SignalR 无法启动连接

我有一个 Angular 应用程序以及使用 SignalR 的 ASP.NET Core 2.2 后端。启动连接时，我收到错误：

Error: Failed to complete negotiation with the server: Error
Error: Failed to start the connection: Error

点击 localhost:5000/chatHub- 时的响应标头

HTTP/1.1 204 No Content
Date: Tue, 19 Feb 2019 08:52:52 GMT
Server: Kestrel
Access-Control-Allow-Headers: x-requested-with
Access-Control-Allow-Methods: POST
Access-Control-Allow-Origin: *

Startup.cs-

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.HttpsPolicy;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using RealChat.Hubs;

namespace RealChat
{
public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors(o => o.AddPolicy("AllowAllPolicy", builder => {
            builder
            .AllowAnyMethod()
            .AllowAnyHeader()
            .AllowAnyOrigin();
        }));

        services.AddSignalR();

        services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }
        else
        {
            // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
            app.UseHsts();
        }

        app.UseCors("AllowAllPolicy");

        app.UseSignalR(routes =>
        {
            routes.MapHub<ChatHub>("/chatHub");
        });

        //app.UseHttpsRedirection();
        app.UseMvc();
    }
}
}

Chathub.cs-

using Microsoft.AspNetCore.SignalR;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace RealChat.Hubs
{
public class ChatHub : Hub
{
    public Task Send(string data)
    {
        return Clients.All.SendAsync("Receive", "Hello!!!");
    }
}
}

应用程序组件.cs-

import { Component, OnInit } from '@angular/core';
import { HubConnection } from '@aspnet/signalr';
import * as signalR from '@aspnet/signalr';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent implements OnInit {

  private _hubConnection: HubConnection;
  msgs: string;

  constructor() { }

  ngOnInit(): void {
    this._hubConnection = new signalR.HubConnectionBuilder()
        .withUrl('http://localhost:5000/chatHub')
        .configureLogging(signalR.LogLevel.Information)
        .build();

    this._hubConnection.start().catch(err => console.error("SignalR startup error"));

    this._hubConnection.on('ReceiveMessage', (data) => {
        console.log(data);
    });
  }

  sendMessage(): void {
      console.log("Sending...\n");
       if (this._hubConnection)
          this._hubConnection.invoke('SendMessage', 'Test');
  }
}

SendMessage() 与 HTML 中的按钮挂钩。 但我永远无法到达那部分！ OnInit() 不断失败，即使 API 返回的响应为 204，应用程序也无法启动。 回复-

info: Microsoft.AspNetCore.Hosting.Internal.WebHost[1]
  Request starting HTTP/1.1 OPTIONS http://localhost:5000/chatHub/negotiate
info: Microsoft.AspNetCore.Cors.Infrastructure.CorsService[4]
  CORS policy execution successful.
info: Microsoft.AspNetCore.Hosting.Internal.WebHost[2]
  Request finished in 0.6942ms 204

我尝试在两端设置相同版本的 SignalR，但它不起作用。是.NET Core本身的问题吗？ 我也尝试过使用 Kestrel 和 IIS Express 但没有成功。 请帮我解决这个问题。

我能够通过在这里查看这个问题来解决这个问题-

https://github.com/aspnet/SignalR/issues/2095#issuecomment-383899098

我将AllowAllPolicy移至配置方法-

app.UseCors(builder =>
            {
                builder.WithOrigins("http://localhost:4200")
                .AllowAnyHeader().AllowAnyMethod().AllowCredentials();
            });