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Using Async in your Mobile Apps - Marek Safar
- 1. Marek Safar Engineer Xamarin msafar@xamarin.com Using Async in Your Apps
- 2. You?
- 3. Wikipedia “Asynchronous events are those occurring independently of the main program flow. Asynchronous actions are actions executed in a non-blocking scheme, allowing the main program flow to continue processing”
- 4. You Should Use Async Today • Your UI always remains responsive • No need to guard against race conditions • No ugly callbacks • Compiler does all the heavy li!ing
- 6. Async in C# • New major feature of C# 5.0 Makes asynchronous programming a first-class citizen in C# Important part of C# – as significant as LINQ • New async modifier keyword introduced • Methods, lambda expressions, and anonymous methods can be asynchronous • New contextual keyword await introduced
- 7. Adding Async to Your Codebase • Decorated with new async modifier • Async method must return one of Task, Task<T> or void type • Async anonymous method and lambdas Delegates returning Task, Task<T> or void • Very few restrictions No ref or out parameters allowed No unsafe code No Main async method No async iterators (aka IAsyncEnumerable<T>)
- 8. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 Example: Calculating a Price Asynchronously public async Task<int> CalculatePriceAsync (Item[] items) { int price; // Calculate price asynchronously and return the value return price; } public override void ViewDidLoad () { button.TouchDown += async (sender, e) => { // This is asynchronous handler }; }
- 9. Await Introduction • Await can be used in async context only Marks a suspension point Can be used anywhere (except catch and finally blocks) Requires awaitable types (Task, Task<T> but can be any custom type) As many as you like inside async block
- 10. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 Calculating a Price Asynchronously public async Task<int> CalculatePriceAsync (Item[] items) { using (var cmd = CreateSqlCommand (Calculations.TotalPrice, items)) { using (var reader = await cmd.ExecuteReaderAsync ()) { int price = ReadPrice (reader); return price; } } } public override void ViewDidLoad () { button.TouchDown += async (sender, e) => { var price = await CalculatePriceAsync (items); priceLabel.Text = price.ToString (); }; }
- 11. Cancelling an Async Task • CancellationToken controls cancellation process • Async method needs explicitly support it Usually another method overload Task SaveAsync (CancellationToken token) • Caller calls Cancel on CancellationTokenSource Async method stops and returns Task with cancelled state Implementation detail how quickly async method terminates
- 12. DEMO
- 13. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 Async Language Features public async Task RealAsync () { // This is real async heavy code (only 7 awaits someone else will // certainly do better) var test = new TestClass (await CallAsync () | await CallAsync ()) { Latitude = await GetLatitudeAsync (await AnotherAsync ()), Roles = from role in await GetRolesAsync () where role == "agent" select role }; .... test [await CallAsync ()] = new int[] { 33, await GetIntAsync () }; ... }
- 14. Best Practices • Use Async naming suffix for asynchronous methods LoadAsync, SendAsync, etc. Only recommended naming convention • Return Task or Task<T> preferably Task for SaveAsync like methods Task<T> for ReadAsync like methods Leave async void to event handlers only • Support cancellation, if possible
- 15. Synchronization Context • Framework abstraction over UI toolkit threading model NSRunLoop, DispatchContext in Xamarin.iOS MainLooper in Xamarin.Android Dispatcher in WPF etc. • Await uses synchronization context to restore back suspended context SynchronizationContext::Post method is used Suspension on UI thread resumes back on same UI thread as “expected”
- 16. Sometimes Things Go Wrong • Async returning Task or Task<T> Task state is set to Faulted Exception is re-thrown when Task is checked (awaited) • Async void Fire and forget asynchrony Exception is thrown on current synchronization context and your app will crash
- 17. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 Exception Handling with Await public override void ViewDidLoad () { button.TouchDown += async (sender, e) => { try { button.Enabled = false; var price = await CalculatePriceAsync (items); priceLabel.Text = price.ToString (); } catch (Exception ex) { // Handles price calculation error priceLabel.Text = "Calculation failed"; Debug.Print (ex.ToString ()); // Simple exception logging } finally { button.Enabled = true; } }; }
- 18. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 Exception Throwing public async Task<int> CalculatePriceAsync (Item[] items) { if (items == null) { // Throw before suspension throw new ArgumentNullException ("items"); } ... var price = await CalculatePriceAsync (items); if (price < 0) { // Throw after suspension throw new ArgumentException ("Invalid items"); } ... }
- 19. Diving Deeper • Await can work with any type which implements the awaiter pattern • Task and Task<T> types do since .NET 4.5 • Any custom type can be made awaitable Has an accessible method called GetAwaiter returning a type which Implements the interface INotifyCompletion Has property IsCompleted of type bool Has method GetResult with no parameters
- 20. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 Custom await Behaviour public async Task<int> SomeAsync () { ... await 3000; // This does not compile unless we provide custom awaiter ... } // Name of the method is important static TaskAwaiter GetAwaiter (this int millisecondsDelay) { return Task.Delay (millisecondsDelay).GetAwaiter (); }
- 21. Async Deadlocks • await restores execution context based on current SynchronizationContext Good for most application code Bad for most library code • Don’t block the UI thread Good old rule still applies in async world Use await when possible
- 22. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 Async Deadlock Example public async Task<int> CalculatePriceAsync (Item[] items) { using (var cmd = CreateSqlCommand (Calculations.TotalPrice, items)) { using (var r = await cmd.ExecuteReaderAsync ()) { ..... } } } // The method itself cannot be async public override bool FinishedLaunching (UIApplication app, NSDictionary options) { .... int price = CalculatePriceAsync (items).Result; // Synchronous wait // This line won’t be reached if flow suspension occurred }
- 23. Controlling the Synchronization Context • ConfigureAwait (bool) Controls captured context behaviour • true value - default behaviour Continue execution on context async was called from Important for UI to switch back to UI context • false value Avoids expensive context switching Avoids possible deadlocks on blocking UI
- 24. 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 Async Deadlock Fixed public async Task<int> CalculatePriceAsync (Item[] items) { using (var cmd = CreateSqlCommand (Calculations.TotalPrice, items)) { using (var r = await cmd.ExecuteReaderAsync ().ConfigureAwait (false)) { // The context is not restored but that’s fine for no UI method } } } // The method itself cannot be async public override bool FinishedLaunching (UIApplication app, NSDictionary options) { .... int price = CalculatePriceAsync (items).Result; // Synchronous wait // Program continues when the wait is signalled }
- 25. Combinators • Asynchronously wait on multiple asynchronous operations Better than individual awaits of multiple tasks The result can be awaited (Task of Tasks) • Task.WhenAll (IEnumerable<Task>) Completes when all of the individual tasks have completed • Task.WhenAny (IEnumerable<Task>) Completes when any of the individual tasks have completed
- 26. The Compiler Magic • Compiler does all the “magic” • Local variable and parameters are li!ed into compiler generated type Compiler converts variables and parameters to fields • Any stack values need to be li!ed too Stack needs to be restored a"er suspension • Try-Catch block over any async block • No suspension when awaited task finished
- 27. Best Practices - Performance • Use ConfigureAwait when you can • Reduce number of local variables Move async block into separate method or lambda expression Pass variables as parameters Avoid deep await usage • Cache Task not task result • Don’t over use async Use async only when appropriate (unreliable tasks, running >50ms) Async has its own overhead
- 28. Get Your Hands on Async • Async is available today Xamarin.iOS Beta release Xamarin.Android Beta release Xamarin Studio async support • Give us feedback on our new async API Forums Bugzilla Mailing list IRC
- 29. Q&A
- 30. THANK YOU