Can you provide an example of using Polly's WaitAndRetry policy with IHttpClientFactory in MvvmCross

Using Polly's WaitAndRetry Policy with IHttpClientFactory in MvvmCross

To implement Polly's `WaitAndRetry` policy with `IHttpClientFactory` in an MvvmCross application, you need to follow these steps:

Step 1: Install Required Packages

First, ensure you have the necessary packages installed in your project. You will need `Microsoft.Extensions.Http.Polly` for integrating Polly with `IHttpClientFactory`. You can install it using NuGet Package Manager or the dotnet CLI:

bash
dotnet add package Microsoft.Extensions.Http.Polly

Step 2: Configure IHttpClientFactory with Polly

In your MvvmCross application, typically in the `App.cs` or wherever you configure services, you need to add the `IHttpClientFactory` with Polly policies. Here’s how you can do it:

csharp
using Microsoft.Extensions.Http;
using Polly;

public class App : MvxApplication
{
    public override void Initialize()
    {
        // Other initialization code...

        // Register services
        var services = Mvx.IoCProvider.Resolve().GetInstance();
        
        // Add HttpClient with Polly policy
        services.AddHttpClient("MyService", client =>
        {
            client.BaseAddress = new Uri("https://example.com/api/"); // Your base URL
        })
        .AddPolicyHandler(GetRetryPolicy());

        // Other service registrations...
    }

    private static IAsyncPolicy GetRetryPolicy()
    {
        return HttpPolicyExtensions
            .HandleTransientHttpError()
            .OrResult(msg => msg.StatusCode == System.Net.HttpStatusCode.NotFound)
            .WaitAndRetryAsync(
                retryCount: 6,
                retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt))
            );
    }
}

Step 3: Use the Configured HttpClient

Now, you can inject and use the `IHttpClientFactory` in your services or view models to make HTTP requests with the retry policy applied:

csharp
using MvvmCross.ViewModels;
using System.Net.Http;
using System.Threading.Tasks;

public class MyViewModel : MvxViewModel
{
    private readonly IHttpClientFactory _httpClientFactory;

    public MyViewModel(IHttpClientFactory httpClientFactory)
    {
        _httpClientFactory = httpClientFactory;
    }

    public async Task MakeApiCall()
    {
        var client = _httpClientFactory.CreateClient("MyService");
        var response = await client.GetAsync("path/to/resource");

        // Handle response...
    }
}

Explanation

- Polly's WaitAndRetry Policy: This policy retries failed requests after a specified delay. The delay increases exponentially with each retry attempt.
- IHttpClientFactory: This is a factory for creating instances of `HttpClient`. It helps manage the lifetime of `HttpClient` instances and avoids socket exhaustion issues.
- MvvmCross Integration: In MvvmCross, you typically configure services in the `App.cs` file. Here, you add the `IHttpClientFactory` with Polly's retry policy.

Benefits

- Resilience: Polly's policies make your application more resilient by handling transient faults.
- Flexibility: You can customize the retry policy to suit your application's needs, such as changing the number of retries or the backoff strategy.
- Decoupling: Using `IHttpClientFactory` decouples the creation and management of `HttpClient` instances from your business logic.