ASP.NET Core 中基于工厂的中间件激活详解

IMiddlewareFactory/IMiddleware是中间件激活的扩展点。

UseMiddleware扩展方法检查中间件的已注册类型是否实现IMiddleware。如果是，则使用在容器中注册的IMiddlewareFactory实例来解析IMiddleware实现，而不使用基于约定的中间件激活逻辑。中间件在应用的服务容器中注册为作用域或瞬态服务。

优点：

按客户端请求（作用域服务的注入）激活
让中间件强类型化

IMiddleware按客户端请求（连接）激活，因此作用域服务可以注入到中间件的构造函数中。

IMiddleware

IMiddleware定义应用的请求管道的中间件。InvokeAsync(HttpContext, RequestDelegate)方法处理请求，并返回代表中间件执行的Task。

使用约定激活的中间件：

public class ConventionalMiddleware
{
  private readonly RequestDelegate _next;

  public ConventionalMiddleware(RequestDelegate next)
  {
    _next = next;
  }

  public async Task InvokeAsync(HttpContext context, AppDbContext db)
  {
    var keyValue = context.Request.Query["key"];

    if (!string.IsNullOrWhiteSpace(keyValue))
    {
      db.Add(new Request()
        {
          DT = DateTime.UtcNow, 
          MiddlewareActivation = "ConventionalMiddleware", 
          Value = keyValue
        });

      await db.SaveChangesAsync();
    }

    await _next(context);
  }
}

使用MiddlewareFactory激活的中间件：

public class FactoryActivatedMiddleware : IMiddleware
{
  private readonly AppDbContext _db;

  public FactoryActivatedMiddleware(AppDbContext db)
  {
    _db = db;
  }

  public async Task InvokeAsync(HttpContext context, RequestDelegate next)
  {
    var keyValue = context.Request.Query["key"];

    if (!string.IsNullOrWhiteSpace(keyValue))
    {
      _db.Add(new Request()
        {
          DT = DateTime.UtcNow, 
          MiddlewareActivation = "FactoryActivatedMiddleware", 
          Value = keyValue
        });

      await _db.SaveChangesAsync();
    }

    await next(context);
  }
}

程序会为中间件创建扩展：

public static class MiddlewareExtensions
{
  public static IApplicationBuilder UseConventionalMiddleware(
    this IApplicationBuilder builder)
  {
    return builder.UseMiddleware<ConventionalMiddleware>();
  }

  public static IApplicationBuilder UseFactoryActivatedMiddleware(
    this IApplicationBuilder builder)
  {
    return builder.UseMiddleware<FactoryActivatedMiddleware>();
  }
}

无法通过UseMiddleware将对象传递给工厂激活的中间件：

public static IApplicationBuilder UseFactoryActivatedMiddleware(
  this IApplicationBuilder builder, bool option)
{
  // Passing 'option' as an argument throws a NotSupportedException at runtime.
  return builder.UseMiddleware<FactoryActivatedMiddleware>(option);
}

将工厂激活的中间件添加到Startup.ConfigureServices的内置容器中：

public void ConfigureServices(IServiceCollection services)
{
  services.AddDbContext<AppDbContext>(options =>
    options.UseInMemoryDatabase("InMemoryDb"));

  services.AddTransient<FactoryActivatedMiddleware>();

  services.AddRazorPages();
}

两个中间件均在Startup.Configure的请求处理管道中注册：

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
  if (env.IsDevelopment())
  {
    app.UseDeveloperExceptionPage();
  }
  else
  {
    app.UseExceptionHandler("/Error");
  }

  app.UseConventionalMiddleware();
  app.UseFactoryActivatedMiddleware();

  app.UseStaticFiles();
  app.UseRouting();

  app.UseEndpoints(endpoints =>
  {
    endpoints.MapRazorPages();
  });
}

IMiddlewareFactory

IMiddlewareFactory提供中间件的创建方法。中间件工厂实现在容器中注册为作用域服务。

可在Microsoft.AspNetCore.Http包中找到默认的IMiddlewareFactory实现（即MiddlewareFactory）。

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