SignalR服务主动发送消息给客户端

背景

当服务端进行数据处理时（数据可能来源于非人工操作，比如各种控制器和信号源），需要将处理结果推送给客户端，进行实时数据显示。

尝试

在上一篇文章中我们创建过服务端，代码吗如下

using Microsoft.AspNetCore.SignalR;

namespace Demo_WarningMonitor.Api.SignalRHubs
{
    /// <summary>
    /// 报警广播
    /// </summary>
    public class WarningBroadcastHub : Hub
    { 
        private ILogger<WarningBroadcastHub> logger; 

        public WarningBroadcastHub(ILogger<WarningBroadcastHub> logger, IConfiguration configuration)
        {
            this.logger = logger; 
        }

        public async Task SendMessage(string user, string message)
        {
            try
            {
                await Clients.All.SendAsync("ReceiveMessage", user, message);
            }
            catch (Exception ex)
            {
                logger.LogError(ex,"发送消息出现异常");
            }
          
        }
    }
}

我想这直接在数据处理服务中i性能调用SendMessage方法就能实现发送消息了，

但是事与愿违，因为这里涉及到一个对象声明周期的问题，在调用Clients.All的时候会提示调用了一个已经释放的资源对象，具体错误如下：System.ObjectDisposedException:"Cannot access a disposed object.

方案

使用IHubContext上下文进行处理

https://learn.microsoft.com/zh-cn/aspnet/core/signalr/hubcontext?view=aspnetcore-6.0

第一步 创建一个类用于处理上下文（WarningBroadcast）

我这里使用了MessageBus的一个中间件，这样就实现了解耦，Messagebus的nuget包

using Demo_WarningMonitor.Api.Models;
using iml6yu.MessageBus;
using iml6yu.MessageBus.Constand;
using Microsoft.AspNetCore.SignalR;

namespace Demo_WarningMonitor.Api.SignalRHubs
{
    public class WarningBroadcast
    {
        private ISubscriber<WarningMessage> warningSubscriber;
        private ISubscriber<string> realtimeDataSubscriber;
        private IConfiguration configuration;
        private string warningChannel;
        private ILogger<WarningBroadcast> logger;
        private IHubContext<WarningBroadcastHub> hubContext;//

        public WarningBroadcast(IConfiguration configuration, IHubContext<WarningBroadcastHub> context, ILogger<WarningBroadcast> logger)//<WarningBroadcastHub> 
        {
            hubContext = context;
            this.logger = logger;
            this.configuration = configuration;
            warningChannel = configuration.GetSection("WarningChannel").Get<string>();
            warningSubscriber = MessageBuses.Subscrib<WarningMessage>(warningChannel);
            warningSubscriber.OnNoticed += Subscriber_OnNoticed;

            realtimeDataSubscriber = MessageBuses.Subscrib<string>(configuration.GetSection("RealtimeDataChannel").Get<string>());
            realtimeDataSubscriber.OnNoticed += RealtimeDataSubscriber_OnNoticed;
        }

        private async void RealtimeDataSubscriber_OnNoticed(MessageChannel channel, MessageBusArges<string> message)
        {
            try
            {
                await hubContext.Clients.All.SendAsync("ReceiveMessage", "HubServiceCenter", message.Data);
            }
            catch (Exception ex)
            {
                logger.LogError(ex, "WarningBroadcast发送消息出现异常");
            }
        }

        /// <summary>
        /// 接收到消息的通知后进行分发处理
        /// </summary>
        /// <param name="channel"></param>
        /// <param name="message"></param>
        /// <exception cref="NotImplementedException"></exception>
        private async void Subscriber_OnNoticed(MessageChannel channel, MessageBusArges<WarningMessage> message)
        {
            try
            {
                Console.ForegroundColor = ConsoleColor.Red;

                var msg = System.Text.Json.JsonSerializer.Serialize(message.Data);
                await hubContext.Clients.All.SendAsync("WarningMessage", "HubServiceCenter", msg);

                Console.WriteLine("发送一次报警信息\r\n" + msg);

                Console.ForegroundColor = ConsoleColor.White;
            }
            catch (Exception ex)
            {
                logger.LogError(ex, "WarningBroadcast发送消息出现异常");
            }
        }
    }
}

这里的重点就是构造函数

第二步：如何实例化这个对象呢

这里涉及到如何获取IHubContext实例，可以查看文章

https://learn.microsoft.com/zh-cn/aspnet/core/signalr/hubcontext?view=aspnetcore-6.0

下面是我实现的方式

 var hubContext = app.Services.GetService<IHubContext<WarningBroadcastHub>>();
            if (hubContext != null)
            {
                new WarningBroadcast(app.Services.GetService<IConfiguration>(), hubContext,app.Services.GetService<ILogger<WarningBroadcast>>());
            }

对此我新建了一个扩展类，完整代码如下

using Microsoft.AspNetCore.SignalR;

namespace Demo_WarningMonitor.Api.SignalRHubs
{
    /// <summary>
    /// 扩展类
    /// </summary>
    public static class WarningBroadcastExt
    { 
        public static WebApplication UseWarningBroadcast(this WebApplication app)
        {
            var hubContext = app.Services.GetService<IHubContext<WarningBroadcastHub>>();
            if (hubContext != null)
            {
                new WarningBroadcast(app.Services.GetService<IConfiguration>(), hubContext,app.Services.GetService<ILogger<WarningBroadcast>>());
            }

            return app;
        }

    }
}

第三步：使用扩展类

在main函数中使用

  app.UseWarningBroadcast();

调用通知

在需要发送给客户端消息的时候使用

MessageBuses.Publish 推送一个消息，订阅者直接收到消息后就会调用await hubContext.Clients.All.SendAsync(“WarningMessage”, “HubServiceCenter”, msg); 给客户端发送消息了

关于MessageBus的使用

demo

发布消息

  MessageBuses.Publish<MyClass1>(new MyClass1());

订阅消息

   ISubscriber<MyClass1> subscriber;
   subscriber = MessageBuses.Subscrib<MyClass1>("newWin");
    subscriber.OnNoticed += (channel,e)=>{
       Debug.WriteLine(e.Data.ToString())  
    };