How can I use Polly to handle retries and errors in MvvmCross

1. Installing Polly

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

bash
Install-Package Polly

2. Basic Retry Policy

Polly allows you to define policies that specify how to handle exceptions and retries. Here's an example of a basic retry policy that retries up to three times with an exponential backoff strategy:

csharp
using Polly;

// Define a retry policy that handles HttpRequestException
var retryPolicy = Policy
    .Handle()
    .OrResult(response => response.StatusCode == HttpStatusCode.InternalServerError)
    .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));

3. Executing Policies in MvvmCross

In your MvvmCross ViewModel, you can use the retry policy to wrap calls to services or APIs. Here's how you might implement it:

csharp
public class MyViewModel : MvxViewModel
{
    private readonly IMyService _myService;

    public MyViewModel(IMyService myService)
    {
        _myService = myService;
    }

    public async Task FetchDataAsync()
    {
        try
        {
            var result = await retryPolicy.ExecuteAsync(async () =>
            {
                // Call your service or API here
                return await _myService.GetDataAsync();
            });

            // Handle successful result
        }
        catch (Exception ex)
        {
            // Handle any exceptions not caught by Polly
            Mvx.LogError(ex.ToString());
        }
    }
}

4. Propagating Errors to the View

To propagate error messages from the ViewModel to the View, you can use MvvmCross's built-in messaging or interaction mechanisms. For example, you can use `MvxInteraction` to request user interaction based on validation errors:

csharp
public class MyViewModel : MvxViewModel
{
    private readonly IMvxInteraction _interaction;

    public MyViewModel(IMvxInteraction interaction)
    {
        _interaction = interaction;
    }

    public async Task FetchDataAsync()
    {
        try
        {
            // ...
        }
        catch (Exception ex)
        {
            // Propagate error to the view
            await _interaction.RequestAsync(new ValidationError { Message = ex.Message });
        }
    }
}

public class ValidationError
{
    public string Message { get; set; }
}

In your View, you can then handle this interaction to display the error message:

csharp
public class MyView : MvxActivity
{
    private readonly MyViewModel _viewModel;

    public MyView()
    {
        _viewModel = ViewModel as MyViewModel;
        _viewModel.InteractionRequest += HandleInteractionRequest;
    }

    private async Task HandleInteractionRequest(MvxInteractionRequest request)
    {
        if (request is MvxInteractionRequest interactionRequest)
        {
            var error = await interactionRequest.GetResponseAsync();
            // Display error message
            Toast.MakeText(this, error.Message, ToastLength.Long).Show();
        }
    }
}

5. Logging Errors

MvvmCross supports logging through various frameworks like NLog or Serilog. You can integrate these with Polly to log errors and retries:

csharp
public class MyViewModel : MvxViewModel
{
    private readonly ILogger _logger;

    public MyViewModel(ILogger logger)
    {
        _logger = logger;
    }

    public async Task FetchDataAsync()
    {
        try
        {
            // ...
        }
        catch (Exception ex)
        {
            _logger.Error(ex, "An error occurred while fetching data.");
        }
    }
}

Conclusion

Polly provides a robust way to handle retries and errors in MvvmCross applications. By defining custom policies and integrating them with MvvmCross's messaging and logging mechanisms, you can create resilient and user-friendly applications that handle transient faults gracefully.