Presented at IT/DevConnections in Las Vegas, October 2013

1. Craig Dunn
Developer Evangelist
Xamarin
craig@xamarin.com
@conceptdev
C# async await
for mobile

2. Mobile Apps
• Need responsive user interfaces
• App features are o!en dependent on:
network access (images, web services)
database functionality or I/O
complex processing on mobile CPUs
any 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

3. Responsive User Interfaces
Don’t design
for desktop

4. Responsive User Interfaces
Instagram Don’t design for desktop
start upload (or here)
so
this
is
FAST
user doesn’t
notice here...

5. Responsive User Interfaces
Don’t design for desktopInstagram
hit-and-hope

6. • DO things in the background
• DON’T block the UI thread
• DON’T make users feel like they’re waiting
Responsive User Interfaces
Don’t design for desktop

7. DEMODEMO1) Download Html string
2) Download Jpeg image
3) Save to Storage
4) Return Html length

8. Old-style
callbacks
1) Download Html string
2) Download Jpeg image
3) Save to Photo Album
5) Error Handling
4) Return Html length
6) InvokeOnMainThread
Callback Hell

9. Old-style
callbacks
1) Download Html string
2) Download Jpeg image
3) Save to Photo Album
5) Error Handling
4) Return Html length
6) InvokeOnMainThread
Callback Hell

10. Old-style
callbacks
1) Download Html string
2) Download Jpeg image
3) Save to Photo Album
5) Error Handling
4) Return Html length
6) InvokeOnMainThread
Callback Hell

11. 1) Download Html string
2) Download Jpeg image
3) Save to Photo Album
5) Error Handling
4) Return Html length
6) InvokeOnMainThread
Callback Hell
Old-style
callbacks

12. 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

13. Old-style callbacks
• Spaghetti code:
Callbacks are the new “GOTO”
Control flow jumps around in ways that are difficult to read
& understand from the source
Error handling is difficult to implement, required in many
different places
Changes in the chain can have unintended consequences
http://tirania.org/blog/archive/2013/Aug-15.html

14. Old-style callbacks
• Execute On Main Thread
We need to schedule updates - can’t just access UI objects
directly from background threads
Dispatcher.Invoke (pre RT)

15. USING ASYNC
async, await, cancellation and error handling

16. Comparison
1) Download Html string
2) Download Jpeg image
3) Save to Photo Album
5) Error Handling
4) Return Html length
6) InvokeOnMainThread
Callback Hell

17. Comparison
1) Download Html string
2) Download Jpeg image
3) Save to Photo Album
5) Error Handling
4) Return Html length
6) InvokeOnMainThread
Callback Hell

18. 1) Download Html string
2) Download Jpeg image
3) Save to Photo Album
5) Error Handling
4) Return Html length
Comparison
Async-ified
6) InvokeOnMainThread

19. 1) Download Html string
2) Download Jpeg image
3) Save to Photo Album
5) Error Handling
4) Return Html length
Comparison
6) InvokeOnMainThread
one place
not required
Async-ified

20. Comparison
1) Download Html string
2) Download Jpeg image
3) Save to Photo Album
5) Error Handling
4) Return Html length
6) InvokeOnMainThread
old new!

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

22. 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 await
and use Tasks

23. How to use: async?
• async modifier on methods, lambdas and anonymous methods
use Async suffix, eg LoadAsync, SendAsync
return 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

24. How to use: await?
• await keyword on awaitable objects (Tasks)
only within an async context
marks a suspension point - control is returned to caller
can’t be used in catch or finally blocks
Task, Task<T> or a custom type
get a reference
to the Task first
... or just await

25. How to use: error handling?
• async Task or Task<T>
Returned Task State == Faulted
Exception re-thrown when task is awaited
• async void methods (event handlers)
Exception is thrown on the current synchronization context
and the app will crash...

26. 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”
• AggregateException
Loop through, handle or throw as required...

27. • Cancellation is optional
usually exposed an Async() method overload that takes a
CancellationToken parameter
calling code uses the token
Cancellation.None means it won’t be cancelled by the
caller (useful when the cancellation parameter isn’t optional)
How to use: cancellation?

28. • Cancellation is optional
cancelled async tasks complete (but time for this to happen
isn’t guaranteed)
How to use: cancellation?

29. • Specifying a timeout is optional
for HttpClient, set Timeout
some Async methods accept a timeout parameter
more generally, use CancellationToken (timeout)
How to use: timeouts?

30. • Progress reporting is optional
provide another Async overload with an IProgress<T>
parameter
basic Progress<T> implementation can be used
exposes EventHandler<T> ProgressChanged
How to use: progress reporting?
send events to
track progress

31. BONUS: Combinators
• Wait on multiple tasks
Task.WhenAll (IEnumerable<Task>)
- requires all the tasks to be completed
Task.WhenAny(IEnumerable<Task>)
- returns when any of the tasks completes
this kind of
loop fine for
small numbers
of Tasks

32. WARNING: Await, and UI, and deadlocks
http://blogs.msdn.com/b/pfxteam/archive/2011/01/13/10115163.aspx

33. MOAR MOBILE
iOS & Android too!

34. .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 apps
only have async APIs

35. 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

36. 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

37. Xamarin APIs
• Xamarin.Mobile
Geolocator.GetPositionAsync()
Contact.SaveThumbnailAsync()
• Xamarin.Auth
FormAuthenticator.SignInAsync()
Request.GetResponseAsync(cancellationToken)
• and many more...
Xamarin cross-platform libraries

38. Xamarin Component Store
• Many components already
offer async APIs, eg. Parse!
Powerful, easy to use components
http://components.xamarin.com

39. DEMODEMO
ParseTodo

40. 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

41. Other tricks...
• Creating your own async Tasks

42. 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

43. Questions?
Developer Evangelist
Xamarin
craig@xamarin.com
@conceptdev
Craig Dunn