Pilz/Pilz.Networking/Api/ApiServer.cs

using Pilz.Extensions.Reflection;
using System;
using System.Net;
using System.Reflection;
using System.Reflection.PortableExecutable;

namespace Pilz.Networking.Api;

public class ApiServer(string apiUrl) : ApiClient, IApiServer
{
    protected readonly Dictionary<string, Delegate> handlers = [];
    protected readonly Dictionary<Type, IMessageSerializer> serializers = [];
    protected readonly HttpListener httpListener = new();

    public string ApiUrl { get; } = apiUrl;

    public IMessageSerializer Serializer { get; set; } = new DefaultMessageSerializer();

    public virtual void Start()
    {
        httpListener.Start();
        Listen();
    }

    public virtual void RegisterHandler<T>(T instance) where T : class
    {
        // Get all public instance methods
        var methods = typeof(T).GetMethods(BindingFlags.Instance | BindingFlags.Public);

        // Register each method
        foreach (var method in methods)
            RegisterHandler(method.CreateDelegate(instance));
    }

    public virtual void RegisterHandler(Delegate handler)
    {
        var method = handler.Method;

        // Sanity checks
        if (method.GetCustomAttribute<MessageHandlerAttribute>() is not MessageHandlerAttribute attribute
            || method.GetParameters().FirstOrDefault() is not ParameterInfo parameter
            || !parameter.ParameterType.IsAssignableTo(typeof(ApiMessage))
            || !method.ReturnType.IsAssignableTo(typeof(ApiMessage)))
            throw new NotSupportedException("The first parameter needs to be of type ApiMessage and must also return an ApiMessage object and the method must have the MessageHAndlerAttribute.");

        // Add handler
        handlers.Add(ApiUrl + attribute.Url, handler);
    }

    protected virtual void Listen()
    {
        while (httpListener.IsListening)
        {
            var context = httpListener.GetContext();

            if (context.Request.HttpMethod != HttpMethod.Post.Method
                || context.Request.ContentType is not string contentType
                || contentType.Contains("application/json")
                || context.Request.AcceptTypes is null
                || context.Request.AcceptTypes.Contains("application/json"))
            {
                close();
                continue;
            }

            // Parse url
            var path = context.Request.Url?.PathAndQuery.Replace(ApiUrl, string.Empty);
            if (string.IsNullOrWhiteSpace(path) || context.Request.ContentLength64 <= 0)
            {
                close();
                continue;
            }

            // Read input content
            using StreamReader input = new(context.Request.InputStream);
            var contentJson = input.ReadToEnd();
            
            // Handle message
            if (HandleMessage(path, contentJson) is not string resultJson)
            {
                close();
                continue;
            }

            // Set response parameters
            context.Response.StatusCode = (int)args.ResponseStatusCode;
            context.Response.StatusDescription = args.ResponseStatusDescription;

            // Write response content
            context.Response.ContentType = ContentTypes.CONTENT_TYPE_JSON;
            using StreamWriter output = new(context.Response.OutputStream);
            output.Write(resultJson);

            close();
            void close() => context.Response.OutputStream.Close();
        }
    }

    protected virtual string? HandleMessage(string url, string json)
    {
        // Get handler
        if (!handlers.TryGetValue(url, out var handler)
            || handler.Method.GetCustomAttribute<MessageHandlerAttribute>() is not MessageHandlerAttribute attribute)
            return null;

        // Get required infos
        var targetType = handler.Method.GetParameters().First().ParameterType;
        var serializer = GetSerializer(attribute.Serializer);

        // Deserialize
        if (serializer.Deserialize(json, targetType) is not ApiMessage message)
            return null;

        // Invoke handler
        if (handler.DynamicInvoke(message) is not ApiMessage result)
            return null;

        // Serializer
        if (serializer.Serialize(result) is not string resultStr)
            return null;

        return resultStr;
    }

    protected virtual IMessageSerializer GetSerializer(Type? t)
    {
        if (t is not null)
        {
            if (serializers.TryGetValue(t, out var s) && s is not null)
                return s;
            else if (Activator.CreateInstance(t) is IMessageSerializer ss)
            {
                serializers.Add(t, ss);
                return ss;
            }
        }
        return Serializer;
    }
}