Reactive Extensions

Ye olde introduction and walk-through,

with plenty o’ code

OK, what is it?

● A library to compose asynchronous and event-based programs using observable collections and LINQ-style query operators.

● Keywords: Compose, LINQ, Asynchronous,Compose, LINQ, AsynchronousCompose, LINQ, Asynchronous

Compose, LINQ, Asynchronous

Compose, LINQ, Asynchronous

Reactive what?● Wikipedia: a programming paradigm oriented

around data flows and the propagation of change

● Interactive: a = b + cMeaning: a will store the current values of b and c

(added), but future updates to b and c will not affect a

● Reactive:Meaning: a will be updated when either b or c (or

both) changes in the future

Reactive vs Interactive, part II(push vs pull)

● Interactive (pull, compare with polling)

● Reactive (push, compare with notifications)

● Key concept: create a query now, receive updates in the future.

The library

● Created by Microsoft● Built on Parallel extensions library (PFX,

meaning fully multi-threaded from day one)● Open source as of November 2012● Based on functional programming● Runs on Windows Forms, WPF, Silverlight,

WP7, WP8, JavaScript, ASP.NET● Is written using extension methods

Why should I use it?● Today’s applications are complex● Managing applications with complex message flows,

threads and states tend to get real messy real soon● An asynchronous and responsive UI is a necessity for

the demanding users (and apps) of today● Using a library that handles threading and

synchronization for you leads to using fewer locks (and having fewer dead-locks or bottlenecks)

● Allows you to orchestrate message flows in an easy manner

Why should I use it?

Events are high-maintenance

Enumerating collections is a blocking operation

Observables are asynchronous by nature,

Observables introduce the notion of time

The assemblies

What you need to know(show me the code)

● The Interfaces

● The ContractOnNext* (OnError | OnCompleted)?

Publish next value

An Error occured

And we’re done

Migrating into the Observable

● Strategy: move tricky problem into observable domain, solve problem easily, move back to regular domain

● Methods exist for converting existing objects to observable objects

Creating Observables( - what is it really?)

● Think 'streams' (of events, data, etc)● Do not implement your own IObservable, use

available combinators, or the factory methods:

Code Demo – Creating ObservablesFrom raw dataFrom eventsFrom async patternsCustom observables

Error handling(you be illin’)

● Dissecting the OnError lambda● Error handling strategy

● Keep it local

Code Demo – Error Handling

Schedulers(time is of the essence)

● Calls are asynchronous, so…● Where do observables run?● Observable.Return(42) OnNext(42) OnCompleted

Code Demo – Schedulers

Unit Testing(you better check yourself, before you wreck yourself)

● TestScheduler - allows testing of asynchronous code in real-time

● Helps you learn and understand the inner workings of the Rx libraries.

● This is what’s available:● NewThreadScheduler● TaskPoolScheduler● DispatcherScheduler● ImmediateScheduler● ThreadPoolScheduler

Code Demo – Testing Schedulers

Resource Disposal(I must clean)

● In Essence: how and when do you Unsubscribe?

– OnCompleted AutoDispose● What happens if you don’t?● Best practices

Code Demo – Unsubscribe

Hot vs Cold● There are two types of streams:

● Hot – producing values all the time, regardless of subscribers– Examples: mouse events, streaming data

● Cold – producing values only when someone is subscribing– Examples: web requests

● Important to know the difference, especially when it comes to side effects

Code Demo – Sharing resources• hot and cold• subjects

Code Demo• Search Twitter• Common Operators

Why do I recommend Rx?

● Allows me to write modular, reusable code, which is composable

● Allows me to orchestrate message flows ● Relieves me of a lot of state management● Allows me to write fully asynchronous UI’s from

day one.● Provides full unit testing of timeouts, delays, etc● Is feature rich => less work for me

Further Reading● introtorx.com● http://social.msdn.microsoft.com/Forums/en-US/rx/threads● http://rxx.codeplex.com/● http://www.reactiveui.net/

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