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Tamir Dresher (@tamir_dresher)

Senior Software ArchitectJ

Testing Time and Concurrency with Rx

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• Author of Rx.NET in Action (manning publications)

• Software architect, consultant and instructor

• Software Engineering Lecturer @ Ruppin Academic Center

• OzCode (www.oz-code.com) Evangelist

@tamir_dreshertamirdr@codevalue.nethttp://www.TamirDresher.com.

About Me

Reactive Extensions (Rx)

Your headache relief pill to Asynchronous Event

based applications

AsyncPush

Triggers

Events

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Reacting to changes

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IEnumerable<Message> LoadMessages(string hashtag)

{

var statuses = facebook.Search(hashtag);

var tweets = twitter.Search(hashtag);

var updates = linkedin.Search(hashtag);

return statuses.Concat(tweets).Concat(updates);

}

Twitter App

Linkedin

Facebook

Pull ModelPull Model

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???? LoadMessages(string hashtag)

{

facebook.Search(hashtag);

twitter.Search(hashtag);

linkedin.Search(hashtag);

}

DoSomething( )msg

Push ModelPush Model

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namespace System{

public interface IObservable<out T>{

IDisposable Subscribe(IObserver<T> observer);}

public interface IObserver<in T>{

void OnNext(T value);void OnError(Exception error);void OnCompleted();

}}

InterfacesInterfaces

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observable observer

Subscribe(observer)

subscription

OnNext(X1)

OnNext(Xn)⁞

⁞

IDisposable

Observables and ObserversObservables and Observers

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OnCompleted()

OnError(Exception)

observable observer

⁞

Observables and ObserversObservables and Observers

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Rx packages

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Push ModelPush Model with Rx Observables

class ReactiveSocialNetworksManager{

//members

public IObservable<Message> ObserveMessages(string hashtag){

:}

}

var mgr = new ReactiveSocialNetworksManager();

mgr.ObserveMessages("Rx").Subscribe(

msg => Console.WriteLine($"Observed:{msg} \t"),ex => { /*OnError*/ },() => { /*OnCompleted*/ });

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Creating Observables

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Observable.Range(1, 10)

.Subscribe(x => Console.WriteLine(x));

Observable.Interval(TimeSpan.FromSeconds(1)).Subscribe(x => Console.WriteLine(x));

Observable.FromEventPattern(SearchBox, "TextChanged")

⁞

⁞1 sec 1 sec

Observables Factories

⁞

Observables Factories

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Observable Queries (Rx Operators)

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Filtering Projection Partitioning Joins Grouping Set

Element Generation Quantifiers Aggregation Error HandlingTime and Concurrency

Where

OfType

Select

SelectMany

Materialize

Skip

Take

TakeUntil

CombineLatest

Concat

join

GroupBy

GroupByUntil

Buffer

Distinct

DistinctUntilChanged

Timeout

TimeInterval

ElementAt

First

Single

Range

Repeat

Defer

All

Any

Contains

Sum

Average

Scan

Catch

OnErrorResumeNext

Using

Rx operators

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Reactive SearchReactive Search

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1. At least 3 characters2. Don’t overflow server (0.5 sec delay)3. Don’t send the same string again4. Discard results if another search was requested

Reactive Search - RulesReactive Search - Rules

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Where(s => s.Length > 2 )

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Where(s => s.Length > 2 )

Throttle(TimeSpan.FromSeconds(0.5))

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Where(s => s.Length > 2 )

Throttle(TimeSpan.FromSeconds(0.5))

DistinctUntilChanged()

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Select(text => SearchAsync(text))

“REA”

“REAC”

Switch()

“REA” results are ignored since we switched to the “REAC” results

Abstracting Time and Concurrency

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Thread Pool

Task Scheduler

other

SchedulersSchedulers

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public interface IScheduler{

DateTimeOffset Now { get; }

IDisposable Schedule<TState>( TState state, Func<IScheduler, TState, IDisposable> action);

IDisposable Schedule<TState>(TimeSpan dueTime, TState state,Func<IScheduler, TState, IDisposable> action);

IDisposable Schedule<TState>(DateTimeOffset dueTime, TState state,Func<IScheduler, TState, IDisposable> action);

}

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//// Runs a timer on the default scheduler//IObservable TimeSpan

//// Every operator that introduces concurrency// has an overload with an IScheduler//IObservable T TimeSpan

IScheduler scheduler);

Parameterizing ConcurrencyParameterizing Concurrency

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textChanged.Throttle(TimeSpan.FromSeconds(0.5),

DefaultScheduler.Instance).DistinctUntilChanged().SelectMany(text => SearchAsync(text)).Switch().Subscribe(/*handle the results*/);

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//// runs the observer callbacks on the specified // scheduler.//IObservable T ObserveOn<T>(IScheduler);

//// runs the observer subscription and unsubsciption on// the specified scheduler.//IObservable T SubscribeOn<T>(IScheduler)

Changing Execution ContextChanging Execution Context

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textChanged.Throttle(TimeSpan.FromSeconds(0.5)).DistinctUntilChanged().Select(text => SearchAsync(text)).Switch().ObserveOn(DispatcherScheduler.Current).Subscribe(/*handle the results*/);

Changing Execution ContextChanging Execution Context

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textChanged.Throttle(TimeSpan.FromSeconds(0.5)).DistinctUntilChanged().Select(text => SearchAsync(text)).Switch().ObserveOnDispatcher().Subscribe(/*handle the results*/);

Changing Execution ContextChanging Execution Context

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Virtual Time

What is time?

Time can be a anything that is sequential and comparable

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“Time is the indefinite continued progress of existence and events ... Time

is a component quantity of various measurements used

to sequence events, to compare the duration of events or the intervals

between them…”

https://en.wikipedia.org/wiki/Time

Virtual Time Scheduler

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public abstract class VirtualTimeSchedulerBase<TAbsolute, TRelative> : IScheduler,

IServiceProvider, IStopwatchProvider

where TAbsolute : IComparable<TAbsolute>

{

public TAbsolute Clock { get; protected set;}

public void Start()

public void Stop()

public void AdvanceTo(TAbsolute time)

public void AdvanceBy(TRelative time)

...

}

public class TestScheduler : VirtualTimeScheduler<long, long>{

...}

Testing Rx

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1. At least 3 characters One letter word, No Search performed

2. Don’t overflow server (0.5 sec delay) 2 words typed, 0.2 sec gap, search only the last

3. Don’t send the same string again 2 words, 1 sec gap, same value, search only first

4. Discard results if another search was requested 2 words, 1 sec gap, slow first search, fast last search, last results shown

Reactive Search - RulesReactive Search – Rules Tests

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Questions and Answers

Q: How can we test the code without a real user interaction?

Q: How can we test the code without a real server?

Q: How can we test the code deterministically without a real asynchronicity and concurrency?

Q: How can we test the code without REALLY waiting for the time to pass?

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A: Separation of concerns. Separate the logic from the view

A: Enable Dependency Injection and mock the service client

A: Leverage the Rx Schedulers and provide a Scheduler you can control via DI

A: Leverage the Rx TestScheduler which provides a virtualization of time

Separating the logic from the view

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SearchView(Presentation, Logic, State)

SearchView(Presentation)

SearchViewModel(Logic, State)

Before

After

Separating the logic from the view

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<Window x:Class="TestableReactiveSearch.SearchView"><DockPanel>

<TextBox x:Name="SearchBox"Text="{Binding SearchTerm …}"DockPanel.Dock="Top“/>

<ListBox x:Name="SearchResults"ItemsSource="{Binding SearchResults}“/>

</DockPanel></Window>

public class SearchViewModel : INotifyPropertyChanged

{

public event PropertyChangedEventHandler PropertyChanged;

public SearchViewModel()

{

// Rx query

}

public string SearchTerm { get { ... } set { ... } }

public IEnumerable<string> SearchResults { get { ... } set { ... } }}

SearchView.xaml

SearchViewModel.cs

Separating the logic from the view – fixing the Rx query

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public SearchViewModel()

{

var terms =

Observable.FromEventPattern<PropertyChangedEventArgs>(this, nameof(PropertyChanged))

.Where(e => e.EventArgs.PropertyName == nameof(SearchTerm))

.Select(_ => SearchTerm);

_subscription =

terms

.Where(txt => txt.Length >= 3)

.Throttle(TimeSpan.FromSeconds(0.5))

.DistinctUntilChanged()

.Select(txt => searchServiceClient.SearchAsync(txt))

.Switch()

.ObserveOnDispatcher()

.Subscribe(

results => SearchResults = results,

err => { Debug.WriteLine(err); },

() => { /* OnCompleted */ });

}

Same query as before

Injecting the Search Service client

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public class SearchViewModel : INotifyPropertyChanged

{

public event PropertyChangedEventHandler PropertyChanged;

public SearchViewModel()

{

// rest of rx query

}

...}

Injecting the Search Service client

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public class SearchViewModel : INotifyPropertyChanged

{

public event PropertyChangedEventHandler PropertyChanged;

public SearchViewModel(ISearchServiceClient searchServiceClient)

{

// rest of rx query

}

...}

Simple test – first try

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[TestMethod]

public void Search_OneLetterWord_NoSearchSentToService()

{

var fakeServiceClient = Substitute.For<ISearchServiceClient>();

var vm = new SearchViewModel(fakeServiceClient);

vm.SearchTerm = "A";

fakeServiceClient.DidNotReceive().SearchAsync("A");

}

Injecting concurrency

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public interface IConcurrencyProvider{

IScheduler TimeBasedOperations { get; }IScheduler Task { get; }IScheduler Thread { get; }IScheduler Dispatcher { get; }

}class ConcurrencyProvider : IConcurrencyProvider{

public ConcurrencyProvider(){

TimeBasedOperations = DefaultScheduler.Instance;Task = TaskPoolScheduler.Default;Thread = NewThreadScheduler.Default;Dispatcher=DispatcherScheduler.Current;

}

public IScheduler TimeBasedOperations { get; }public IScheduler Task { get; }public IScheduler Thread { get; }public IScheduler Dispatcher { get; }

}

Injecting concurrency

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public SearchViewModel(ISearchServiceClient searchServiceClient,

IConcurrencyProvider concurrencyProvider)

{

var terms =

Observable.FromEventPattern<PropertyChangedEventArgs>(this, nameof(PropertyChanged))

.Where(e => e.EventArgs.PropertyName == nameof(SearchTerm)).Select(_=>SearchTerm);

_subscription =

terms

.Where(txt => txt.Length >= 3)

.Throttle(TimeSpan.FromSeconds(0.5), concurrencyProvider.Thread)

.DistinctUntilChanged()

.Select(txt => searchServiceClient.SearchAsync(txt))

.Switch()

.ObserveOn(concurrencyProvider.Dispatcher)

.Subscribe(

results => SearchResults = results,

err => { Debug.WriteLine(err); },

() => { /* OnCompleted */ });

}

Simplest Rx Test

Install-Package Microsoft.Reactive.Testing

To simplify the Rx testing, derive your test class from ReactiveTest

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using Microsoft.Reactive.Testing;

[TestClass]

public class SearchViewModelTests : ReactiveTest

{

// Test Methods

}

Test 1: Search is sent after half a sec

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const long ONE_SECOND = TimeSpan.TicksPerSecond;

[TestMethod]

public void MoreThanThreeLetters_HalfSecondGap_SearchSentToService()

{

var fakeServiceClient = Substitute.For<ISearchServiceClient>();

var fakeConcurrencyProvider = Substitute.For<IConcurrencyProvider>();

var testScheduler = new TestScheduler();

fakeConcurrencyProvider.ReturnsForAll<IScheduler>(testScheduler);

var vm = new SearchViewModel(fakeServiceClient, fakeConcurrencyProvider);

testScheduler.Start();

vm.SearchTerm = "reactive";

testScheduler.AdvanceBy(ONE_SECOND / 2);

fakeServiceClient.Received().SearchAsync("reactive");

}

TestScheduler

TestScheduler provides two methods for creating observables:CreateColdObservable – Creates an observable that emits its value relatively to when each observer subscribes.

CreateHotObservable – Creates and observable that emits its values regardless to the observer subscription time, and each emission is configured to the absolute scheduler clock

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var testScheduler = new TestScheduler();

ITestableObservable<int> coldObservable = testScheduler.CreateColdObservable<int>(OnNext<int>(20, 1),OnNext<int>(40, 2),OnCompleted<int>(60)

);

Test 2: first search is discarded if another search happens

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public void TwoValidWords_SlowSearchThenFastSearch_SecondSearchResultsOnly()

{

var fakeServiceClient = Substitute.For<ISearchServiceClient>();

var fakeConcurrencyProvider = Substitute.For<IConcurrencyProvider>();

var testScheduler = new TestScheduler();

fakeConcurrencyProvider.ReturnsForAll<IScheduler>(testScheduler);

fakeServiceClient.SearchAsync("first").Returns(testScheduler.CreateColdObservable(

OnNext<IEnumerable<string>>(2 * ONE_SECOND, new[] {"first"}), ...);

fakeServiceClient.SearchAsync("second").Returns(testScheduler.CreateColdObservable(

OnNext<IEnumerable<string>>(1, new[] { "second" }), ...);

var vm = new SearchViewModel(fakeServiceClient, fakeConcurrencyProvider);

testScheduler.Start();

vm.SearchTerm = "first";

testScheduler.AdvanceBy(ONE_SECOND);

vm.SearchTerm = "second";

testScheduler.AdvanceBy(5 * ONE_SECOND);

Assert.AreEqual("second", vm.SearchResults.First());

}

Summary

Pull vs. Push model

Rx operators

Building Rx queries

Rx Concurrency Model

Virtual Time

Testing Time and Concurrency with TestScheduler

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Your headache relief pill to Asynchronous and

Event based applications

AsyncPush

Triggers

Events

Reactive ExtensionsReactive Extensions

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www.reactivex.io github.com/Reactive-Extensionswww.manning.com/dresher

Thank You

Tamir Dresher (@tamir_dresher)

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