Craig DunnDeveloper EvangelistXamarincraig@xamarin.com@conceptdev

C# async awaitfor mobile

Mobile Apps

• Need responsive user interfaces• App features are o!en dependent on:

network access (images, web services)database functionality or I/Ocomplex processing on mobile CPUsany stuff that takes some time

• You want to run these on a different thread to keep the UI responsive... they should be ASYNCHRONOUS!

Fast!

Long running tasks!

Threads!

All

Responsive User Interfaces

Don’t designfor desktop

Responsive User InterfacesInstagram Don’t design for desktop

start upload (or here)

so this is FAST

user doesn’t notice here...

Responsive User InterfacesDon’t design for desktopInstagram

hit-and-hope

• DO things in the background

• DON’T block the UI thread

• DON’T make users feel like they’re waiting

Responsive User InterfacesDon’t design for desktop

DEMODEMO1) Download Html string2) Download Jpeg image3) Save to Storage4) Return Html length

Old-style callbacks

1) Download Html string2) Download Jpeg image3) Save to Photo Album

5) Error Handling4) Return Html length

6) InvokeOnMainThread

Callback Hell

Old-style callbacks

1) Download Html string2) Download Jpeg image3) Save to Photo Album

5) Error Handling4) Return Html length

6) InvokeOnMainThread

Callback Hell

Old-style callbacks

1) Download Html string2) Download Jpeg image3) Save to Photo Album

5) Error Handling4) Return Html length

6) InvokeOnMainThread

Callback Hell

1) Download Html string2) Download Jpeg image3) Save to Photo Album

5) Error Handling4) Return Html length

6) InvokeOnMainThread

Callback Hell

Old-style callbacks

What is “async”?

• Tasks running outside of the main program flow• Code runs on another thread, so the UI doesn’t block/freeze• Completion runs on the calling thread, so if you started on the

UI that’s where you’ll be a!er the async task is complete

• async and await syntax in C# 5 takes Task support to the next level!

Frameworks need to support it on long running tasks

Task Parallel Library (TPL) has been around a while

Old-style callbacks

• Spaghetti code:

Callbacks are the new “GOTO”Control flow jumps around in ways that are difficult to read & understand from the sourceError handling is difficult to implement, required in many different placesChanges in the chain can have unintended consequences

http://tirania.org/blog/archive/2013/Aug-15.html

Old-style callbacks

• Execute On Main ThreadWe need to schedule updates - can’t just access UI objects directly from background threadsDispatcher.Invoke (pre RT)

USING ASYNCasync, await, cancellation and error handling

Comparison

1) Download Html string2) Download Jpeg image3) Save to Photo Album

5) Error Handling4) Return Html length

6) InvokeOnMainThread

Callback Hell

Comparison

1) Download Html string2) Download Jpeg image3) Save to Photo Album

5) Error Handling4) Return Html length

6) InvokeOnMainThread

Callback Hell

1) Download Html string2) Download Jpeg image3) Save to Photo Album

5) Error Handling4) Return Html length

Comparison

Async-ified

6) InvokeOnMainThread

1) Download Html string2) Download Jpeg image3) Save to Photo Album

5) Error Handling4) Return Html length

Comparison

6) InvokeOnMainThreadone place

not required

Async-ified

Comparison

1) Download Html string2) Download Jpeg image3) Save to Photo Album

5) Error Handling4) Return Html length

6) InvokeOnMainThread

old new!

Compiler Magic

http://msdn.microso!.com/en-us/library/vstudio/hh191443.aspx

Compiler Magic• async keyword informs the compiler that this method needs to be

“munged”• await keyword indicates a suspension point where a callback

needs to be generated, along with error handling• Continuations are generated a!er each suspension point• Error handling (support for enclosing try-catch) is taken care of

• All you need to remember is async and awaitand use Tasks

How to use: async?• async modifier on methods, lambdas and anonymous methods

use Async suffix, eg LoadAsync, SendAsyncreturn Task or Task<T> preferably

- Task for methods that don’t return a value

- Task<T> to return a value

- void for event handlers only!

void for event handlers

How to use: await?• await keyword on awaitable objects (Tasks)

only within an async contextmarks a suspension point - control is returned to callercan’t be used in catch or finally blocksTask, Task<T> or a custom type

get a reference to the Task first

... or just await

How to use: error handling?• async Task or Task<T>

Returned Task State == FaultedException re-thrown when task is awaited

• async void methods (event handlers)Exception is thrown on the current synchronization context and the app will crash...

How to use: error handling?• “If a task is the parent of attached child tasks, or if you are

waiting on multiple tasks, then multiple exceptions could be thrown”

• AggregateExceptionLoop through, handle or throw as required...

• Cancellation is optionalusually exposed an Async() method overload that takes a CancellationToken parametercalling code uses the tokenCancellation.None means it won’t be cancelled by the caller (useful when the cancellation parameter isn’t optional)

How to use: cancellation?

• Cancellation is optionalcancelled async tasks complete (but time for this to happen isn’t guaranteed)

How to use: cancellation?

• Specifying a timeout is optionalfor HttpClient, set Timeout

some Async methods accept a timeout parameter

more generally, use CancellationToken (timeout)

How to use: timeouts?

• Progress reporting is optionalprovide another Async overload with an IProgress<T> parameterbasic Progress<T> implementation can be usedexposes EventHandler<T> ProgressChanged

How to use: progress reporting?

send events to track progress

BONUS: Combinators• Wait on multiple tasks

Task.WhenAll (IEnumerable<Task>)

- requires all the tasks to be completedTask.WhenAny(IEnumerable<Task>)

- returns when any of the tasks completes

this kind of loop fine for small numbersof Tasks

WARNING: Await, and UI, and deadlocks

http://blogs.msdn.com/b/pfxteam/archive/2011/01/13/10115163.aspx

MOAR MOBILEiOS & Android too!

.NET BCL APIs

• Lots of .NET APIs support async; for example:HttpClient.GetStringAsync()HttpClient.PostAsync()FileStream.ReadAsync()FileStream.CopyToAsync()

• and many more...

Windows Store and Phone appsonly have async APIs

Xamarin.iOS

• updated iOS APIs to be async; some examples:ALAssetsLibrary.WriteImageToSavedPhotosAlbumAsync()ALAsset.SetImageDataAsync()SKStoreProductViewController.LoadProductAsync()CLGeocoder.GeocodeAddress()NSUrlConnection.SendRequestAsync()

and many more... 174 async iOS native APIs

Xamarin.Android

• updated Android APIs to be async; some examples:Android.Net.Http.AndroidHttpClient.ExecuteAsync()Android.Bluetooth.BluetoothServerSocket.AcceptAsync()Android.Graphics.BitmapFactory.DecodeFileAsync()Android.Locations.Geocoder.GetFromLocationAsync()Java.IO.File.ListAsync()

and many more... 337 async Android native APIs

Xamarin APIs

• Xamarin.Mobile Geolocator.GetPositionAsync()Contact.SaveThumbnailAsync()

• Xamarin.Auth

FormAuthenticator.SignInAsync()Request.GetResponseAsync(cancellationToken)

• and many more...Xamarin cross-platform libraries

Xamarin Component Store

• Many components already offer async APIs, eg. Parse!

Powerful, easy to use componentshttp://components.xamarin.com

DEMODEMOParseTodo

Other tricks...• Creating your own async Tasks

eg. how you could async-ify WebClient

http://developer.nokia.com/Community/Wiki/Asynchronous_Programming_For_Windows_Phone_8

Other tricks...

• Creating your own async Tasks

Don’t await... give it a try• Available in .NET 4.5 for Windows Store apps, Windows 8,

Windows Phone, etc.

• Also available programming in C# iOS & Android using Xamarin!

xamarin.com/download

Questions?Developer EvangelistXamarincraig@xamarin.com@conceptdev

Craig Dunn