Are there any examples of using Polly with MvvmCross for both sync and async operations

Introduction to Polly and MvvmCross

- Polly: A popular .NET library for implementing resilience and transient fault handling strategies such as retry, circuit-breaker, and fallback policies. It supports both synchronous and asynchronous operations[3][5].

- MvvmCross: A cross-platform MVVM framework that supports Xamarin.iOS, Xamarin.Android, Xamarin.Forms, and more. It provides features like data binding, navigation, and dependency injection[4][7].

Using Polly for Synchronous Operations

While Polly primarily focuses on asynchronous operations, you can still use it for synchronous operations by wrapping your synchronous code in an asynchronous method. However, for purely synchronous operations, you would typically use `Policy` instead of `AsyncPolicy`.

Here's an example of using Polly for synchronous operations in an MvvmCross context:

csharp
using Polly;

public class MyViewModel : MvxViewModel
{
    private readonly ISomeService _someService;

    public MyViewModel(ISomeService someService)
    {
        _someService = someService;
    }

    public void DoSomethingSync()
    {
        var policy = Policy.Handle()
            .Retry(3, (exception, retryCount) =>
            {
                // Handle retry logic here
            });

        policy.Execute(() =>
        {
            _someService.DoSomethingSync();
        });
    }
}

Using Polly for Asynchronous Operations

For asynchronous operations, Polly provides `AsyncPolicy` and `AsyncPolicy`, which are more suitable for handling async code. Here's how you can use Polly with MvvmCross for async operations:

csharp
using Polly;

public class MyViewModel : MvxViewModel
{
    private readonly ISomeService _someService;

    public MyViewModel(ISomeService someService)
    {
        _someService = someService;
    }

    public async Task DoSomethingAsync()
    {
        var policy = Policy.Handle()
            .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));

        await policy.ExecuteAsync(async () =>
        {
            await _someService.DoSomethingAsync();
        });
    }
}

Combining Polly with MvxNotifyTask

MvvmCross's `MvxNotifyTask` can be used to manage async operations and handle exceptions gracefully. You can combine Polly with `MvxNotifyTask` to enhance exception handling and retries:

csharp
using MvvmCross.ViewModels;
using Polly;

public class MyViewModel : MvxViewModel
{
    private readonly ISomeService _someService;

    public MyViewModel(ISomeService someService)
    {
        _someService = someService;
    }

    private MvxNotifyTask _myTaskNotifier;

    public MvxNotifyTask MyTaskNotifier
    {
        get => _myTaskNotifier;
        private set => SetProperty(ref _myTaskNotifier, value);
    }

    public async Task DoSomethingAsync()
    {
        var policy = Policy.Handle()
            .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));

        MyTaskNotifier = MvxNotifyTask.Create(async () =>
        {
            await policy.ExecuteAsync(async () =>
            {
                await _someService.DoSomethingAsync();
            });
        });
    }
}

Handling Exceptions with Polly and MvvmCross

When using Polly with MvvmCross, you can handle exceptions by specifying the types of exceptions to handle and implementing retry logic. For example, you might handle `HttpRequestException` for network-related issues or `SqlException` for database operations.

csharp
var policy = Policy.Handle()
    .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)),
    onRetryAsync: async (exception, retryCount, context) =>
    {
        // Handle retry logic here
    });

Conclusion

Integrating Polly with MvvmCross enhances your application's resilience by providing robust handling of transient faults in both synchronous and asynchronous operations. By using Polly's retry policies alongside MvvmCross's async management features like `MvxNotifyTask`, you can ensure that your cross-platform applications remain responsive and reliable.