利用.NET Core类库System.Reflection.DispatchProxy实现简易Aop
背景
Aop即是面向切面编程，众多Aop框架里Castle是最为人所知的，另外还有死去的Spring.NET，当然，.NET Core社区新秀AspectCore在性能与功能上都非常优秀，已经逐渐被社区推崇和有越来越多的人使用。感谢柠檬同学的礼物！
如果大家出于自身需求或者学习，想实现一个Aop，是不是觉得一来就要使用Emit去做？最近我了解到了System.Reflection.DispatchProxy这个corefx类库，已经实现了动态代理功能。
System.Reflection.DispatchProxy
下面演示一下它的使用方法：
class Program
{
static void Main(string[] args)
{
//创建代理类，并把SampleProxy作为拦截器注入
var sampleProxy = (targetInterface)SampleProxy.Create<targetInterface, SampleProxy>();
//执行接口方法
sampleProxy.Write("here is invoke by proxy");
}
}
//需要被生成代理实例的接口
public interface targetInterface
{
//这个方法会被代理类实现
void Write(string writesomeshing);
}
public class SampleProxy : DispatchProxy
{
/// <summary>
/// 拦截调用
/// </summary>
/// <param name="method">所拦截的方法信息</param>
/// <param name="parameters">所拦截方法被传入的参数指</param>
/// <returns></returns>
protected override object Invoke(MethodInfo targetMethod, object[] args)
{
Console.WriteLine(args[0]);
return null;
}
}
改造成一款简易AOP
为什么？
System.Reflection.DispatchProxy只有一个Api，就是objecct Create<T,TProxy>() where TProxy:DispatchProxy,约束了只能传入泛型参数，并不能从方法传入类型，这就会带来很多问题。而更可气的是，给官方提了issue之后，还是不给增加这个api……
改造方法
幸好，在那个issue下，issue作者提供了一个解决方案，就是用反射来构造这个泛型方法。我还在这基础上，封装了一下，加入了传入拦截器实例和传入拦截器构造方法参数的功能。
/// <summary>
/// 拦截器接口
/// </summary>
public interface IInterceptor
{
/// <summary>
/// 拦截器调用
/// </summary>
/// <param name="target">代理实例</param>
/// <param name="method">所拦截的方法</param>
/// <param name="parameters">所拦截方法传入的参数值</param>
/// <returns>返回值会传递给方法返回值</returns>
object Intercept(object target, MethodInfo method, object[] parameters);
}
拦截器要实现这个接口，下面是对DispatchProxy的封装，实现更多创建代理实例的方法
public class ProxyGenerator : DispatchProxy
{
private IInterceptor interceptor { get; set; }
/// <summary>
/// 创建代理实例
/// </summary>
/// <param name="targetType">所要代理的接口类型</param>
/// <param name="interceptor">拦截器</param>
/// <returns>代理实例</returns>
public static object Create(Type targetType, IInterceptor interceptor)
{
object proxy = GetProxy(targetType);
((ProxyGenerator)proxy).CreateInstance(interceptor);
return proxy;
}
/// <summary>
/// 创建代理实例
/// </summary>
/// <param name="targetType">所要代理的接口类型</param>
/// <param name="interceptorType">拦截器类型</param>
/// <param name="parameters">拦截器构造函数参数值</param>
/// <returns>代理实例</returns>
public static object Create(Type targetType, Type interceptorType, params object[] parameters)
{
object proxy = GetProxy(targetType);
((ProxyGenerator)proxy).CreateInstance(interceptorType, parameters);
return proxy;
}
/// <summary>
/// 创建代理实例 TTarget:所要代理的接口类型 TInterceptor:拦截器类型
/// </summary>
/// <param name="parameters">拦截器构造函数参数值</param>
/// <returns>代理实例</returns>
public static TTarget Create<TTarget, TInterceptor>(params object[] parameters) where TInterceptor : IInterceptor
{
var proxy = GetProxy(typeof(TTarget));
((ProxyGenerator)proxy).CreateInstance(typeof(TInterceptor), parameters);
return (TTarget)proxy;
}
private static object GetProxy(Type targetType)
{
var callexp = Expression.Call(typeof(DispatchProxy), nameof(DispatchProxy.Create), new[] { targetType, typeof(ProxyGenerator) });
return Expression.Lambda<Func<object>>(callexp).Compile()();
}
private void CreateInstance(Type interceptorType, object[] parameters)
{
var ctorParams = parameters.Select(x => x.GetType()).ToArray();
var paramsExp = parameters.Select(x => Expression.Constant(x));
var newExp = Expression.New(interceptorType.GetConstructor(ctorParams), paramsExp);
this.interceptor = Expression.Lambda<Func<IInterceptor>>(newExp).Compile()();
}
private void CreateInstance(IInterceptor interceptor)
{
this.interceptor = interceptor;
}
protected override object Invoke(MethodInfo method, object[] parameters)
{
return this.interceptor.Intercept(method, parameters);
}
}
使用方法
class Program
{
static void Main(string[] args)
{
var poxy1 = (targetInterface)ProxyGenerator.Create(typeof(targetInterface), new SampleProxy("coreproxy1"));
poxy1.Write("here was invoked"); //---> "here was invoked by coreproxy1"
var poxy2 = (targetInterface)ProxyGenerator.Create(typeof(targetInterface), typeof(SampleProxy), "coreproxy2");
poxy2.Write("here was invoked"); //---> "here was invoked by coreproxy2"
var poxy3 = ProxyGenerator.Create<targetInterface, SampleProxy>("coreproxy3");
poxy3.Write("here was invoked"); //---> "here was invoked by coreproxy3"
}
}
public class SampleProxy : IInterceptor
{
private string proxyName { get; }
public SampleProxy(string name)
{
this.proxyName = name;
}
public object Intercept(MethodInfo method, object[] parameters)
{
Console.WriteLine(parameters[0] + " by " + proxyName);
return null;
}
}
public interface targetInterface
{
void Write(string writesome);
}
总结
总结一下就是，微软爸爸给我们的这个轮子还是即轻便又很好用的。
本文的实例代码可以在我的github上找到：https://github.com/ElderJames/CoreProxy
