Pilz/Pilz.Net/Api/ApiServer.cs

using Pilz.Extensions.Reflection;
using System.Net;
using System.Reflection;
using static Pilz.Net.Api.IApiServer;

namespace Pilz.Net.Api;

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

    public event OnCheckAuthenticationEventHandler? OnCheckAuthentication;

    protected record PrivateApiResult(ApiResult Original, string? ResultJson);

    public string ApiUrl { get; } = apiUrl;

    public virtual bool EnableAuth { get; set; }

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

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

    public virtual void Stop()
    {
        httpListener.Stop();
    }

    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(ApiResult)))
            throw new NotSupportedException("The first parameter needs to be of type ApiMessage and must return an ApiResult object and the method must have the MessageHandlerAttribute.");

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

    protected virtual void Listen()
    {
        while (httpListener.IsListening)
            CheckContext();
    }

    protected virtual void CheckContext()
    {
        var context = httpListener.GetContext();
        void close() => context.Response.OutputStream.Close();

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

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

        // Read input content
        using StreamReader input = new(context.Request.InputStream);
        var contentJson = input.ReadToEnd();

        // Get auth key
        if (context.Request.Headers.Get("API-AUTH-KEY") is not string authKey)
            authKey = null!;

        // Handle message
        if (HandleMessage(path, contentJson, authKey) is not PrivateApiResult result)
        {
            close();
            return;
        }

        // Set response parameters
        context.Response.StatusCode = (int)result.Original.StatusCode;

        // Write response content
        if (result.ResultJson is not null)
        {
            context.Response.ContentType = "application/json";
            using StreamWriter output = new(context.Response.OutputStream);
            output.Write(result);
        }

        close();
    }

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

        // Check authentication
        if (attribute.RequiesAuth && (string.IsNullOrWhiteSpace(authKey) || !CheckAuthentication(authKey)))
            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 ApiResult result)
            return null;

        // Return result without message
        if (result.Message is null)
            return new(result, null);

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

        // Return result with message
        return new(result, 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;
    }

    protected virtual bool CheckAuthentication(string authKey)
    {
        if (OnCheckAuthentication != null)
        {
            var args = new ApiAuthCheckEventArgs(authKey);
            OnCheckAuthentication?.Invoke(this, args);
            return args.Valid;
        }
        return false;
    }
}