How do I configure Polly to retry and log errors in a Xamarin.Forms app

Step 1: Install Polly

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

bash
Install-Package Polly

Step 2: Define the Retry Policy

You can define a retry policy using Polly's `Policy.Handle` and `WaitAndRetryAsync` methods. This policy will handle exceptions and retry operations a specified number of times with a defined delay between retries.

csharp
using Polly;

// Define a retry policy that handles HttpRequestException and retries up to 3 times
// with an exponential backoff strategy
var retryPolicy = Policy.Handle()
    .WaitAndRetryAsync(
        retryCount: 3,
        retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt))
    );

Step 3: Implement Logging

To log errors after the final retry attempt, you can use a fallback policy or wrap the execution with a logging mechanism. For logging, you can use a library like Serilog.

First, install Serilog:

bash
Install-Package Serilog

Then, configure Serilog in your app's startup logic:

csharp
using Serilog;

// Configure Serilog
Log.Logger = new LoggerConfiguration()
    .MinimumLevel.Debug()
    .WriteTo.Console()
    .CreateLogger();

Step 4: Execute the Policy with Logging

Now, you can execute the policy and log any final errors. Here’s how you can do it:

csharp
// Execute the policy and log any final errors
try
{
    await retryPolicy.ExecuteAsync(async () =>
    {
        // Your API call or operation here
        var response = await httpClient.GetAsync("https://example.com/api/data");
        // Handle the response
    });
}
catch (HttpRequestException ex)
{
    // Log the final error
    Log.Error(ex, "Final retry attempt failed.");
}

Alternatively, you can use Polly's `onRetryAsync` callback to log each retry attempt:

csharp
var retryPolicy = Policy.Handle()
    .WaitAndRetryAsync(
        retryCount: 3,
        retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)),
        onRetryAsync: (exception, ts, retryCount, context) =>
        {
            Log.Warning(exception, $"Retry {retryCount} after {ts.TotalSeconds} seconds.");
        }
    );

And for logging the final error, you can wrap the execution in a try-catch block as shown above.

Step 5: Integrate with MVVM Pattern

In a Xamarin.Forms MVVM app, you can integrate Polly by defining the retry policies in a service layer. For example, you can create an `INetworkService` that handles API calls with Polly policies:

csharp
public interface INetworkService
{
    Task GetDataAsync(string url);
}

public class NetworkService : INetworkService
{
    private readonly IHttpClientFactory _httpClientFactory;
    private readonly AsyncRetryPolicy _retryPolicy;

    public NetworkService(IHttpClientFactory httpClientFactory)
    {
        _httpClientFactory = httpClientFactory;
        _retryPolicy = Policy.Handle()
            .WaitAndRetryAsync(
                retryCount: 3,
                retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt))
            );
    }

    public async Task GetDataAsync(string url)
    {
        var httpClient = _httpClientFactory.CreateClient();
        return await _retryPolicy.ExecuteAsync(async () => await httpClient.GetAsync(url));
    }
}

This way, your view models can inject `INetworkService` and use it to make API calls with built-in retry logic.

Conclusion

By following these steps, you can effectively configure Polly to retry and log errors in your Xamarin.Forms app, enhancing its resilience and user experience when dealing with transient faults.