How does MvxNotifyTask interact with Polly in MvvmCross

MvxNotifyTask Overview

- Purpose: MvxNotifyTask allows you to execute asynchronous operations while keeping track of the task's state (e.g., running, completed, faulted). It raises property-changed notifications, enabling data binding in views to reflect the task's status[1].
- Exception Handling: If a task fails, MvxNotifyTask captures the exception without crashing the application, making it available for further handling through its `Exception` property[1].

Polly Overview

- Purpose: Polly is a library that provides a simple way to implement retry policies, circuit breakers, and other resilience strategies for handling transient faults in asynchronous operations.
- Usage: Typically, you wrap your asynchronous code with Polly's policies to automatically retry failed operations or break the circuit if failures exceed a threshold.

Combining MvxNotifyTask with Polly

To integrate Polly with MvxNotifyTask, you can wrap your asynchronous operations with Polly's policies before passing them to MvxNotifyTask. Here's how you might structure this:

1. Define a Polly Policy: Create a retry policy or another resilience strategy using Polly. For example, you might define a policy that retries an operation up to three times if it fails.

2. Wrap the Operation with Polly: Use the defined policy to wrap your asynchronous operation. This ensures that if the operation fails, Polly will automatically retry it according to the policy.

3. Pass to MvxNotifyTask: Once wrapped with Polly, pass the operation to MvxNotifyTask. This allows you to monitor the task's state and handle any exceptions that might still occur despite Polly's retries.

Here's a simplified example of how you might implement this:

csharp
using MvvmCross.Core.ViewModels;
using Polly;

public class MyViewModel : MvxViewModel
{
    private readonly ISomeService _someService;
    private readonly Policy _retryPolicy;

    public MyViewModel(ISomeService someService)
    {
        _someService = someService;
        _retryPolicy = Policy.Handle()
            .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));
    }

    public IMvxCommand MyCommand { get; private set; }

    private MvxNotifyTask _myTaskNotifier;

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

    public override async Task Initialize()
    {
        await base.Initialize();
        MyCommand = new MvxCommand(() =>
        {
            MyTaskNotifier = MvxNotifyTask.Create(async () =>
            {
                await _retryPolicy.ExecuteAsync(async () =>
                {
                    await MyMethodAsync();
                });
            }, onException: ex => OnException(ex));
        });
    }

    private async Task MyMethodAsync()
    {
        await _someService.DoSomethingAsync();
    }

    private void OnException(Exception exception)
    {
        // Handle the exception
    }
}

In this example, `MyMethodAsync` is wrapped with a Polly retry policy before being passed to MvxNotifyTask. This ensures that if `MyMethodAsync` fails, Polly will retry it up to three times before MvxNotifyTask captures any remaining exceptions.

Conclusion

While MvxNotifyTask and Polly serve different purposes, combining them enhances the robustness of your asynchronous operations by providing both task state monitoring and resilience against transient faults. This approach allows you to leverage the strengths of both libraries to build more reliable and fault-tolerant applications.