用最少的代码打造一个Mini版的gRPC框架

在《用最少的代码模拟gRPC四种消息交换模式》中，我使用很简单的代码模拟了gRPC四种消息交换模式（Unary、Client Streaming、Server Streaming和Duplex Streaming），现在我们更近一步，试着使用极简的方式打造一个gRPC框架（github地址）。这个gRPC是对ASP.NET Core gRPC实现原理的模拟，并不是想重新造一个轮子。

一、“标准”的gRPC定义、承载和调用
二、将gRPC方法抽象成委托
三、将委托转换成RequestDelegate
   UnaryCallHandler
   ClientStreamingCallHandler
   ServerStreamingCallHandler
   DuplexStreamingCallHandler
四、路由注册
五、为gRPC服务定义一个接口
六、重新定义和承载服务

一、“标准”的gRPC定义、承载和调用

可能有些读者朋友们对ASP.NET Core gRPC还不是太熟悉，所以我们先来演示一下如何在一个ASP.NET Core应用中如何定义和承载一个简单的gRPC服务，并使用自动生成的客户端代码进行调用。我们新建一个空的解决方案，并在其中添加如下所示的三个项目。

我们在类库项目Proto中定义了如下所示Greeter服务，并利用其中定义的四个操作分别模拟四种消息交换模式。HelloRequest 和HelloReply 是它们涉及的两个ProtoBuf消息。

syntax = "proto3";
import "google/protobuf/empty.proto";

service Greeter {
  rpc SayHelloUnary (HelloRequest) returns ( HelloReply);
  rpc SayHelloServerStreaming (google.protobuf.Empty) returns (stream HelloReply);
  rpc SayHelloClientStreaming (stream HelloRequest) returns (HelloReply);
  rpc SayHelloDuplexStreaming (stream HelloRequest) returns (stream HelloReply);
}

message HelloRequest {
  string name = 1;
}

message HelloReply {
  string message = 1;
}

ASP.NET Core项目中定义了如下的GreeterServce服务实现了定义的四个操作，基类GreeterBase是针对上面这个.proto文件生成的类型。

public class GreeterService: GreeterBase
{
    public override Task<HelloReply> SayHelloUnary(HelloRequest request, ServerCallContext context)
    => Task.FromResult(new HelloReply { Message = $"Hello, {request.Name}" });

    public override async Task<HelloReply> SayHelloClientStreaming(IAsyncStreamReader<HelloRequest> reader, ServerCallContext context)
    {
        var list = new List<string>();
        while (await reader.MoveNext(CancellationToken.None))
        {
            list.Add(reader.Current.Name);
        }
        return new HelloReply { Message = $"Hello, {string.Join(",", list)}" };
    }

    public  override async Task SayHelloServerStreaming(Empty request, IServerStreamWriter<HelloReply> responseStream, ServerCallContext context)
    {
        await responseStream.WriteAsync(new HelloReply { Message = "Hello, Foo!" });
        await Task.Delay(1000);
        await responseStream.WriteAsync(new HelloReply { Message = "Hello, Bar!" });
        await Task.Delay(1000);
        await responseStream.WriteAsync(new HelloReply { Message = "Hello, Baz!" });
    }

    public override async Task SayHelloDuplexStreaming(IAsyncStreamReader<HelloRequest> reader, IServerStreamWriter<HelloReply> writer, ServerCallContext context)
    {
        while (await reader.MoveNext())
        {
            await writer.WriteAsync(new HelloReply { Message = $"Hello {reader.Current.Name}" });
        }
    }
}

具体的服务承载代码如下。我们采用Minimal API的形式，通过调用IServiceCollection接口的AddGrpc扩展方法注册相关服务，并调用MapGrpcService<TService>将定义在GreeterServce中的四个方法映射我对应的路由终结点。

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddGrpc();
builder.WebHost.ConfigureKestrel(kestrel => kestrel.ConfigureEndpointDefaults(options => options.Protocols = HttpProtocols.Http2));
var app = builder.Build();
app.MapGrpcService<GreeterService>();
app.Run();

在控制台项目Client中，我们利用生成出来的客户端类型GreeterClient分别一对应的服务交换模式调用了四个gRPC方法。

var channel = GrpcChannel.ForAddress("http://localhost:5000");
var client = new GreeterClient(channel);

Console.WriteLine("Unary");
await UnaryCallAsync();

Console.WriteLine("\nServer Streaming");
await ServerStreamingCallAsync();

Console.WriteLine("\nClient Streaming");
await ClientStreamingCallAsync();

Console.WriteLine("\nDuplex Streaming");
await DuplexStreamingCallAsync();


Console.ReadLine();


async Task UnaryCallAsync()
{
    var request = new HelloRequest { Name = "foobar" };
    var reply = await client.SayHelloUnaryAsync(request);
    Console.WriteLine(reply.Message);
}


async Task ServerStreamingCallAsync()
{
    var streamingCall = client.SayHelloServerStreaming(new Empty());
    var reader = streamingCall.ResponseStream;
    while (await reader.MoveNext(CancellationToken.None))
    {
        Console.WriteLine(reader.Current.Message);
    }
}


async Task ClientStreamingCallAsync()
{
    var streamingCall = client.SayHelloClientStreaming();
    var writer = streamingCall.RequestStream;

    await writer.WriteAsync(new HelloRequest { Name = "Foo" });
    await Task.Delay(1000);
    await writer.WriteAsync(new HelloRequest { Name = "Bar" });
    await Task.Delay(1000);
    await writer.WriteAsync(new HelloRequest { Name = "Baz" });
    await writer.CompleteAsync();

    var reply = await streamingCall.ResponseAsync;
    Console.WriteLine(reply.Message);
}

async Task DuplexStreamingCallAsync()
{
    var streamingCall = client.SayHelloDuplexStreaming();
    var writer = streamingCall.RequestStream;
    var reader = streamingCall.ResponseStream;
    _ = Task.Run(async () =>
    {
        await writer.WriteAsync(new HelloRequest { Name = "Foo" });
        await Task.Delay(1000);
        await writer.WriteAsync(new HelloRequest { Name = "Bar" });
        await Task.Delay(1000);
        await writer.WriteAsync(new HelloRequest { Name = "Baz" });
        await writer.CompleteAsync();
    });
    await foreach (var reply in reader.ReadAllAsync())
    {
        Console.WriteLine(reply.Message);
    }
}

如下所示的是客户端控制台上的输出结果。

二、将gRPC方法抽象成委托

通过上面的演示我们也知道，承载的gRPC类型最终会将其实现的方法注册成路由终结点，这一点其实和MVC是一样的。但是gRPC的方法和定义在Controller类型中的Action方法不同之处在于，前者的签名其实是固定的。如果我们将请求和响应消息类型使用Request和Reply来表示，四种消息交换模式的方法签名就可以写成如下的形式。

Task<Reply> Unary(Request request, ServerCallContext context);
Task<Reply> ClientStreaming(IAsyncStreamReader<Request> reader, ServerCallContext context);
Task ServerStreaming(Empty request, IServerStreamWriter<Reply> responseStream, ServerCallContext context);
Task DuplexStreaming(IAsyncStreamReader<Request> reader, IServerStreamWriter<Reply> writer, ServerCallContext context);

“流式”方法中用来读取请求和写入响应的IAsyncStreamReader<T>和IServerStreamWriter<T>定义如下。

public interface IAsyncStreamReader<out T>
{
    T Current { get; }
    Task<bool> MoveNext(CancellationToken cancellationToken = default);
}


public interface IAsyncStreamWriter<in T>
{
    Task WriteAsync(T message, CancellationToken cancellationToken = default);
}

public interface IServerStreamWriter<in T> : IAsyncStreamWriter<T>
{
}

public interface IClientStreamWriter<in T> : IAsyncStreamWriter<T>
{
    Task CompleteAsync();
}

表示服务端调用上下文的ServerCallContext 类型具有丰富的成员，但是它的本质就是对HttpContext上下文的封装，所以我们对它进行了简化。如下面的代码片段所示，我们给予这个上下文类型两个属性成员，一个是表示请求上下文的HttpContext，另一个则是用来设置响应状态StatusCode，后者对应的枚举定义了完整的gRPC状态码。

public class ServerCallContext
{
    public StatusCode StatusCode { get; set; } = StatusCode.OK;
    public HttpContext HttpContext { get; }
    public ServerCallContext(HttpContext httpContext)=> HttpContext = httpContext;
}

public enum StatusCode
{
    OK = 0,
    Cancelled = 1,
    Unknown = 2,
    InvalidArgument = 3,
    DeadlineExceeded = 4,
    NotFound = 5,
    AlreadyExists = 6,
    PermissionDenied = 7,
    Unauthenticated = 0x10,
    ResourceExhausted = 8,
    FailedPrecondition = 9,
    Aborted = 10,
    OutOfRange = 11,
    Unimplemented = 12,
    Internal = 13,
    Unavailable = 14,
    DataLoss = 0xF
}

既然方法签名固定，意味着我们可以将四种gRPC方法定义成如下四个对应的委托，泛型参数TService、TRequest和TResponse分别表示服务、请求和响应类型。

public delegate Task<TResponse> UnaryMethod<TService, TRequest, TResponse>(TService service, TRequest request, ServerCallContext context)
    where TService : class
    where TRequest : IMessage<TRequest>
    where TResponse : IMessage<TResponse>;

public delegate Task<TResponse> ClientStreamingMethod<TService, TRequest, TResponse>(TService service, IAsyncStreamReader<TRequest> reader, ServerCallContext context)
    where TService : class
    where TRequest : IMessage<TRequest>
    where TResponse : IMessage<TResponse>;

public delegate Task ServerStreamingMethod<TService, TRequest, TResponse>(TService service, TRequest request, IServerStreamWriter<TResponse> writer, ServerCallContext context)
    where TService : class
    where TRequest : IMessage<TRequest>
    where TResponse : IMessage<TResponse>;

public delegate Task DuplexStreamingMethod<TService, TRequest, TResponse>(TService service, IAsyncStreamReader<TRequest> reader, IServerStreamWriter<TResponse> writer, ServerCallContext context)
    where TService : class
    where TRequest : IMessage<TRequest>
    where TResponse : IMessage<TResponse>;

我们知道路由的本质就是创建一组路由模式（Pattern）和对应处理器之间的映射关系。路由模式很简单，对应的路由模板为“{ServiceName}/{MethodName}”，并且采用Post请求方法。对应的处理器最终体现为一个RequestDelegate。那么只要我们能够将上述四种委托类型都转换成RequestDelegate委托，一切都迎刃而解了。

三、将委托转换成RequestDelegate

为了将四种委托类型转化成RequestDelegate，我们将后者实现为一个ServiceCallHandler类型，并为其定义了如下两个基类。ServerCallHandlerBase的HandleCallAsync方法正好与RequestDelegate委托的签名一致，所以这个方法最终会用来处理gRPC请求。不同的消息交换模式采用不同的请求处理方式，只需实现抽象方法HandleCallAsyncCore就可以了。HandleCallAsync方法在调用此抽象方法之前将响应的ContentType设置成gRPC标准的响应类型“application/grpc”。在此之后将状态码设置为“grpc-status”首部，它将在HTTP2的DATA帧发送完毕后，以HEADERS帧发送到客户端。这两项操作都是gRPC协议的一部分。

public abstract class ServerCallHandlerBase
{
    public async Task HandleCallAsync(HttpContext httpContext)
    {
        try
        {
            var serverCallContext = new ServerCallContext(httpContext);
            var response = httpContext.Response;
            response.ContentType = "application/grpc";
            await HandleCallAsyncCore(serverCallContext);
            SetStatus(serverCallContext.StatusCode);
        }
        catch
        {
            SetStatus(StatusCode.Unknown);
        }
        void SetStatus(StatusCode statusCode)
        {
            httpContext.Response.AppendTrailer("grpc-status", ((int)statusCode).ToString());
        }
    }
    protected abstract Task HandleCallAsyncCore(ServerCallContext serverCallContext);
}

public abstract class ServerCallHandler<TService, TRequest, TResponse> : ServerCallHandlerBase
    where TService : class
    where TRequest : IMessage<TRequest>
    where TResponse : IMessage<TResponse>
{
    protected ServerCallHandler(MessageParser<TRequest> requestParser)=> RequestParser = requestParser;
    public MessageParser<TRequest> RequestParser { get; }
}

ServerCallHandler<TService, TRequest, TResponse>派生自ServerCallHandlerBase，并利用三个泛型参数TService、TRequest、TResponse来表示服务、请求和响应类型，RequestParser用来提供发序列化请求消息的MessageParser<TRequest>对象。针对四种消息交换模式的ServiceCallHandler类型均继承这个泛型基类。

UnaryCallHandler

基于Unary消息交换模式的ServerCallHandler的具体类型为UnaryCallHandler<TService, TRequest, TResponse>，它由上述的UnaryMethod<TService, TRequest, TResponse>委托构建而成。在重写的HandleCallAsyncCore方法中，我们利用HttpContext提供的IServiceProvider对象将服务实例创建出来后，从请求主体中将请求消息读取出来，然后交给指定的委托对象进行处理并得到响应消息，该响应消息最终用来对当前请求予以回复。

internal class UnaryCallHandler<TService, TRequest, TResponse> : ServerCallHandler<TService, TRequest, TResponse>
    where TService : class
    where TRequest : IMessage<TRequest>
    where TResponse : IMessage<TResponse>
{
    private readonly UnaryMethod<TService, TRequest, TResponse> _handler;

    public UnaryCallHandler(UnaryMethod<TService, TRequest, TResponse> handler, MessageParser<TRequest> requestParser):base(requestParser)
    => _handler = handler;
        protected override async Task HandleCallAsyncCore(ServerCallContext serverCallContext)
    {
        using var scope = serverCallContext.HttpContext.RequestServices.CreateScope();
        var service = ActivatorUtilities.CreateInstance<TService>(scope.ServiceProvider);
        var httpContext = serverCallContext.HttpContext;
        var request = await httpContext.Request.BodyReader.ReadSingleMessageAsync<TRequest>(RequestParser);
        var reply = await _handler(service, request!, serverCallContext);
        await httpContext.Response.BodyWriter.WriteMessageAsync(reply);
    }
}

请求消息是通过如下这个ReadSingleMessageAsync<TMessage>方法读取出来的。按照gRPC协议，通过网络传输的请求和响应消息都会在前面追加5个字节，第一个字节表示消息是否经过加密，后面四个字节是一个以大端序表示的整数，表示消息的长度。对于其他消息交换模式，也是调用Buffers的TryReadMessage<TRequest>方法从缓冲区中读取请求消息。

public static async Task<TMessage> ReadSingleMessageAsync<TMessage>(this PipeReader reader, MessageParser<TMessage> parser) where TMessage:IMessage<TMessage>
{
    while (true)
    {
        var result = await reader.ReadAsync();
        var buffer = result.Buffer;
        if (Buffers.TryReadMessage(parser, ref buffer, out var message))
        {
            return message!;
        }
        reader.AdvanceTo(buffer.Start, buffer.End);
        if (result.IsCompleted)
        {
            break;
        }
    }
    throw new IOException("Fails to read message.");
}

internal static class Buffers
{
    public static readonly int HeaderLength = 5;
    public static bool TryReadMessage<TRequest>(MessageParser<TRequest> parser, ref ReadOnlySequence<byte> buffer, out TRequest? message) where TRequest: IMessage<TRequest>
    {
        if (buffer.Length < HeaderLength)
        {
            message = default;
            return false;
        }

        Span<byte> lengthBytes = stackalloc byte[4];
        buffer.Slice(1, 4).CopyTo(lengthBytes);
        var length = BinaryPrimitives.ReadInt32BigEndian(lengthBytes);
        if (buffer.Length < length + HeaderLength)
        {
            message = default;
            return false;
        }

        message = parser.ParseFrom(buffer.Slice(HeaderLength, length));
        buffer = buffer.Slice(length + HeaderLength);
        return true;
    }
}

如下这个WriteMessageAsync扩展方法负责输出响应消息。

public static ValueTask<FlushResult> WriteMessageAsync(this PipeWriter writer, IMessage message)
{
    var length = message.CalculateSize();
    var span = writer.GetSpan(5 + length);
    span[0] = 0;
    BinaryPrimitives.WriteInt32BigEndian(span.Slice(1, 4), length);
    message.WriteTo(span.Slice(5, length));
    writer.Advance(5 + length);
    return writer.FlushAsync();
}

ClientStreamingCallHandler

ClientStreamingCallHandler<TService, TRequest, TResponse>代表Client Streaming模式下的ServerCallHandler，它由对应的ClientStreamingMethod<TService, TRequest, TResponse>委托创建而成。在重写的HandleCallAsyncCore方法中，除了服务实例，它还需要一个用来以“流”的方式读取请求的IAsyncStreamReader<TRequest>对象，它们都将作为参数传递给指定的委托，后者执行后会返回最终的响应消息。此消息同样通过上面这个WriteMessageAsync扩展方法予以回复。

internal class ClientStreamingCallHandler<TService, TRequest, TResponse> : ServerCallHandler<TService, TRequest, TResponse>
    where TService : class
    where TRequest : IMessage<TRequest>
    where TResponse : IMessage<TResponse>
{
    private readonly ClientStreamingMethod<TService, TRequest, TResponse> _handler;
    public ClientStreamingCallHandler(ClientStreamingMethod<TService, TRequest, TResponse> handler, MessageParser<TRequest> requestParser)
        :base(requestParser)
    {
        _handler = handler;
    }
    protected override async Task HandleCallAsyncCore(ServerCallContext serverCallContext)
    {
        using var scope = serverCallContext.HttpContext.RequestServices.CreateScope();
        var service = ActivatorUtilities.CreateInstance<TService>(scope.ServiceProvider);
        var reader = serverCallContext.HttpContext.Request.BodyReader;
        var writer = serverCallContext.HttpContext.Response.BodyWriter;
        var streamReader = new HttpContextStreamReader<TRequest>(serverCallContext.HttpContext, RequestParser);
        var response = await _handler(service, streamReader, serverCallContext);
        await writer.WriteMessageAsync(response);
    }
}

IAsyncStreamReader<T>接口的实现类型为如下这个HttpContextStreamReader<T>。在了解了请求消息在网络中的结构之后，对于实现在该类型中针对请求的读取操作，应该不难理解。

public class HttpContextStreamReader<T> : IAsyncStreamReader<T> where T : IMessage<T>
{
    private readonly PipeReader _reader;
    private readonly MessageParser<T> _parser;
    private ReadOnlySequence<byte> _buffer;
    public HttpContextStreamReader(HttpContext httpContext, MessageParser<T> parser)
    {
        _reader = httpContext.Request.BodyReader;
        _parser = parser;
    }
    public T Current { get; private set; } = default!;
    public async Task<bool> MoveNext(CancellationToken cancellationToken)
    {
        var completed = false;
        if (_buffer.IsEmpty)
        {
            var result = await _reader.ReadAsync(cancellationToken);
            _buffer = result.Buffer;
            completed = result.IsCompleted;
        }
        if (Buffers.TryReadMessage(_parser, ref _buffer, out var mssage))
        {
            Current = mssage!;
            _reader.AdvanceTo(_buffer.Start, _buffer.End);
            return true;
        }
        _reader.AdvanceTo(_buffer.Start, _buffer.End);
        _buffer = default;
        return !completed && await MoveNext(cancellationToken);
    }
}

ServerStreamingCallHandler

ServerStreamingCallHandler<TService, TRequest, TResponse>代表Server Streaming模式下的ServerCallHandler，它由对应的ServerStreamingMethod<TService, TRequest, TResponse>委托创建而成。在重写的HandleCallAsyncCore方法中，除了服务实例，它还需要一个用来以“流”的方式写入响应的IAsyncStreamWriter<TResponse>对象，它们都将作为参数传递给指定的委托。

internal class ServerStreamingCallHandler<TService, TRequest, TResponse> : ServerCallHandler<TService, TRequest, TResponse>
    where TService : class
    where TRequest : IMessage<TRequest>
    where TResponse : IMessage<TResponse>
{
    private readonly ServerStreamingMethod<TService, TRequest, TResponse> _handler;
    public ServerStreamingCallHandler(ServerStreamingMethod<TService, TRequest, TResponse> handler, MessageParser<TRequest> requestParser):base(requestParser)
        => _handler = handler;
    protected override async Task HandleCallAsyncCore(ServerCallContext serverCallContext)
    {
        using var scope = serverCallContext.HttpContext.RequestServices.CreateScope();
        var service = ActivatorUtilities.CreateInstance<TService>(scope.ServiceProvider);
        var httpContext = serverCallContext.HttpContext;
        var streamWriter = new HttpContextStreamWriter<TResponse>(httpContext);
        var request = await httpContext.Request.BodyReader.ReadSingleMessageAsync(RequestParser);
        await _handler(service, request, streamWriter, serverCallContext);
    }
}

IAsyncStreamWriter<T>接口的实现类型为如下这个HttpContextStreamWriter<T>，它直接调用上面定义的WriteMessageAsync扩展方法将指定的消息写入响应主体的输出流。

public class HttpContextStreamWriter<T> : IServerStreamWriter<T> where T : IMessage<T>
{
    private readonly PipeWriter _writer;
    public HttpContextStreamWriter(HttpContext httpContext) => _writer = httpContext.Response.BodyWriter;
    public Task WriteAsync(T message, CancellationToken cancellationToken = default)
    {
        cancellationToken.ThrowIfCancellationRequested();
        return _writer.WriteMessageAsync(message).AsTask();
    }
}

DuplexStreamingCallHandler

DuplexStreamingCallHandler<TService, TRequest, TResponse>代表Duplex Streaming模式下的ServerCallHandler，它由对应的DuplexStreamingMethod<TService, TRequest, TResponse>委托创建而成。在重写的HandleCallAsyncCore方法中，除了服务实例，它还需要分别创建以“流”的方式读/写请求/响应的IAsyncStreamReader<TRequest>和IAsyncStreamWriter<TResponse>对象，对应的类型分别为上面定义的HttpContextStreamReader<TRequest>和HttpContextStreamWriter<TResponse>。

internal class DuplexStreamingCallHandler<TService, TRequest, TResponse> : ServerCallHandler<TService, TRequest, TResponse>
    where TService : class
    where TRequest : IMessage<TRequest>
    where TResponse : IMessage<TResponse>
{
    private readonly DuplexStreamingMethod<TService, TRequest, TResponse> _handler;
    public DuplexStreamingCallHandler(DuplexStreamingMethod<TService, TRequest, TResponse> handler, MessageParser<TRequest> requestParser) :base(requestParser)
        => _handler = handler;
    protected override async Task HandleCallAsyncCore(ServerCallContext serverCallContext)
    {
        using var scope = serverCallContext.HttpContext.RequestServices.CreateScope();
        var service = ActivatorUtilities.CreateInstance<TService>(scope.ServiceProvider);
        var reader = serverCallContext.HttpContext.Request.BodyReader;
        var writer = serverCallContext.HttpContext.Response.BodyWriter;
        var streamReader = new HttpContextStreamReader<TRequest>(serverCallContext.HttpContext, RequestParser);
        var streamWriter = new HttpContextStreamWriter<TResponse>(serverCallContext.HttpContext);
        await _handler(service, streamReader, streamWriter, serverCallContext);
    }
}

四、路由注册

目前我们将针对四种消息交换模式的gRPC方法抽象成对应的泛型委托，并且可以利用它们创建ServerCallHandler，后者可以提供作为路由终结点处理器的RequestDelegate委托。枚举和对应ServerCallHandler之间的映射关系如下所示：

• UnaryMethod<TService, TRequest, TResponse>：UnaryCallHandler<TService, TRequest, TResponse>
• ClientStreamingMethod<TService, TRequest, TResponse>：ClientStreamingCallHandler<TService, TRequest, TResponse>
• ServerStreamingMethod<TService, TRequest, TResponse>：ServerStreamingCallHandler<TService, TRequest, TResponse>
• DuplexStreamingMethod<TService, TRequest, TResponse>：DuplexStreamingCallHandler<TService, TRequest, TResponse>

现在我们将整个路由注册的流程串起来，为此我们定义了如下这个IServiceBinder<TService>接口，它提供了两种方式将定义在服务类型TService中的gRPC方法注册成对应的路由终结点。

public interface IServiceBinder<TService> where TService : class
{
    IServiceBinder<TService> AddUnaryMethod<TRequest, TResponse>(string methodName, Func<TService, Func<TRequest, ServerCallContext, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>;

    IServiceBinder<TService> AddClientStreamingMethod<TRequest, TResponse>(string methodName, Func<TService, Func<IAsyncStreamReader<TRequest>, ServerCallContext, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>;

    IServiceBinder<TService> AddServerStreamingMethod<TRequest, TResponse>(string methodName, Func<TService, Func<TRequest, IServerStreamWriter<TResponse>, ServerCallContext, Task>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>;

    IServiceBinder<TService> AddDuplexStreamingMethod<TRequest, TResponse>(string methodName, Func<TService, Func<IAsyncStreamReader<TRequest>, IServerStreamWriter<TResponse>, ServerCallContext, Task>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>;


    IServiceBinder<TService> AddUnaryMethod<TRequest, TResponse>(Expression<Func<TService, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>;
    IServiceBinder<TService> AddClientStreamingMethod<TRequest, TResponse>( Expression<Func<TService, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>;

    IServiceBinder<TService> AddServerStreamingMethod<TRequest, TResponse>( Expression<Func<TService, Task>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>;

    IServiceBinder<TService> AddDuplexStreamingMethod<TRequest, TResponse>( Expression<Func<TService, Task>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>;
}

路由终结点由路由模式和处理器两个元素组成，路由模式主要体现在由gRPC服务和操作名称组成的路由模板，我们默认使用服务类型的名称和方法名称（提出Async后缀）。为了能够对这两个名称进行定制，我们定义了如下两个特性GrpcServiceAttribute和GrpcMethodAttribute，它们可以分别标注在服务类型和操作方法上来指定一个任意的名称。

[AttributeUsage(AttributeTargets.Class)]
public class GrpcServiceAttribute: Attribute
{
    public string? ServiceName { get; set; }

}

[AttributeUsage(AttributeTargets.Method)]
public class GrpcMethodAttribute : Attribute
{
    public string? MethodName { get; set; }
}

如下所示的ServiceBinder<TService> 是对IServiceBinder<TService> 接口的实现，它是对一个IEndpointRouteBuilder 对象的封装。对于实现的第一组方法，我们利用提供的方法名称与解析TService类型得到的服务名称合并，进而得到路由终结点的URL模板。这些方法还提供了一个针对gRPC方法签名的Func<TService,Func<…>>委托，我们利用它来将提供用于构建对应ServiceCallHandler的委托。我们最终利用IEndpointRouteBuilder 对象完成针对路由终结点的注册。

public class ServiceBinder<TService> : IServiceBinder<TService> where TService : class
{
    private readonly IEndpointRouteBuilder _routeBuilder;
    public ServiceBinder(IEndpointRouteBuilder routeBuilder) => _routeBuilder = routeBuilder;

    public IServiceBinder<TService> AddUnaryMethod<TRequest, TResponse>(string methodName, Func<TService, Func<TRequest, ServerCallContext, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>
    {
        Task<TResponse> GetMethod(TService service, TRequest request, ServerCallContext context) => methodAccessor(service)(request, context);
        var callHandler = new UnaryCallHandler<TService, TRequest, TResponse>(GetMethod, parser);
        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);
        return this;
    }

    public IServiceBinder<TService> AddClientStreamingMethod<TRequest, TResponse>(string methodName, Func<TService, Func<IAsyncStreamReader<TRequest>, ServerCallContext, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>
    {
        Task<TResponse> GetMethod(TService service, IAsyncStreamReader<TRequest> reader, ServerCallContext context) => methodAccessor(service)(reader, context);
        var callHandler = new ClientStreamingCallHandler<TService, TRequest, TResponse>(GetMethod, parser);
        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);
        return this;
    }

    public IServiceBinder<TService> AddServerStreamingMethod<TRequest, TResponse>(string methodName, Func<TService, Func<TRequest, IServerStreamWriter<TResponse>, ServerCallContext, Task>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>
    {
        ServerStreamingMethod<TService, TRequest, TResponse> handler = (service, request, writer, context) => methodAccessor(service)(request, writer, context);
        var callHandler = new ServerStreamingCallHandler<TService, TRequest, TResponse>(handler, parser);
        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);
        return this;
    }

    public IServiceBinder<TService> AddDuplexStreamingMethod<TRequest, TResponse>(string methodName, Func<TService, Func<IAsyncStreamReader<TRequest>, IServerStreamWriter<TResponse>, ServerCallContext, Task>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>
    {
        DuplexStreamingMethod<TService, TRequest, TResponse> handler = (service, reader, writer, context) => methodAccessor(service)(reader, writer, context);
        var callHandler = new DuplexStreamingCallHandler<TService, TRequest, TResponse>(handler, parser);
        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);
        return this;
    }

    private static string GetPath(string methodName)
    {
        var serviceName = typeof(TService).GetCustomAttribute<GrpcServiceAttribute>()?.ServiceName ?? typeof(TService).Name;
        if (methodName.EndsWith("Async"))
        {
            methodName = methodName.Substring(0, methodName.Length - 5);
        }
        return $"{serviceName}/{methodName}";
    }

    public IServiceBinder<TService> AddUnaryMethod<TRequest, TResponse>(Expression<Func<TService, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>
    {
        var method = CreateDelegate<UnaryMethod<TService, TRequest,TResponse>>(methodAccessor, out var methodName);
        var serviceName = typeof(TService).GetCustomAttribute<GrpcServiceAttribute>()?.ServiceName ?? typeof(TService).Name;
        var callHandler = new UnaryCallHandler<TService, TRequest, TResponse>(method, parser);
        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);
        return this;
    }

    public IServiceBinder<TService> AddClientStreamingMethod<TRequest, TResponse>( Expression<Func<TService, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>
    {
        var method = CreateDelegate<ClientStreamingMethod<TService, TRequest, TResponse>>(methodAccessor, out var methodName);
        var serviceName = typeof(TService).GetCustomAttribute<GrpcServiceAttribute>()?.ServiceName ?? typeof(TService).Name;
        var callHandler = new ClientStreamingCallHandler<TService, TRequest, TResponse>(method, parser);
        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);
        return this;
    }

    public IServiceBinder<TService> AddServerStreamingMethod<TRequest, TResponse>(Expression<Func<TService, Task>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>
    {
        var method = CreateDelegate<ServerStreamingMethod<TService, TRequest, TResponse>>(methodAccessor, out var methodName);
        var serviceName = typeof(TService).GetCustomAttribute<GrpcServiceAttribute>()?.ServiceName ?? typeof(TService).Name;
        var callHandler = new ServerStreamingCallHandler<TService, TRequest, TResponse>(method, parser);
        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);
        return this;
    }

    public IServiceBinder<TService> AddDuplexStreamingMethod<TRequest, TResponse>(Expression<Func<TService, Task>> methodAccessor, MessageParser<TRequest> parser)
        where TRequest : IMessage<TRequest>
        where TResponse : IMessage<TResponse>
    {
        var method = CreateDelegate<DuplexStreamingMethod<TService, TRequest, TResponse>>(methodAccessor, out var methodName);
        var serviceName = typeof(TService).GetCustomAttribute<GrpcServiceAttribute>()?.ServiceName ?? typeof(TService).Name;
        var callHandler = new DuplexStreamingCallHandler<TService, TRequest, TResponse>(method, parser);
        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);
        return this;
    }

    private TDelegate CreateDelegate<TDelegate>(LambdaExpression expression, out string methodName) where TDelegate : Delegate
    {
        var method = ((MethodCallExpression)expression.Body).Method;
        methodName = method.GetCustomAttribute<GrpcMethodAttribute>()?.MethodName ?? method.Name;
        return (TDelegate)Delegate.CreateDelegate(typeof(TDelegate), method);
    }
}

由于第二组方法提供的针对gRPC方法调用的表达式，所以我们可以得到描述方法的MethodInfo对象，该对象不但解决了委托对象的创建问题，还可以提供方法的名称，所以这组方法无需提供gRPC方法的名称。但是提供的表达式并不能严格匹配方法的签名，所以无法提供编译时的错误检验，所以各有优缺点。

五、为gRPC服务定义一个接口

由于路由终结点的注册是针对服务类型进行的，所以我们决定让服务类型自身来完成所有的路由注册工作。在这里我们使用C# 11中一个叫做“静态接口方法”的特性，为服务类型定义如下这个IGrpcService<TService>接口，服务类型TService定义的所有gRPC方法的路由注册全部在静态方法Bind中完成，该方法将上述的IServiceBinder<TService>作为参数。

public interface  IGrpcService<TService> where TService:class
{
     static abstract void Bind(IServiceBinder<TService> binder);
}

我们定义了如下这个针对IEndpointRouteBuilder 接口的扩展方法完成针对指定服务类型的路由注册。为了与现有的方法区别开来，我特意将其命名为MapGrpcService2。该方法根据指定的IEndpointRouteBuilder 对象将ServiceBinder<TService>对象创建出来，并作为参数调用服务类型的静态Bind方法。到此为止，整个Mini版的gRPC服务端框架就构建完成了，接下来我们看看它能否工作。

public static class EndpointRouteBuilderExtensions
{
    public static IEndpointRouteBuilder MapGrpcService2<TService>(this IEndpointRouteBuilder routeBuilder) where TService : class, IGrpcService<TService>
    {

        var binder = new ServiceBinder<TService>(routeBuilder);
        TService.Bind(binder);
        return routeBuilder;
    }
}

六、重新定义和承载服务

我们开篇演示了ASP.NET Core gRPC的服务定义、承载和调用。如果我们上面构建的Mini版gRPC框架能够正常工作，意味着客户端代码可以保持不变，我们现在就来试试看。我们在Server项目中将GreeterService服务类型改成如下的形式，它不再继承任何基类，只实现IGrpcService<GreeterService>接口。针对四种消息交换模式的四个方法的实现方法保持不变，在实现的静态Bind方法中，我们采用两种形式完成了针对这四个方法的路由注册。

[GrpcService(ServiceName = "Greeter")]
public class GreeterService: IGrpcService<GreeterService>
{
    public Task<HelloReply> SayHelloUnaryAsync(HelloRequest request, ServerCallContext context)
        => Task.FromResult(new HelloReply { Message = $"Hello, {request.Name}" });

    public async Task<HelloReply> SayHelloClientStreamingAsync(IAsyncStreamReader<HelloRequest> reader, ServerCallContext context)
    {
        var list = new List<string>();
        while (await reader.MoveNext(CancellationToken.None))
        {
            list.Add(reader.Current.Name);
        }
        return new HelloReply { Message = $"Hello, {string.Join(",", list)}" };
    }

    public  async Task SayHelloServerStreamingAsync(Empty request, IServerStreamWriter<HelloReply> responseStream, ServerCallContext context)
    {
        await responseStream.WriteAsync(new HelloReply { Message = "Hello, Foo!" });
        await Task.Delay(1000);
        await responseStream.WriteAsync(new HelloReply { Message = "Hello, Bar!" });
        await Task.Delay(1000);
        await responseStream.WriteAsync(new HelloReply { Message = "Hello, Baz!" });
    }

    public async Task SayHelloDuplexStreamingAsync(IAsyncStreamReader<HelloRequest> reader, IServerStreamWriter<HelloReply> writer, ServerCallContext context)
    {
        while (await reader.MoveNext())
        {
            await writer.WriteAsync(new HelloReply { Message = $"Hello {reader.Current.Name}" });
        }
    }

    public static void Bind(IServiceBinder<GreeterService> binder)
    {
         binder
               .AddUnaryMethod<HelloRequest, HelloReply>(it =>it.SayHelloUnaryAsync(default!,default!), HelloRequest.Parser)
               .AddClientStreamingMethod<HelloRequest, HelloReply>(it => it.SayHelloClientStreamingAsync(default!, default!), HelloRequest.Parser)
               .AddServerStreamingMethod<Empty, HelloReply>(nameof(SayHelloServerStreamingAsync), it => it.SayHelloServerStreamingAsync, Empty.Parser)
               .AddDuplexStreamingMethod<HelloRequest, HelloReply>(nameof(SayHelloDuplexStreamingAsync), it => it.SayHelloDuplexStreamingAsync, HelloRequest.Parser);
    }
}

服务承载程序直接将针对MapGrpcService<GreeterService>方法的调用换成MapGrpcService2<GreeterService>。由于整个框架根本不需要预先注册任何的服务，所以针对AddGrpc扩展方法的调用也可以删除。

using GrpcMini;
using Microsoft.AspNetCore.Server.Kestrel.Core;

var builder = WebApplication.CreateBuilder(args);
builder.WebHost.ConfigureKestrel(kestrel => kestrel.ConfigureEndpointDefaults(options => options.Protocols = HttpProtocols.Http2));
var app = builder.Build();
app.MapGrpcService2<Server.Greeter>();
app.Run();

再次运行我们的程序，客户端依然可以得到相同的输出。