比如新建一个CommHub类并继承Hub,我这里初始化了_context，但是在调用的时候提示：
Error when dispatching 'OnConnectedAsync' on hub.
System.InvalidOperationException: Unable to resolve service for type 'XXK.DataContext' while attempting to activate 'XXK.Hubs.CommHub'.
at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.GetService(IServiceProvider sp, Type type, Type requiredBy, Boolean isDefaultParameterRequired)
at lambda_method191(Closure , IServiceProvider , Object[] )
at Microsoft.AspNetCore.SignalR.Internal.DefaultHubActivator1.Create() at Microsoft.AspNetCore.SignalR.Internal.DefaultHubDispatcher1.OnConnectedAsync(HubConnectionContext connection)
at Microsoft.AspNetCore.SignalR.Internal.DefaultHubDispatcher1.OnConnectedAsync(HubConnectionContext connection) at Microsoft.AspNetCore.SignalR.HubConnectionHandler1.RunHubAsync(HubConnectionContext connection)
1.Create() at Microsoft.AspNetCore.SignalR.Internal.DefaultHubDispatcher1.OnConnectedAsync(HubConnectionContext connection) at Microsoft.AspNetCore.SignalR.Internal.DefaultHubDispatcher1.OnConnectedAsync(HubConnectionContext connection) at Microsoft.AspNetCore.SignalR.HubConnectionHandler1.RunHubAsync(HubConnectionContext connection)