So I want to send a message to a specific client via SignalR. That client is not Clients.Caller - currently I can only identify it by let's call it "ID", a property in the context: this.Context.Items["ID"]
So to find a client by its ID, how do I...access all clients or contexts? Or should I save this ID in a different way? This is not the connection ID, it is an ID that maps to something in the database.
Basically I'd like to go Clients.Single(c => c.Items["ID"] == specificId).SendMessage(msg);
You can send the ID down to the clients using context.Clients.User(...) or context.Clients.All(). Then in JavaScript, read the ID and compare it to what's on the page. If it's a match, carry out some action; else ignore.
As an example, let's say your app's processing a specific record on an edit screen. The record ID is on the page as a form field. You send a SignalR message down from C# with the ID. You do a comparison in JavaScript between the ID and the form field value; if they match, you display a toaster message, perform other processing, etc.
In the OnConnectedAsync method in your hub, you can group a user by their "ID" each time they connect.
E.G:
public async override Task OnConnectedAsync()
{
var currentUserId = /*get your current users id here*/
await Groups.AddToGroupAsync(Context.ConnectionId, $"user-{currentUserId}");
await base.OnConnectedAsync();
}
Then when you need to send that person a message, you can just broadcast it to their 'personal' group. (One person may have multiple connections, so the group approach work perfectly for us).
E.G:
Clients.Group($"user-{userId}").SendAsync("UserSpecificMessage", message);
When users disconnect, SignalR will handle the removing of connections from the groups automatically.
In using this approach, we do not have to unnecessarily broadcast a message to every single client with the intention of only one client filtering out the message based on the id.