Can you provide examples of using Polly policies in MvvmCross ViewModels

1. Installing Polly

First, ensure you have Polly installed in your project. You can install it via NuGet Package Manager:

bash
Install-Package Polly

Or using .NET CLI:

bash
dotnet add package Polly

2. Basic Retry Policy Example

Let's create a simple retry policy in an MvvmCross ViewModel. This policy will retry an operation up to three times if it fails.

csharp
using Polly;
using MvvmCross.ViewModels;

public class MyViewModel : MvxViewModel
{
    private readonly IMyService _myService;

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

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

        await retryPolicy.ExecuteAsync(async () =>
        {
            // Operation that might fail
            await _myService.DoSomethingAsync();
        });
    }
}

3. Circuit Breaker Policy Example

A circuit breaker policy can prevent cascading failures by preventing further requests when a service is down.

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

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

    public async Task DoSomethingAsync()
    {
        var circuitBreakerPolicy = Policy.Handle()
            .CircuitBreakerAsync(
                exceptionsAllowedBeforeBreaking: 2,
                durationOfBreak: TimeSpan.FromMinutes(1));

        await circuitBreakerPolicy.ExecuteAsync(async () =>
        {
            // Operation that might fail
            await _myService.DoSomethingAsync();
        });
    }
}

4. Fallback Policy Example

A fallback policy allows you to specify a default action or value when the main operation fails.

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

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

    public async Task GetDataAsync()
    {
        var fallbackPolicy = Policy.Handle()
            .FallbackAsync(() => "Default value");

        return await fallbackPolicy.ExecuteAsync(async () =>
        {
            // Operation that might fail
            return await _myService.GetDataAsync();
        });
    }
}

5. Combining Policies

You can combine multiple policies to achieve more complex behavior. For example, you might want to retry a few times before falling back to a default value.

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

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

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

        var fallbackPolicy = Policy.Handle()
            .FallbackAsync(() => "Default value");

        var combinedPolicy = Policy.WrapAsync(retryPolicy, fallbackPolicy);

        return await combinedPolicy.ExecuteAsync(async () =>
        {
            // Operation that might fail
            return await _myService.GetDataAsync();
        });
    }
}

Conclusion

Polly policies can significantly enhance the reliability of your MvvmCross applications by providing a robust mechanism for handling transient faults. By integrating retry, circuit breaker, fallback, and other policies into your ViewModels, you can ensure that your app remains responsive even in the face of service failures or network issues.