source: TFP-WebServer/WebServer/src/UrlHandlers/ApiHandler.cs@ 488

using System;
using System.Collections.Generic;
using System.Net;
using System.Reflection;
using Webserver.WebAPI;

namespace Webserver.UrlHandlers {
        public class ApiHandler : AbsHandler {
                private readonly Dictionary<string, AbsWebAPI> apis = new CaseInsensitiveStringDictionary<AbsWebAPI> ();

                public ApiHandler () : base (null) {
                }
                
                private static readonly Type[] apiWithParentCtorTypes = { typeof (Web) };
                private static readonly object[] apiWithParentCtorArgs = new object[1];
                private static readonly Type[] apiEmptyCtorTypes = { };
                private static readonly object[] apiEmptyCtorArgs = { };

                public override void SetBasePathAndParent (Web _parent, string _relativePath) {
                        base.SetBasePathAndParent (_parent, _relativePath);

                        apiWithParentCtorArgs[0] = _parent;

                        ReflectionHelpers.FindTypesImplementingBase (typeof (AbsWebAPI), apiFoundCallback);

                        // Permissions that don't map to a real API
                        addApi (new Null ("viewallclaims"));
                        addApi (new Null ("viewallplayers"));
                }

                private void apiFoundCallback (Type _type) {
                        ConstructorInfo ctor = _type.GetConstructor (BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic, null,
                                apiWithParentCtorTypes, null);
                        if (ctor != null) {
                                AbsWebAPI apiInstance = (AbsWebAPI)ctor.Invoke (apiWithParentCtorArgs);
                                addApi (apiInstance);
                                return;
                        }

                        ctor = _type.GetConstructor (BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic, null,
                                apiEmptyCtorTypes, null);
                        if (ctor != null) {
                                AbsWebAPI apiInstance = (AbsWebAPI)ctor.Invoke (apiEmptyCtorArgs);
                                addApi (apiInstance);
                        }
                }

                private void addApi (AbsWebAPI _api) {
                        apis.Add (_api.Name, _api);
                        parent.OpenApiHelpers.LoadOpenApiSpec (_api);
                }

                private static readonly UnityEngine.Profiling.CustomSampler apiHandlerSampler = UnityEngine.Profiling.CustomSampler.Create ("API_Handler");

                private bool HandleCors (RequestContext _context) {
                        _context.Request.Headers.TryGetValue ("Origin", out string origin);
                        _context.Response.AddHeader ("Access-Control-Allow-Origin", origin ?? "*");

                        if (_context.Method != ERequestMethod.OPTIONS) {
                                return false;
                        }

                        if (!_context.Request.Headers.TryGetValue ("Access-Control-Request-Method", out _)) {
                                return false;
                        }

                        _context.Response.AddHeader ("Access-Control-Allow-Methods", "GET, PUT, POST, DELETE, OPTIONS, HEAD");
                        _context.Response.AddHeader ("Access-Control-Allow-Headers", "X-SDTD-API-TOKENNAME, X-SDTD-API-SECRET");
                        _context.Response.AddHeader ("Access-Control-Allow-Credentials", "true");
                        return true;
                }

                public override void HandleRequest (RequestContext _context) {

                        string apiName;
                        string subPath = null;

                        int pathSeparatorIndex = _context.RequestPath.IndexOf ('/', urlBasePath.Length);
                        if (pathSeparatorIndex >= 0) {
                                apiName = _context.RequestPath.Substring (urlBasePath.Length, pathSeparatorIndex - urlBasePath.Length);
                                subPath = _context.RequestPath.Substring (pathSeparatorIndex + 1);
                        } else {
                                apiName = _context.RequestPath.Substring (urlBasePath.Length);
                        }
                        
                        if (!apis.TryGetValue (apiName, out AbsWebAPI api)) {
                                Log.Warning ($"[Web] In {nameof(ApiHandler)}.HandleRequest(): No handler found for API \"{apiName}\"");
                                _context.Response.StatusCode = (int) HttpStatusCode.NotFound;
                                return;
                        }

                        // CORS specific stuff
                        if (HandleCors (_context)) {
                                return;
                        }
                        // CORS end
                        
                        _context.RequestPath = subPath;

                        if (!api.Authorized (_context)) {
                                _context.Response.StatusCode = (int) HttpStatusCode.Forbidden;
                                if (_context.Connection != null) {
                                        //Log.Out ($"{nameof(ApiHandler)}: user '{user.SteamID}' not allowed to execute '{apiName}'");
                                }

                                return;
                        }

                        try {
                                apiHandlerSampler.Begin ();
                                api.HandleRequest (_context);
                                apiHandlerSampler.End ();
                        } catch (Exception e) {
                                Log.Error ($"[Web] In {nameof(ApiHandler)}.HandleRequest(): Handler {api.Name} threw an exception:");
                                Log.Exception (e);
                                _context.Response.StatusCode = (int) HttpStatusCode.InternalServerError;
                        }
                }
        }
}