hnabbasi/polly-mvvm

Using Polly in your Xamarin.Forms MVVM app.

Polly-MVVM

Blog: Enhance Your Xamarin.Forms App’s User Experience With Polly

Polly has been around for a while and is a great tool for making your app more responsive and have a strategy around transient faults i.e. dropped network connection, weak transmission resulting in errors, etc. Polly has been document extensively on their page linked above, but in this session, I will demonstrate how to simplify the use of Polly in your MVVM app. You can build from here and make the strategies as complex as needed.

Polly works with any .NET application. Which means that you can apply these resilience policies to your .NET API layer as well.

Architecture

I am going to structure my code in several layers/nodes/tiers however you want to call them. This will keep the code decoupled based on behavior. I will have the following layers,

• Models
• Common
• Services
• Core
• iOS
• Android

In my opinion, the best framework to write MVVM Xamarin.Forms is Prism.Forms. It provides services to handle navigation, dependency injection, events, etc. So I will be using Prism.Forms for this demo.

Components

These are the main components of the app that make use of Polly in a suitable manner possible.

IHttpClient

We will use the mono managed HttpClient for this demo. But since we can swap the implementation of the client for numerous reasons, I am going to keep the client out of my main service using IHttpClient abstraction.

INetworkService

This service will handle our Polly policies. I want to keep all my policies in one place so that I can easily manage them and test them.

IApiService

This service will handle all API calls. We can choose to use Polly policies or not in this service. The developer has the freedom to call APIs as he needs because not all calls need a policy e.g. POST that can cause duplications or errors. All my functional services will inject IApiService and will provide APIs to make calls.

Resilience Policies

For this demo, I will implement Retry and WaitAndRetry policies. But use these as a starting point and build on top to get more complex policies for your app.

Scope

Polly offers a lot of cool ways to make your app's network resilience awesome. In this demo, I will cover how to use Polly in a MVVM-friendly manner that lets you test your service code independently. I will also cover some policies in this demo, and some more in my future demos.

In This Demo

• Using Polly in a service
• Inject policies service
• Retry policy
• WaitAndRetry policy

Future Demos

• PolicyRegistry
• Combined policies
• Circuit Breaker
• Cache
• Fallback