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ReactiveX - Operators

Introduction

Each language-specific implementation of ReactiveX implements a set of operators. Although there is much overlap between implementations, there are also some operators that are only implemented in certain implementations. Also, each implementation tends to name its operators to resemble those of similar methods that are already familiar from other contexts in that language.

Chaining Operators

Most operators operate on an Observable and return an Observable. This allows you 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.

There are other patterns, like the Builder Pattern, in which a variety of methods of a particular class operate on an item of that same class by modifying that object through the operation of the method. These patterns also allow you to chain the methods in a similar way. But while in the Builder Pattern, the order in which the methods appear in the chain does not usually matter, with the Observable operators order matters.

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.

The Operators of ReactiveX

This page first lists what could be considered the “core” operators in ReactiveX, and links to pages that have more in-depth information on how these operators work and how particular language-specific ReactiveX versions have implemented these operators.

Next is a “decision tree” that may help you choose the operator that is most appropriate to your use case.

Finally, there is an alphabetical list of most of the operators available in the many language-specific implementations of ReactiveX. These link to the page that documents the core operator that most closely resembles the language-specific operator (so, for instance, the Rx.NET “SelectMany” operator links to the documentation of the FlatMap ReactiveX operator, of which “SelectMany” is the Rx.NET implementation).

If you want to implement your own operator, see Implementing Your Own Operators.

Contents

Operators By Category 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
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
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
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.

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
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
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
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

Backpressure Operators

backpressure operators — strategies for coping with Observables that produce items more rapidly than their observers consume them

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

A Decision Tree of Observable Operators

This tree can help you find the ReactiveX Observable operator you’re looking for.

I want to create a new Observable

that emits a particular item

Just

that was returned from a function called at subscribe-time: Start
that was returned from an Action, Callable, Runnable, or something of that sort, called at subscribe-time: From
after a specified delay: Timer

that pulls its emissions from a particular Array, Iterable, or something like that

From

by retrieving it from a Future

Start

that obtains its sequence from a Future

From

that emits a sequence of items repeatedly

Repeat

from scratch, with custom logic

Create

for each observer that subscribes

Defer

that emits a sequence of integers

Range

at particular intervals of time

Interval

after a specified delay: Timer

that completes without emitting items

Empty

that does nothing at all

Never

I want to create an Observable by combining other Observables

and emitting all of the items from all of the Observables in whatever order they are received

Merge

and emitting all of the items from all of the Observables, one Observable at a time

Concat

by combining the items from two or more Observables sequentially to come up with new items to emit

whenever each of the Observables has emitted a new item: Zip
whenever any of the Observables has emitted a new item: CombineLatest
whenever an item is emitted by one Observable in a window defined by an item emitted by another: Join
by means of Pattern and Plan intermediaries: And/Then/When

and emitting the items from only the most-recently emitted of those Observables

Switch

I want to emit the items from an Observable after transforming them

one at a time with a function

Map

by emitting all of the items emitted by corresponding Observables

FlatMap

one Observable at a time, in the order they are emitted: ConcatMap

based on all of the items that preceded them

Scan

by attaching a timestamp to them

Timestamp

into an indicator of the amount of time that lapsed before the emission of the item

TimeInterval

I want to shift the items emitted by an Observable forward in time before reemitting them

Delay

I want to transform items and notifications from an Observable into items and reemit them

by wrapping them in Notification objects

Materialize

which I can then unwrap again with: Dematerialize

I want to ignore all items emitted by an Observable and only pass along its completed/error notification

IgnoreElements

I want to mirror an Observable but prefix items to its sequence

StartWith

only if its sequence is empty: DefaultIfEmpty

I want to collect items from an Observable and reemit them as buffers of items

Buffer

containing only the last items emitted: TakeLastBuffer

I want to split one Observable into multiple Observables

Window

so that similar items end up on the same Observable: GroupBy

I want to retrieve a particular item emitted by an Observable:

the last item emitted before it completed: Last
the sole item it emitted: Single
the first item it emitted: First

I want to reemit only certain items from an Observable

by filtering out those that do not match some predicate

Filter

that is, only the first item

First

that is, only the first items

Take

that is, only the last item

Last

that is, only item n

ElementAt

that is, only those items after the first items

that is, after the first n items: Skip
that is, until one of those items matches a predicate: SkipWhile
that is, after an initial period of time: Skip
that is, after a second Observable emits an item: SkipUntil

that is, those items except the last items

that is, except the last n items: SkipLast
that is, until one of those items matches a predicate: TakeWhile
that is, except items emitted during a period of time before the source completes: SkipLast
that is, except items emitted after a second Observable emits an item: TakeUntil

by sampling the Observable periodically

Sample

by only emitting items that are not followed by other items within some duration

Debounce

by suppressing items that are duplicates of already-emitted items

Distinct

if they immediately follow the item they are duplicates of: DistinctUntilChanged

by delaying my subscription to it for some time after it begins emitting items

DelaySubscription

I want to reemit items from an Observable only on condition that it was the first of a collection of Observables to emit an item

Amb

I want to evaluate the entire sequence of items emitted by an Observable

and emit a single boolean indicating if all of the items pass some test: All
and emit a single boolean indicating if the Observable emitted any item (that passes some test): Contains
and emit a single boolean indicating if the Observable emitted no items: IsEmpty
and emit a single boolean indicating if the sequence is identical to one emitted by a second Observable: SequenceEqual
and emit the average of all of their values: Average
and emit the sum of all of their values: Sum
and emit a number indicating how many items were in the sequence: Count
and emit the item with the maximum value: Max
and emit the item with the minimum value: Min
by applying an aggregation function to each item in turn and emitting the result: Scan

I want to convert the entire sequence of items emitted by an Observable into some other data structure

I want an operator to operate on a particular Scheduler

SubscribeOn

when it notifies observers: ObserveOn

I want an Observable to invoke a particular action when certain events occur

I want an Observable that will notify observers of an error

Throw

if a specified period of time elapses without it emitting an item: Timeout

I want an Observable to recover gracefully

from a timeout by switching to a backup Observable

Timeout

from an upstream error notification

Catch

by attempting to resubscribe to the upstream Observable: Retry

I want to create a resource that has the same lifespan as the Observable

Using

I want to subscribe to an Observable and receive a Future that blocks until the Observable completes

Start

I want an Observable that does not start emitting items to subscribers until asked

Publish

and then only emits the last item in its sequence: PublishLast
and then emits the complete sequence, even to those who subscribe after the sequence has begun: Replay
but I want it to go away once all of its subscribers unsubscribe: RefCount
and then I want to ask it to start: Connect