.NET中的SignalR是什么？如何用它在Web应用中实现实时通信？

var builder = WebApplication.CreateBuilder(args);

// 添加 SignalR 服务
builder.Services.AddSignalR();

var app = builder.Build();

// 配置中间件
app.UseRouting();
app.MapHub<ChatHub>("/chat"); // 映射 Hub 路由

app.Run();

using Microsoft.AspNetCore.SignalR;

public class ChatHub : Hub
{
    public async Task SendMessage(string user, string message)
    {
        await Clients.All.SendAsync("ReceiveMessage", user, message);
    }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/microsoft-signalr/6.0.1/signalr.min.js"></script>

const connection = new signalR.HubConnectionBuilder()
    .withUrl("/chat")
    .build();

// 启动连接
connection.start().catch(err => console.error(err));

// 接收服务器推送的消息
connection.on("ReceiveMessage", function (user, message) {
    const msg = document.createElement("div");
    msg.textContent = `${user}: ${message}`;
    document.getElementById("messages").appendChild(msg);
});

// 发送消息到服务器
function sendMessage() {
    const user = document.getElementById("user").value;
    const message = document.getElementById("message").value;
    connection.invoke("SendMessage", user, message).catch(err => console.error(err));
}

<input id="user" placeholder="用户名" />
<input id="message" placeholder="消息" />
<button onclick="sendMessage()">发送</button>

<div id="messages"></div>