托构建管道

public delegate Task RequestDelegate(HttpContext context);

public class HttpContext
{

}

public class ApplicationBuilder
{
    private readonly List<Func<RequestDelegate, RequestDelegate>> _componen

    public void Use(Func<RequestDelegate, RequestDelegate> middleware)
    {
        _components.Add(middleware);
    }

    public void Use(Func<HttpContext, Func<Task>, Task> middleware)
    {
        _components.Add((next) =>
        {
            return async c =>
            {
                await middleware(c, () => next(c));
            };
        });
    }

    public void Use(Func<HttpContext, RequestDelegate, Task> middleware)
    {
        _components.Add((next) =>
        {
            return async c =>
            {
                await middleware(c, next);
            };
        });
    }

    public void Run(RequestDelegate handler)
    {
        _components.Add((next) =>
        {
            return async c =>
            {
                await handler(c);
            };
        });
    }
    //构建管道
    public RequestDelegate Build()
    {
        RequestDelegate app = c =>
        {
            throw new InvalidOperationException("无效的管道");
        };
        for (int i = _components.Count - 1; i > -1; i--)
        {
            app = _components[i](app);
        }
        return app;
    }
}

接口方式

2.通过接口构建管道

有点类型动态代理，动态代理是通过Invocation进行反射，而下面的方式是通过接口的方式。反射更加灵活，性能不行。

public interface IChain
{
    Task NextAsync();
}
public class FilterChain : IChain
{
    private readonly IFilter _filter;
    private readonly HttpContext _context;
    private readonly IChain _next;
    public FilterChain(IFilter filter, HttpContext context, IChain next)
    {
        _filter = filter;
        _context = context;
        _next = next;
    }
    public async Task NextAsync()
    {
        await _filter.InvokeAsync(_context, _next);
    }
}
public class ServletChain : IChain
{
    private readonly IServlet _servlet;
    private readonly HttpContext _context;

    public ServletChain(IServlet servlet, HttpContext context)
    {
        _servlet = servlet;
        _context = context;
    }

    public async Task NextAsync()
    {
        await _servlet.DoPostAsync(_context);
    }
}
public interface IFilter
{
    Task InvokeAsync(HttpContext context, IChain chain);
}
public class Filter1 : IFilter
{
    public async Task InvokeAsync(HttpContext context, IChain chain)
    {
        Console.WriteLine("身份认证开始");
        await chain.NextAsync();
        Console.WriteLine("身份认证结束");
    }
}

public class Filter2 : IFilter
{
    public async Task InvokeAsync(HttpContext context, IChain chain)
    {
        Console.WriteLine("授权认证开始");
        await chain.NextAsync();
        Console.WriteLine("授权认证结束");
    }
}

public interface IServlet
{
    Task DoPostAsync(HttpContext context);
}

public class HelloServlet : IServlet
{
    public Task DoPostAsync(HttpContext context)
    {
        Console.WriteLine("Hello World");
        return Task.CompletedTask;
    }
}

public class WebHost
{
    private readonly List<IFilter> _filters = new List<IFilter>();

    public void AddFilter(IFilter filter)
    {
        _filters.Add(filter);
    }

    public void Exeucte(HttpContext context, IServlet servlet)
    {
        //自行处理filter为空的情况,就是直接执行serlvet就好了
        var stack = new Stack<IFilter>(_filters);
        var filter = stack.Pop();
        var chain = GetFilterChain(context, servlet,stack);
        filter.InvokeAsync(context, chain);
    }

    private IChain GetFilterChain(HttpContext context, IServlet servlet, Stack<IFilter> filters)
    {
        if (filters.Any())
        {
            var filter = filters.Pop();
            var chain = GetFilterChain(context, servlet, filters);
            return new FilterChain(filter, context, chain);
        }
        else
        {
            return new ServletChain(servlet, context);
        }
    }
}

AOP总结

1.代理分为静态代理和动态代理，静态代理需要自己编写代理类，动态代理由框架生成代理类。

2.代理和管道都需要通过接口（委托）进行链接，串联，形成链式调用。

3.动态代理慎用，因为涉及到反射技术，而且对异步支持不友好。

4.静态代理常用于加强已有类型，比如接口要求一个IList，我们已经拥有了一个list实列，我们需要在在list.Add方法时打印日志，此时我们可以不改变原有的list，通过静态代理实现IList接口来进行对原有的list加强。这个方法在更改框架的时候很有用。我们可以对原有的HttpContext，进行加强。

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