How can I log errors handled by Polly in MvvmCross

1. Configure Polly for Error Logging**

Polly provides a way to handle exceptions and log them using its telemetry features. You can create a custom `TelemetryListener` to capture and log events, including exceptions.

First, define a custom telemetry listener that will handle logging:

csharp
using Polly;

public class PollyTelemetryLogger : ITelemetryListener
{
    private readonly ILogger _logger;

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

    public void OnTelemetryEvent(TelemetryEvent telemetryEvent)
    {
        if (telemetryEvent.Exception != null)
        {
            _logger.LogError(telemetryEvent.Exception, "Polly handled exception");
        }
    }
}

2. Integrate Polly with MvvmCross Logging**

In MvvmCross, you can use `Microsoft.Extensions.Logging` for logging purposes. Ensure that you have configured logging in your MvvmCross setup.

Here’s how you can integrate Polly with MvvmCross logging:

1. Configure Logging in MvvmCross Setup**

In your `MvxSetup` class, ensure that logging is properly configured. You can use Serilog or another logging provider with `Microsoft.Extensions.Logging`.

csharp
   public class Setup : MvxSetup
   {
       protected override void InitializeFirstChance()
       {
           base.InitializeFirstChance();
           // Configure logging here, e.g., using Serilog
           Log.Logger = new LoggerConfiguration()
               .MinimumLevel.Debug()
               .WriteTo.Console()
               .CreateLogger();
           // Register logger factory
           RegisterLoggerFactory();
       }

       private void RegisterLoggerFactory()
       {
           Services.AddLogging(loggingBuilder =>
           {
               loggingBuilder.AddSerilog(dispose: true);
           });
       }
   }
   

2. Use Polly with Custom Telemetry Listener**

Now, you can use Polly with your custom telemetry listener to log errors:

csharp
   public class MyService
   {
       private readonly AsyncRetryPolicy _retryPolicy;
       private readonly ILogger _logger;

       public MyService(ILogger logger)
       {
           _logger = logger;

           _retryPolicy = Policy.Handle()
               .WaitAndRetryAsync(
                   retryCount: 3,
                   retryAttempt => TimeSpan.FromMilliseconds(retryAttempt * 500),
                   onRetryAsync: (exception, ts, retryCount, context) =>
                   {
                       _logger.LogError(exception, "Retry attempt {retryCount}", retryCount);
                   })
               .AddTelemetryListener(new PollyTelemetryLogger(_logger));
       }

       public async Task DoSomethingAsync()
       {
           await _retryPolicy.ExecuteAsync(async () =>
           {
               // Code that might throw an exception
           });
       }
   }
   

3. Logging in ViewModels**

If you need to log errors directly in your ViewModels, you can inject an `ILogger` instance and use it to log messages:

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

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

    private async Task SomeMethodAsync()
    {
        try
        {
            // Code that might throw an exception
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Error in SomeMethodAsync");
        }
    }
}

Conclusion

By integrating Polly with MvvmCross's logging capabilities, you can effectively log errors handled by Polly. This approach ensures that your application remains robust and provides valuable insights into transient faults and exceptions.