SignalR Core - 从 WebAPI 检查连接状态

Question

I am using Microsoft.AspNetCore.SignalR.Client to open a connection from my WebAPI project to connect and call methods in my SignalR Hub project. These are separate projects hosted on separate servers.

How do I check whether the connection has been started, so I don't try to start it twice?

I connect from WebAPI using the following code:

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);
    }
}

Since my WebAPI will be calling into SignalR quite a bit, I want to create the single connection between WebAPI and SignalR and not close/open the connection each time. At the moment I create the ChatApi class as a singleton and initialize the hub connection in the constructor.

How would I check if the connection is started before calling await _connection.StartAsync(); ?

Using: Microsoft.AspNetCore.SignalR.Client v1.0.0-preview1-final

Answer 1

There is no ConnectionState property.

You need to track the state yourself, by subscribing to the Closed event on HubConnection .

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
    }
}

Usage:

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

Reference: SignalR/benchmarkapps/Crankier/Client.cs

Answer 2

In .NET Core 2.1 at least, you can check the State property of your HubConnection:

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