Using Async in your Mobile Apps - Marek Safar

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Marek SafarEngineerXamarinmsafar@xamarin.com

Using Async in Your Apps

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”

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

Async in C#

• New major feature of C# 5.0Makes 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

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 lambdasDelegates returning Task, Task<T> or void

• Very few restrictionsNo ref or out parameters allowedNo unsafe codeNo Main async methodNo async iterators (aka IAsyncEnumerable<T>)

Example: Calculating a Price Asynchronously public async Task<int> CalculatePriceAsync (Item[] items){int price;// Calculate price asynchronously and return the valuereturn price;

public override void ViewDidLoad (){button.TouchDown += async (sender, e) => {

// This is asynchronous handler};

Await Introduction

• Await can be used in async context onlyMarks a suspension pointCan 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

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 ();

Cancelling an Async Task

• CancellationToken controls cancellation process• Async method needs explicitly support it

Usually another method overloadTask SaveAsync (CancellationToken token)

• Caller calls Cancel on CancellationTokenSourceAsync method stops and returns Task with cancelled stateImplementation detail how quickly async method terminates

Async Language Featurespublic 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 () };...

Best Practices

• Use Async naming suffix for asynchronous methodsLoadAsync, SendAsync, etc.Only recommended naming convention

• Return Task or Task<T> preferablyTask for SaveAsync like methodsTask<T> for ReadAsync like methodsLeave async void to event handlers only

• Support cancellation, if possible

Synchronization Context

• Framework abstraction over UI toolkit threading modelNSRunLoop, DispatchContext in Xamarin.iOSMainLooper in Xamarin.AndroidDispatcher in WPFetc.

• Await uses synchronization context to restore back suspended context

SynchronizationContext::Post method is usedSuspension on UI thread resumes back on same UI thread as “expected”

Sometimes Things Go Wrong

• Async returning Task or Task<T>Task state is set to FaultedException is re-thrown when Task is checked (awaited)

• Async voidFire and forget asynchronyException is thrown on current synchronization context and your app will crash

Exception Handling with Awaitpublic 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 errorpriceLabel.Text = "Calculation failed";Debug.Print (ex.ToString ()); // Simple exception logging

} finally {button.Enabled = true;

Exception Throwingpublic async Task<int> CalculatePriceAsync (Item[] items){if (items == null) {

// Throw before suspensionthrow new ArgumentNullException ("items");

}...var price = await CalculatePriceAsync (items);if (price < 0) {

// Throw after suspensionthrow new ArgumentException ("Invalid items");

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 whichImplements the interface INotifyCompletionHas property IsCompleted of type boolHas method GetResult with no parameters

Custom await Behaviourpublic async Task<int> SomeAsync (){ ... await 3000; // This does not compile unless we provide custom awaiter ...}

// Name of the method is importantstatic TaskAwaiter GetAwaiter (this int millisecondsDelay){ return Task.Delay (millisecondsDelay).GetAwaiter ();}

Async Deadlocks

• await restores execution context based on current SynchronizationContext

Good for most application codeBad for most library code

• Don’t block the UI threadGood old rule still applies in async worldUse await when possible

Async Deadlock Examplepublic async Task<int> CalculatePriceAsync (Item[] items){using (var cmd = CreateSqlCommand (Calculations.TotalPrice, items)) {

using (var r = await cmd.ExecuteReaderAsync ()) {.....

// The method itself cannot be asyncpublic override bool FinishedLaunching (UIApplication app, NSDictionary options){....int price = CalculatePriceAsync (items).Result; // Synchronous wait// This line won’t be reached if flow suspension occurred

Controlling the Synchronization Context

• ConfigureAwait (bool)Controls captured context behaviour

• true value - default behaviourContinue execution on context async was called fromImportant for UI to switch back to UI context

• false valueAvoids expensive context switchingAvoids possible deadlocks on blocking UI

Async Deadlock Fixedpublic 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 asyncpublic override bool FinishedLaunching (UIApplication app, NSDictionary options){....int price = CalculatePriceAsync (items).Result; // Synchronous wait// Program continues when the wait is signalled

Combinators

• Asynchronously wait on multiple asynchronous operationsBetter than individual awaits of multiple tasksThe 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

The Compiler Magic

• Compiler does all the “magic”• Local variable and parameters are li!ed into compiler generated

typeCompiler converts variables and parameters to fields

• Any stack values need to be li!ed tooStack needs to be restored a"er suspension

• Try-Catch block over any async block• No suspension when awaited task finished

Best Practices - Performance

• Use ConfigureAwait when you can• Reduce number of local variables

Move async block into separate method or lambda expressionPass variables as parametersAvoid 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

Get Your Hands on Async

• Async is available todayXamarin.iOS Beta releaseXamarin.Android Beta releaseXamarin Studio async support

• Give us feedback on our new async APIForumsBugzillaMailing listIRC

THANK YOU

Using Async in your Mobile Apps - Marek Safar

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