What is Rx

Reactive Extensions (Rx) is a library for composing asynchronous and event-based programs using observable collections and LINQ-style query operators.

Data sequences can take many forms, such as a stream of data from a file or web service, web services requests, system notifications, or a series of events such as user input.

Reactive Extensions represents all these data sequences as observable sequences. The application can subscribe to these observable sequences to receive asynchronous messages as new data arrive. The Rx library is available for desktop application development in NET, it also releases for Silverlight, Windows Phone 7 and JavaScript. For more information on these, see Differences Between Versions of Rx topic.

Pushing vs. Pulling Data

In interactive programming, the application actively polls a data source for more information by retrieving data from a sequence that represents the source. Such behavior is represented by the iterator pattern of IEnumerable<T>/IEnumerator<T>. The IEnumerable<T> interface exposes a single method GetEnumerator() which returns an IEnumerator<T> to iterate through this collection.  The IEnumerator<T> allows us to get the current item (by returning the Current property), and determine whether there are more items to iterate (by calling the MoveNext method). 

The application is active in the data retrieval process: besides getting an enumerator by calling GetEnumerator, it also decides about the pace of the retrieval by calling MoveNext at its own convenience. This enumeration pattern is synchronous, which means your application might be blocked while polling the data source. Such pulling pattern is similar to visiting your library and checking out a book. After you are done with the book, you pay another visit to check out another one.

On the other hand, in reactive programming, the application is offered more information by subscribing to a data stream (called observable sequence in Rx), and any update is handed to it from the source. The application is passive in the data retrieval process: apart from subscribing to the observable source, it does not actively poll the source, but merely react to the data being pushed to it. When the event has completed, the source will send a notice to the subscriber. In this way, your application will not be blocked by waiting for the source to update.

This is the push pattern employed by Reactive Extensions. This is similar to joining a book club in which you register your interest in a particular genre, and books that match your interest are automatically sent to you as they are published. You do not need to stand in a line to acquire something that you want. Employing a push pattern is especially helpful in heavy UI environment in which the UI thread cannot be blocked while the application is waiting for some events. This is also essential in programming environments such as Silverlight which has its own set of asynchronous requirements. In summary, by using Rx, you can make your application more responsive.

The push model implemented by Rx is represented by the observable pattern of IObservable<T>/IObserver<T>. The IObservable<T> interface is a dual of the familiar IEnumerable<T> interface. It abstracts a collection of data, and keeps a list of IObserver<T> implementations that are interested in the data collection. The IObservable will notify all the observers automatically of any state changes. To register an interest through a subscription, you use the Subscribe method of IObservable, which takes on an IObserver and returns an IDisposable. This gives you the ability to track your subscription and be able to dispose the subscription. You can essentially treat the observable sequence (such as a sequence of mouseover events) as if it were a normal collection. Rx’s LINQ implementation over observable sequences allows developers to compose complex event processing queries over push-based sequences such as .NET events, APM-based (“IAsyncResult”) computations, Task<T>-based computations,  Windows 7 Sensor and Location APIs, SQL StreamInsight temporal event streams, F# first-class events, and asynchronous workflows. For more information on these two interfaces, see Exploring The Major Interfaces in Rx

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