详解ASP.NET Core 中基于工厂的中间件激活的实现方法
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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本文标题：详解ASP.NET Core 中基于工厂的中间件激活的实现方法
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