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Reactive Extensions (Rx)

ReactiveX

The Reactive Extensions for .NET, or Rx, are designed for working with asynchronous and event-based sources of information. Rx provides services that help orchestrate and synchronize the way code reacts to data from these kinds of sources.

Rx's fundamental abstraction, IObservable<T>, represents a sequence of items, and its operators are defined as extension methods for this interface.

This might sound a lot like LINQ to Objects, and there are similarities, not only does IObservable<T> have a lot in common with IEnumerable<T>, but Rx also supports almost all of the standard LINQ operators.

The difference is that in Rx, sequences are less passive. Unlike IEnumerable<T>, Rx sources do not wait to be asked for their items, nor can the consumer of an Rx source demand to be given the next item. Instead, Rx uses a push model in which the source notifies its recipients when items are available.

Because Rx implements standard LINQ operators, it's possible to write queries against a live source. Rx goes beyond standard LINQ, adding its own operators that take into account the temporal nature of a live event source.

Fundamental Interfaces

The two most important types in Rx are the IObservable<T> and IObserver<T> interfaces.
They are important enough to be in the System namespace. The other parts of Rx are in the System.Reactive NuGet package.

C#
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public interface IObservable<out T>
{
    IDisposable Subscribe(IObserver<T> observer);
}

public interface IObserver<in T>
{
    void OnCompleted();
    void OnError(Exception error);
    void OnNext(T value);
}

The fundamental abstraction in Rx, IObservable<T>, is implemented by event sources. Instead of using the event keyword, it models events as a sequence of items.
An IObservable<T> provides items to subscribers as and when it’s ready to do so.

It's possible to subscribe to a source by passing an implementation of IObserver<T> to the Subscribe method. The source will invoke OnNext when it wants to report events, and it can call OnCompleted to indicate that there will be no further activity. If the source wants to report an error, it can call OnError. Both OnCompleted and OnError indicate the end of the stream, an observable should not call any further methods on the observer after that.

Operators

Chaining Operators

Most operators operate on an Observable and return an Observable. This allows to apply these operators one after the other, in a chain. Each operator in the chain modifies the Observable that results from the operation of the previous operator.
A chain of Observable operators do not operate independently on the original Observable that originates the chain, but they operate in turn, each one operating on the Observable generated by the operator immediately previous in the chain.

Creating Observables

Operators that originate new Observables.

  • Create: create an Observable from scratch by calling observer methods programmatically
  • Defer: do not create the Observable until the observer subscribes, and create a fresh Observable for each observer
  • Empty/Never/Throw: create Observables that have very precise and limited behavior
  • From*: convert some other object or data structure into an Observable
  • Interval: create an Observable that emits a sequence of integers spaced by a particular time interval
  • Return (aka Just): convert an object or a set of objects into an Observable that emits that or those objects
  • Range: create an Observable that emits a range of sequential integers
  • Repeat: create an Observable that emits a particular item or sequence of items repeatedly
  • Start: create an Observable that emits the return value of a function
  • Timer: create an Observable that emits a single item after a given delay

Transforming Observables

Operators that transform items that are emitted by an Observable.

  • Buffer: periodically gather items from an Observable into bundles and emit these bundles rather than emitting the items one at a time
  • SelectMany (aka FlatMap): transform the items emitted by an Observable into Observables, then flatten the emissions from those into a single Observable
  • GroupBy: divide an Observable into a set of Observables that each emit a different group of items from the original Observable, organized by key
  • Select (aka Map): transform the items emitted by an Observable by applying a function to each item
  • Scan: apply a function to each item emitted by an Observable, sequentially, and emit each successive value
  • Window: periodically subdivide items from an Observable into Observable windows and emit these windows rather than emitting the items one at a time

Filtering Observables

Operators that selectively emit items from a source Observable.

  • Throttle (aka Debounce): only emit an item from an Observable if a particular timespan has passed without it emitting another item
  • Distinct: suppress duplicate items emitted by an Observable
  • ElementAt: emit only item n emitted by an Observable
  • Where (aka Filter): emit only those items from an Observable that pass a predicate test
  • First: emit only the first item, or the first item that meets a condition, from an Observable
  • IgnoreElements: do not emit any items from an Observable but mirror its termination notification
  • Last: emit only the last item emitted by an Observable
  • Sample: emit the most recent item emitted by an Observable within periodic time intervals
  • Skip: suppress the first n items emitted by an Observable
  • SkipLast: suppress the last n items emitted by an Observable
  • Take: emit only the first n items emitted by an Observable
  • TakeLast: emit only the last n items emitted by an Observable

Combining Observables

Operators that work with multiple source Observables to create a single Observable

  • And/Then/When: combine sets of items emitted by two or more Observables by means of Pattern and Plan intermediaries
  • CombineLatest: when an item is emitted by either of two Observables, combine the latest item emitted by each Observable via a specified function and emit items based on the results of this function
  • Join: combine items emitted by two Observables whenever an item from one Observable is emitted during a time window defined according to an item emitted by the other Observable
  • Merge: combine multiple Observables into one by merging their emissions
  • StartWith: emit a specified sequence of items before beginning to emit the items from the source Observable
  • Switch: convert an Observable that emits Observables into a single Observable that emits the items emitted by the most-recently-emitted of those Observables
  • Zip: combine the emissions of multiple Observables together via a specified function and emit single items for each combination based on the results of this function

Error Handling Operators

Operators that help to recover from error notifications from an Observable

  • Catch: recover from an onError notification by continuing the sequence without error
  • Retry: if a source Observable sends an onError notification, resubscribe to it in the hopes that it will complete without error

Observable Utility Operators

A toolbox of useful Operators for working with Observables

  • Delay: shift the emissions from an Observable forward in time by a particular amount
  • Do: register an action to take upon a variety of Observable lifecycle events
  • Materialize/Dematerialize: represent both the items emitted and the notifications sent as emitted items, or reverse this process
  • ObserveOn: specify the scheduler on which an observer will observe this Observable
  • Synchronize (aka Serialize): force an Observable to make serialized calls and to be well-behaved
  • Subscribe: operate upon the emissions and notifications from an Observable
  • SubscribeOn: specify the scheduler an Observable should use when it is subscribed to
  • TimeInterval: convert an Observable that emits items into one that emits indications of the amount of time elapsed between those emissions
  • Timeout: mirror the source Observable, but issue an error notification if a particular period of time elapses without any emitted items
  • Timestamp: attach a timestamp to each item emitted by an Observable
  • Using: create a disposable resource that has the same lifespan as the Observable

Conditional and Boolean Operators

Operators that evaluate one or more Observables or items emitted by Observables

  • All: determine whether all items emitted by an Observable meet some criteria
  • Amb: given two or more source Observables, emit all of the items from only the first of these Observables to emit an item
  • Contains: determine whether an Observable emits a particular item or not
  • DefaultIfEmpty: emit items from the source Observable, or a default item if the source Observable emits nothing
  • SequenceEqual: determine whether two Observables emit the same sequence of items
  • SkipUntil: discard items emitted by an Observable until a second Observable emits an item
  • SkipWhile: discard items emitted by an Observable until a specified condition becomes false
  • TakeUntil: discard items emitted by an Observable after a second Observable emits an item or terminates
  • TakeWhile: discard items emitted by an Observable after a specified condition becomes false

Mathematical and Aggregate Operators

Operators that operate on the entire sequence of items emitted by an Observable

  • Average: calculates the average of numbers emitted by an Observable and emits this average
  • Concat: emit the emissions from two or more Observables without interleaving them
  • Count: count the number of items emitted by the source Observable and emit only this value
  • Max: determine, and emit, the maximum-valued item emitted by an Observable
  • Min: determine, and emit, the minimum-valued item emitted by an Observable
  • Aggregate (aka Reduce): apply a function to each item emitted by an Observable, sequentially, and emit the final value
  • Sum: calculate the sum of numbers emitted by an Observable and emit this sum

Connectable Observable Operators

Specialty Observables that have more precisely-controlled subscription dynamics

  • Connect: instruct a connectable Observable to begin emitting items to its subscribers
  • Publish: convert an ordinary Observable into a connectable Observable
  • RefCount: make a Connectable Observable behave like an ordinary Observable
  • Replay: ensure that all observers see the same sequence of emitted items, even if they subscribe after the Observable has begun emitting items

Operators to Convert Observables

  • To*: convert an Observable into another object or data structure