Note  For these types of exceptions, the value of EnableDetailedErrors will not be taken into account. They will always be serialized and sent to the client as they are.

Sending additional information

Just as it happened in the world of persistent connections, the clients of a hub can send additional information in requests by using the mechanisms provided by web protocols. This information can be retrieved at the server with the Context property of the hub.

For example, a JavaScript client can enter information in a cookie, and its value will be available at the server side because it will travel with each request made to the server:

//Client (JavaScript) code document.cookie = "Username=phil"; $.connection.hub.start();

…

//Server code (Hub)

public Task Send(string message)

{

Cookie cookie; var username =

Context.RequestCookies.TryGetValue("Username", out cookie) ? cookie.Value

: Context.ConnectionId;

return Clients.All.Message(username + " >> " + message);

}

Another option is to enter additional values into the query string of the different requests originated at the client. For this, we can set arbitrary values in the qs (“query string”) property of the hub at the client before opening the connection, and these values will be available later at the server:

// Client code

$.connection.hub.qs = "username=phil"; // Or, alternatively:

$.connection.hub.qs = { username: "phil" };

$.connection.hub.start();

// Server code (Hub)

public Task Send(string message)

{

var username = Context.QueryString["Username"] ?? "Unknown"; return Clients.All.Message(username + " dice " + message);

}

It should be again noted that this information will travel when requests are made to the server. In a transport such as WebSockets, there will normally be only one request that will remain open, while

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other transports such as long polling will open many more connections. Therefore, the value obtained at the server will be the one sent from the client at the time of the most recent request.

Receiving messages sent from the server

We have seen that when we make calls from the server to methods that exist in clients, what actually happens is that the specifications of that call are “packaged” into a data structure and sent to all its recipients using push. See Figure 5-14.

FIGURE 5-14  Calls from the server to client-side methods.

From the point of view of the client, what SignalR does is interpret the data packet received and invoke the relevant methods. That is, it processes the events received.

In the JavaScript client, methods that can be “executed” from the server must be defined in the client property of the proxy object:

var alertHub = $.connection.alertService; alertHub.client.showAlert = function (msg) {

alert(msg);

};

alertHub.client.newUser = function (userId) { alert("New user with id: " + userId);

};

It is necessary to make these specifications before the connection is opened; otherwise, they will not work. It is also important to highlight that the name of the method used at the server must match

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the name at the client exactly, except that the match is case-insensitive. The following commands at the server will execute the logic expected at the client:

public override Task OnConnected()

{

return Clients.All.NewUser(Context.ConnectionId);

}

// Is equivalent to

public override Task OnConnected()

{

return Clients.All.newuser(Context.ConnectionId);

}

However, if a nonexistent method is invoked from the server, there will be no errors on either end.

The server will send the clients the data packet with the command specification, and the clients will not execute any action upon its reception, because the name of the method received will not match any existing one.

Logging

The client component of SignalR for JavaScript allows registering a trace with the most relevant events that occur during the lifetime of the connection, which can be very helpful when debugging the applications. To activate this trace, we just have to add the following line to the initialization code:

$.connection.hub.logging = true;

From that moment on, it will be possible to query the trace in the browser’s console, as shown in Figure 5-15 and Figure 5-16. Note that, as well as the negotiation process, we can view the events thrown from the server—that is, the methods invoked on the client side.

FIGURE 5-15  Log of the JavaScript client in Google Chrome.

FIGURE 5-16  Log of the JavaScript client in Internet Explorer 11 Developer Tools.

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Actually, if we need to, we can even include custom information in this log easily (see Figure 5-17):

$.connection.hub.start()

.done(function () {

$.connection.hub.log("My id: " + $.connection.hub.id);

});

FIGURE 5-17  Custom information in the SignalR trace.

Note  The identifier assigned to the current client is available in the $.connection.hub.id property.

The information logged in this way will always have the prefix “SignalR:”, so it is easy to identify it in the trace.

State maintenance

We have previously seen that it is possible to define variables at the client that can be queried or modified directly from the server:

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