Xamarin ♥ Azure SignalR Service

Azure SignalR Service is a service in Microsoft Azure that makes it possible for developers to build applications with real-time communications without having to think about how to host it. Azure will handle all that for us.

If you are interested to learn more about SignarlR service and Xamarin apps there is a chapter in the Xamarin.Forms project, the book that I have written together with Johan Karlsson that cover the subject. In the book, you will get step-by-step guidance on how to set up the backend and how to write the app from "file new" to a full app. Read more about the book here, https://www.packtpub.com/application-development/xamarinforms-projects.

This post will cover:

  • How to set up a simple backend built with Azure functions and Azure SignalR service
  • How to use SignalR in a Xamarin App.

Setting up a backend

For the backend, we need to create two services in the Azure portal.

  1. Create a SignalRServcie in a new resource group
  2. Create a Function App in the same resouce group.

Open Visual Studio and create a new Azure Functions projects.

Install the NuGet package, Microsoft.Azure.WebJobs.Extensions.SignalRService. We need to check the "Show prereleases" checkbox to find the package, because it is still just a pre-release.

Create a new function with the name GetSignalRConnectionInfo.

public class GetSignalRConnectionInfo
{
        [FunctionName("GetSignalRConnectionInfo")]
        public static SignalRConnectionInfo Run(
            [HttpTrigger(AuthorizationLevel.Anonymous)] HttpRequest req,
            [SignalRConnectionInfo(HubName = "chat")] SignalRConnectionInfo connectionInfo)
        {
            return connectionInfo;
        }
}

The SignalRConnectionInfo attribute is used to read the information from the configuration and to generate an access token. If the function should be able to read the information from the configuration we need to add it. What we should add is the connection string that we will find in the Azure Portal in the Keys tab under the SignalR Service. It will be added with the key AzureSignalRConnectionString to the ApplicationSettings of the Function App or to the local.settings.json file in our Visual Studio project if we want to run the function locally. It is also with the attribute we can set the name of the hub that we want to use, we can have multiple hubs in a SignalR Service.

Next step is to create a function that will add messages to the SignalR Service. Create a new function with the name SendMessages.

public class SendMessages
{
        [FunctionName("SendMessages")]
        public async static Task Run(
            [HttpTrigger(AuthorizationLevel.Anonymous, "post")] object message,
            [SignalR(HubName = "chat")] IAsyncCollector<SignalRMessage> signalRMessages)
        {
            await signalRMessages.AddAsync(
                new SignalRMessage
                {
                    Target = "chatMessage",
                Arguments = new[] { message }
                });
        }
}

Use the SignalR attribute and an IAsyncCollector parameter to get a collection that you can use for adding messages to the SignalR Service. Target is a key that we can listen after in the app.

If we want to add some logic, for example, some type of validation we can do it here, before we add the message to the collection. In the app we are building in the book can the users upload photos. Then we use Azure Cognitive Services to make sure that the photos not are classified as adult photos.

Authentication

If we want to add authentication to our SignalR Service we should add it to the function that returns the connection info.

Building the app

To build an app with Xamarin we need to install the Microsoft.AspNetCore.SignalR.Client NuGet package.

Create a connection and start listening for messages

First we will create a model for connection information:

public class ConnectionInfo
{
     public string Url { get; set; }
     public string AccessToken { get; set; }
}

Now we can make an HTTP call to the function we created for getting connection information. When we get an answer we will serialize it to a ConnectionInfo model.

var result = await httpClient.GetStringAsync("https://{name-of-your-function}.azurewebsites.net/api/GetSignalRConnectionInfo");
var info = JsonConvert.DeserializeObject<Models.ConnectionInfo>(result);

Now we have all the information that we need to connect to the SignalR Service. To create a connection we will use the HubConnectionBuilder.

var connectionBuilder = new HubConnectionBuilder();
connectionBuilder.WithUrl(info.Url, (Microsoft.AspNetCore.Http.Connections.Client.HttpConnectionOptions obj) =>
{
     obj.AccessTokenProvider = () => Task.Run(() => info.AccessToken);
});
 
var hub = connectionBuilder.Build();

To subscribe for messages we will use the On<object> method on the hub. We will pass the key that we specified in the message and an Action, in this case, we using an expression for the action.

In the expression body, we will handle the message, the message will come as a string so we need to convert the object to a string and then we can deserialize it to the type of message that we will receive. In this case, I have created a model with the name Message.

The last thing to do to start to listen for messages is to call the StartAsync method on the hub.

hub.On<object>("chatMessage", (message) =>
{
      var json = message.ToString();
      var obj = JsonConvert.DeserializeObject<Message>(json);
 
      //Handle the message here
});
 
await hub.StartAsync();

Sending a message

To sending a message is just a simple POST to the SendMessages function.

public async Task SendMessage(Message message)
{
     var json = JsonConvert.SerializeObject(message);
 
     var content = new StringContent(json, Encoding.UTF8, "application/json");
 
     var response = await httpClient.PostAsync("https://xfbook.azurewebsites.net/api/SendMessages", content);
}

OnSuspend and OnResume

It is important to handle the suspend and resumes events. The best way to that is to override OnSuspend and OnResume in App.xaml.cs. I recommended that you create a Dispose method in the same class as we have the other code that is related to SignalR. In this method, we will stop listening for messages and dispose the hub.

public async Task Dispose()
{
     if(hub != null)
     {
          await hub.StopAsync();
          await hub.DisposeAsync();
     }
}

And then we can call the Dispose method from the OnSuspend method. In the OnResume method, we can start the connection again.

Xamarin Month

This blog post is a part of Xamarin Month. Follow this link to read all the other blog post of the Xamarin Month.

2 thoughts on “Xamarin ♥ Azure SignalR Service

  1. Hi,
    First thanks for that post. Second, I just ordered you book at amazon.

    The call to the SendMessages Function is wrong specified. Instead of “messages” the fx name is “SendMessages”‘

    Why is it important to Stop SignalR communication when App goes into background?
    I wanna have the communication still established to notify my clients in realtime. On signalr message received in XF client I show a local notification in status bar, the user taps it and gets navigated to the signalR page in the app.

    Best
    Eric

    • dhindrik says:

      Thank you for finding the typo.

      If you want it to work in the background I should recommend you to take a look at backgrounding for each platform. If you stop SignalR communication yourself you will do it in a more controlled way. The connection will probably be killed when the app goes to background.

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