SignalR Core - 从 WebAPI 检查连接状态

Question

我正在使用Microsoft.AspNetCore.SignalR.Client 从我的 WebAPI 项目打开一个连接，以连接和调用我的 SignalR Hub 项目中的方法。这些是托管在不同服务器上的独立项目。

如何查看连接是否已经启动，以免尝试启动两次？

我使用以下代码从 WebAPI 连接：

public class ChatApi
{
    private readonly HubConnection _connection;

    public ChatApi()
    {
        var connection = new HubConnectionBuilder();
        _connection = connection.WithUrl("https://localhost:44302/chathub").Build();
    }

    public async Task SendMessage(Msg model)
    {
        await _connection.StartAsync();
        await _connection.SendAsync("Send", model);
    }
}

由于我的 WebAPI 会大量调用 SignalR，因此我想在 WebAPI 和 SignalR 之间创建单个连接，而不是每次都关闭/打开连接。目前我将ChatApi 类创建为单例，并在构造函数中初始化集线器连接。

在调用await _connection.StartAsync();之前如何检查连接是否已启动？

使用：Microsoft.AspNetCore.SignalR.Clientv1.0.0-preview1-final

Answer 1

Microsoft.AspNetCore.SignalR.Client v1.1.0+

HubConnection 上有一个 State 属性。

public class ChatApi
{
    private readonly HubConnection _connection;

    public ChatApi()
    {
        _connection = new HubConnectionBuilder()
            .WithUrl("https://localhost:44302/chathub")
            .Build();
    }

    public async Task StartIfNeededAsync()
    {
        if (_connection.State == HubConnectionState.Disconnected)
        {
            await _connection.StartAsync();
        }
    }

    public async Task SendMessage(Msg model)
    {
        await StartIfNeededAsync();
        await _connection.SendAsync("Send", model);
    }
}

参考资料：

• https://docs.microsoft.com/en-us/aspnet/core/signalr/dotnet-client
• aspnet/SignalR/pull/2204
• dotnet/aspnetcore@7625bbc

Microsoft.AspNetCore.SignalR.Client

（在撰写本文时，最新版本是 1.0.0-preview2-final。）

没有ConnectionState 属性。

您需要自己跟踪状态，方法是订阅HubConnection 上的Closed 事件。

public class ChatApi
{
    private readonly HubConnection _connection;

    private ConnectionState _connectionState = ConnectionState.Disconnected;

    public ChatApi()
    {
        var connection = new HubConnectionBuilder();
        _connection = connection.WithUrl("https://localhost:44302/chathub").Build();

        // Subscribe to event
        _connection.Closed += (ex) =>
        {
            if (ex == null)
            {
                Trace.WriteLine("Connection terminated");
                _connectionState = ConnectionState.Disconnected;
            }
            else
            {
                Trace.WriteLine($"Connection terminated with error: {ex.GetType()}: {ex.Message}");
                _connectionState = ConnectionState.Faulted;
            }
        };
    }

    public async Task StartIfNeededAsync()
    {
        if (_connectionState == ConnectionState.Connected)
        {
            return;
        }

        try
        {
            await _connection.StartAsync();
            _connectionState = ConnectionState.Connected;
        }
        catch (Exception ex)
        {
            Trace.WriteLine($"Connection.Start Failed: {ex.GetType()}: {ex.Message}");
            _connectionState = ConnectionState.Faulted;
            throw;
        }
    }

    private enum ConnectionState
    {
        Connected,
        Disconnected,
        Faulted
    }
}

用法：

public async Task SendMessage(Msg model)
{
    await StartIfNeededAsync();
    await _connection.SendAsync("Send", model);
}

参考资料：

• aspnet/SignalR/blob/1fc282a/benchmarkapps/Crankier/Client.cs
• aspnet/SignalR/issues/2127（此答案后 8 天开放）

Answer 2

至少在 .NET Core 2.1 中，您可以检查 HubConnection 的 State 属性：

if (_connection.State == HubConnectionState.Disconnected) {
        await _connection.StartAsync();
}