Android无法连接Blazor Server中运行的SignalR问题排查

问题描述 #

我正尝试使用Android连接.NET 6 Core托管的Blazor WASM应用的SignalR服务，请帮我指出操作有误的地方？

我的Android代码 #

String hubUrl = "http://192.168.254.173:5050/notification?parameter=1010072";
if (isReachable) {
    hubConnection = HubConnectionBuilder.create(hubUrl).build();
    if (hubConnection.getConnectionState() != HubConnectionState.CONNECTED) {
        hubConnection.start();
        hubConnection.on("MessageModelToClients", (messageModel) -> {
            Log.i("TAG", messageModel.getMessage());
        }, MessageModel.class);
        if (hubConnection.getConnectionState() == HubConnectionState.CONNECTED) {
            Log.i("TAG", "Connected");
        } else {
            Log.e("TAG", "Failed to establish connection to SignalR hub.");
        }
    }
} else {
    System.out.println("IP address is not reachable.");
}

Gradle配置 #

implementation ("com.microsoft.signalr:signalr:7.0.0")

Blazor WASM服务端NotificationHub代码 #

public class NotificationHub : Hub 
{
    ....
    
    public override async Task OnConnectedAsync()
    {
        Console.WriteLine("Connecting...");
        var userId= Context.GetHttpContext()!.Request.Query["userid"];
        var connectionId = Context.ConnectionId;
        AddUser(connectionId, userId);
        var messageModel = new MessageModel
        {
            IsPrivate = false,
            Message = $"Logged in User: {userId}",
            Recipient = string.Empty,
            Sender = SERVER_NAME
        };
        await SendConnectedUsers();
        await SendObjectToAll(messageModel);

        Console.WriteLine($"CONNECTED: {connectionId}");

        await base.OnConnectedAsync();
    }
    
    
    ...
}

Program.cs配置 #

builder.Services.AddSignalR();
app.UseEndpoints(endpoints =>
{
    endpoints.MapHub<NotificationHub>("/notification");
});

错误点分析 #

1. 异步连接未等待完成
   Android端hubConnection.start()是异步方法，调用后不会立即建立连接。直接同步检查连接状态会误判为失败，必须通过start().subscribe()监听连接结果，示例修正：

   hubConnection.start().subscribe(() -> {
       Log.i("TAG", "Connected");
   }, error -> {
       Log.e("TAG", "连接失败: " + error.getMessage());
   });
   

2. 查询参数名不匹配
   服务端Hub获取的是userid参数，但Android端URL传的是parameter=1010072，参数名不一致导致服务端无法正确获取用户ID，需将URL改为：

   http://192.168.254.173:5050/notification?userid=1010072
   

3. 消息监听注册时机错误
   应在启动连接之前注册hubConnection.on()，否则连接建立后服务端发送的初始消息（如OnConnectedAsync中的推送）会丢失。调整顺序为：先注册监听，再启动连接。

4. SDK版本不匹配
   Android使用的SignalR SDK是7.0.0，而服务端是.NET 6，跨版本可能存在协议兼容问题，建议改用6.x版本的SignalR SDK：

   implementation ("com.microsoft.signalr:signalr:6.0.25")
   

5. 缺少CORS配置检查
   若服务端未正确配置CORS允许Android客户端域名/IP访问，会导致连接失败。需在服务端Program.cs中添加CORS配置：

   builder.Services.AddCors(options =>
   {
       options.AddPolicy("SignalRCors", policy =>
       {
           policy.WithOrigins("http://你的Android客户端IP或域名")
                 .AllowAnyHeader()
                 .AllowAnyMethod()
                 .AllowCredentials();
       });
   });
   
   app.UseCors("SignalRCors");
   

内容的提问来源于stack exchange，提问作者Ibanez1408