Conquer the network! Roy Cornelissen Xpirit @roycornelissen #TECHDAYSNL #XAMARIN

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Agenda

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8 fallacies of 1. The network is reliable. 2. Latency is zero. 3. Bandwidth is infinite. 4. The network is secure. 5. Topology doesn't change. 6. There is one administrator. 7. Transport cost is zero. 8. The network is homogeneous. Deutsch - 1994 Gosling - 1997 distributed computing

Threads Don’t occupy the UI Thread with network I/O or CPU bound work like serialization

Common pitfalls with async/await

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Calling .Run(), .Wait() or .Result on a Task async Task MyMethod(int parameter) { } MyMethod(10).Run(); This will execute the task on the same thread, blocking the UI thread var result = await MyMethod(10); // async all the way! Pitfall

Unnecessarily returning to the UI Thread await DoLotsOfNetworkCalls(); public async Task DoLotsOfNetworkCalls() { await httpClient.GetAsync(“…”); // do some calculations or de-serialization await httpClient.GetAsync(“…”); } After each await, execution will continue on the original thread, causing interruptions and even possibly deadlocks. await httpClient.GetAsync(“…”).ConfigureAwait(false); // use ConfigureAwait(false) in your library code Pitfall

Creating async void methods async void MyMethod(int parameter) { } Awaiting this method will fail This is a Fire & Forget call, you cannot do any proper error handling async Task MyMethod(int parameter) { } async void OnClick(object sender, EventArgs e) { } // Exception: async event handlers! Pitfall

Not awaiting async UI methods Navigation.PushAsync(new DetailView()); await Navigation.PushAsync(new DetailView()); Can cause deadlocks, freezing UI or white screens Pitfall

Demo App RESTful api Repo: https://www.github.com/xpiritbv/ConquerTheNetwork API: http://vslivesampleservice.azurewebsites.net/api/cities

Goals • Easily access RESTful API’s • Fast response to the user • Work offline • Deal with errors

Standing on the shoulders of giants

Easily access Restful Api’s

Easily access Restful Api’s Refit • Install-Package Refit Turns a RESTful API into a typesafe interface public interface IGitHubApi { [Get("/users/{user}")] Task GetUser(string user); } Component by: Paul Betts, http://github.com/paulcbetts/Refit

Faster response to the user

Faster response Akavache • Install-Package Akavache On device cache based on SQLite local database. You must include SQLite in the Windows app as well. Component by: Paul Betts, http://github.com/paulcbetts/Akavache

SQLITE-NET A lightweight ORM suitable for mobile application development

SQLite.org SQLite • Cross-platform format; stored as a single file • 32 or 64 bit; big or little endian • Most of SQL92 • Missing some OUTER joins • Only supports TABLE RENAME & ADDCOLUMN • Views are read only

Configuration – SQLite just needs to know where the database file is stored. – The location must be writable! • If you bundle a database file in your app, copy it out first! • Different platforms “care” about different things • Android: choose internal or external storage • iOS: choose Documents, Library or other

public class Session { [PrimaryKey, AutoIncrement, Column("_id")] public int Id { get; set; } public string SpeakerName { get; set; } public string Title { get; set; } public string Abstract { get; set; } public string Location { get; set; } public DateTime Begins { get; set; } public DateTime Ends { get; set; } } Model Classes

db.CreateTable (); db.Insert (session); // after creating the session object Executing Commands

• The SQLite library is not thread safe • Don’t use the same Connection across threads • Always use lock () statements lock (locker){ // Do your query or insert here } Threading

Core OS layer OS API layer Mono stack Application layer Faster network requests

Core OS layer OS API layer Mono stack Application layer ModernHttpClient Faster network requests

ModernHttpClient • Install-Package ModernHttpClient Leverages the native OS network stack by swapping in a NativeMessageHandler: • NSURLSession on iOS • OkHttp on Android Beware: exceptions are not the same as with plain HttpClient (e.g. Java web exceptions) Beware: iOS App Transport Security (use TLS2.0 edit info.plist) Component by: Paul Betts, http://github.com/paulcbetts/ModernHttpClient Faster network requests

Choose network stack in csproj options Faster network requests (alternative)

Use AndroidClientHandler Uses native java.net.URLConnection Faster network requests (alternative) var client = new HttpClient ( new Xamarin.Android.Net.AndroidClientHandler ());

Sometimes you want to work offline…

Work Offline No network: why bother? Prevent nasty waits and errors

Connectivity Plugin • Install-Package Xam.Plugin.Connectivity Provides a cross platform way to check the connectivity of the device before trying to issue a network request. Component by: James Montemagno, https://github.com/jamesmontemagno/Xamarin.Plugins Work Offline

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What could go wrong?

What do you do? Retry • How many times? • At what interval? • What if the server is already busy? Circuit breaker • Elegant pattern for failing fast after network errors

Polly • Install-Package Polly Implement error handling policies by declaratively specifying retry, circuit breaker etc. Polly will do the heavy lifting. Polly by: Michael Wolfenden, https://github.com/michael-wolfenden/ Handle errors

Fody.AsyncErrorHandler • Install-Package Fody.AsyncErrorHandler Globally handle errors that happen in async (background) code. Useful for catching otherwise hidden errors. Uses a Fody IL weaver. Fody.AsyncErrorHandler by: Simon Cropp, https://github.com/Fody/AsyncErrorHandler Handle errors

Easily access RESTful API’s: Refit Fast response to user: Caching, ModernHttpClient, async Work offline: Akavache, Connectivity Plugin Deal with errors: Polly, Fody.AsyncErrorHandler Goals

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@roycornelissen roycornelissen.wordpress.com [email protected]

@roycornelissen roycornelissen.wordpress.com [email protected] bit.ly/XpiritMagazine