Are there any examples of using Polly with MvvmCross for Android and iOS

Introduction to Polly

Polly is a popular .NET library for resilience and transient-fault-handling. It allows you to define policies for handling exceptions, such as retrying failed requests or falling back to a different approach.

Introduction to MvvmCross

MvvmCross is a cross-platform MVVM framework that supports Xamarin.iOS, Xamarin.Android, Xamarin.Forms, and other platforms. It provides features like data binding, navigation, and dependency injection, making it ideal for developing cross-platform apps.

Integrating Polly with MvvmCross

To integrate Polly with MvvmCross for Android and iOS, follow these steps:

1. Install Polly NuGet Package:
First, you need to install the Polly NuGet package in your MvvmCross project. You can do this by running the following command in the NuGet Package Manager Console:

bash
   Install-Package Polly
   

2. Create a Service for Web Requests:
Create a service class in your Core project that will handle web requests. This service will use Polly to define retry policies.

csharp
   using Polly;
   using System.Net.Http;
   using System.Threading.Tasks;

   public class WebRequestService
   {
       private readonly HttpClient _httpClient;

       public WebRequestService(HttpClient httpClient)
       {
           _httpClient = httpClient;
       }

       public async Task GetDataAsync(string url)
       {
           var retryPolicy = Policy.Handle()
               .OrResult(r => !r.IsSuccessStatusCode)
               .WaitAndRetryAsync(
                   retryCount: 3,
                   retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)),
                   (exception, span) => Console.WriteLine($"Retrying in {span.TotalSeconds} seconds..."));

           return await retryPolicy.ExecuteAsync(async () =>
           {
               var response = await _httpClient.GetAsync(url);
               response.EnsureSuccessStatusCode(); // Throws if not successful
               return await response.Content.ReadAsStringAsync();
           });
       }
   }
   

3. Register the Service in MvvmCross:
You need to register the `WebRequestService` in the MvvmCross IoC container. This is typically done in the `AppStart.cs` file of your Core project.

csharp
   using MvvmCross.Platform;

   public class AppStart : MvxAppStart
   {
       public override void Start(object hint = null)
       {
           var httpClient = Mvx.Resolve().CreateClient("MyClient");
           var webRequestService = new WebRequestService(httpClient);

           Mvx.RegisterSingleton(webRequestService);

           base.Start(hint);
       }
   }
   

4. Use the Service in Your ViewModel:
Now, you can use the `WebRequestService` in your ViewModels to fetch data with retry policies applied.

csharp
   using MvvmCross.ViewModels;

   public class MyViewModel : MvxViewModel
   {
       private readonly IWebRequestService _webRequestService;

       public MyViewModel(IWebRequestService webRequestService)
       {
           _webRequestService = webRequestService;
       }

       public async Task LoadDataAsync()
       {
           try
           {
               var data = await _webRequestService.GetDataAsync("https://example.com/api/data");
               // Process data
           }
           catch (Exception ex)
           {
               // Handle any exceptions not caught by Polly
               Console.WriteLine($"Error loading data: {ex.Message}");
           }
       }
   }
   

Handling Errors in Views

For error notifications, you can use MvvmCross Interactions to pass validation errors or other messages from the ViewModel to the View. For example, if you want to display an error message next to a text input field in Android, you can use `TextInputLayout.SetError()` as shown in the Stack Overflow answer[2].

Conclusion

Integrating Polly with MvvmCross allows you to robustly handle transient faults in web requests across Android and iOS platforms. By defining retry policies with Polly and using MvvmCross for cross-platform development, you can create resilient and maintainable apps.