Framework Overview

This document contains a high-level description of the different components within the ReactiveCocoa framework, and an attempt to explain how they work together and divide responsibilities. This is meant to be a starting point for learning about new modules and finding more specific documentation.

For examples and help understanding how to use RAC, see the README or the Design Guidelines.

Streams

A stream, represented by the RACStream abstract class, is any series of object values.

Values may be available immediately or in the future, but must be retrieved sequentially. There is no way to retrieve the second value of a stream without evaluating or waiting for the first value.

Streams are monads. Among other things, this allows complex operations to be built on a few basic primitives (-bind: in particular). RACStream also implements the equivalent of the Monoid and MonadZip typeclasses from Haskell.

RACStream isn't terribly useful on its own. Most streams are treated as signals or sequences instead.

Signals

A signal, represented by the RACSignal class, is a push-driven stream.

Signals generally represent data that will be delivered in the future. As work is performed or data is received, values are sent on the signal, which pushes them out to any subscribers. Users must subscribe to a signal in order to access its values.

Signals send three different types of events to their subscribers:

The next event provides a new value from the stream. RACStream methods only operate on events of this type. Unlike Cocoa collections, it is completely valid for a signal to include nil.
The error event indicates that an error occurred before the signal could finish. The event may include an NSError object that indicates what went wrong. Errors must be handled specially – they are not included in the stream's values.
The completed event indicates that the signal finished successfully, and that no more values will be added to the stream. Completion must be handled specially – it is not included in the stream of values.

The lifetime of a signal consists of any number of next events, followed by one error or completed event (but not both).

Subscription

A subscriber is anything that is waiting or capable of waiting for events from a signal. Within RAC, a subscriber is represented as any object that conforms to the RACSubscriber protocol.

A subscription is created through any call to -subscribeNext:error:completed:, or one of the corresponding convenience methods. Technically, most RACStream and RACSignal operators create subscriptions as well, but these intermediate subscriptions are usually an implementation detail.

Subscriptions retain their signals, and are automatically disposed of when the signal completes or errors. Subscriptions can also be disposed of manually.

Subjects

A subject, represented by the RACSubject class, is a signal that can be manually controlled.

Subjects can be thought of as the "mutable" variant of a signal, much like NSMutableArray is for NSArray. They are extremely useful for bridging non-RAC code into the world of signals.

For example, instead of handling application logic in block callbacks, the blocks can simply send events to a shared subject instead. The subject can then be returned as a RACSignal, hiding the implementation detail of the callbacks.

Some subjects offer additional behaviors as well. In particular, RACReplaySubject can be used to buffer events for future subscribers, like when a network request finishes before anything is ready to handle the result.

Commands

A command, represented by the RACCommand class, creates and subscribes to a signal in response to some action. This makes it easy to perform side-effecting work as the user interacts with the app.

Usually the action triggering a command is UI-driven, like when a button is clicked. Commands can also be automatically disabled based on a signal, and this disabled state can be represented in a UI by disabling any controls associated with the command.

On OS X, RAC adds a rac_command property to NSButton for setting up these behaviors automatically.

Connections

A connection, represented by the RACMulticastConnection class, is a subscription that is shared between any number of subscribers.

Signals are cold by default, meaning that they start doing work each time a new subscription is added. This behavior is usually desirable, because it means that data will be freshly recalculated for each subscriber, but it can be problematic if the signal has side effects or the work is expensive (for example, sending a network request).

A connection is created through the -publish or -multicast: methods on RACSignal, and ensures that only one underlying subscription is created, no matter how many times the connection is subscribed to. Once connected, the connection's signal is said to be hot, and the underlying subscription will remain active until all subscriptions to the connection are disposed.

Sequences

A sequence, represented by the RACSequence class, is a pull-driven stream.

Sequences are a kind of collection, similar in purpose to NSArray. Unlike an array, the values in a sequence are evaluated lazily (i.e., only when they are needed) by default, potentially improving performance if only part of a sequence is used. Just like Cocoa collections, sequences cannot contain nil.

Sequences are similar to Clojure's sequences (lazy-seq in particular), or the List type in Haskell.

RAC adds a -rac_sequence method to most of Cocoa's collection classes, allowing them to be used as RACSequences instead.

Disposables

The RACDisposable class is used for cancellation and resource cleanup.

Disposables are most commonly used to unsubscribe from a signal. When a subscription is disposed, the corresponding subscriber will not receive any further events from the signal. Additionally, any work associated with the subscription (background processing, network requests, etc.) will be cancelled, since the results are no longer needed.

For more information about cancellation, see the RAC Design Guidelines.

Schedulers

A scheduler, represented by the RACScheduler class, is a serial execution queue for signals to perform work or deliver their results upon.

Schedulers are similar to Grand Central Dispatch queues, but schedulers support cancellation (via disposables), and always execute serially. With the exception of the +immediateScheduler, schedulers do not offer synchronous execution. This helps avoid deadlocks, and encourages the use of signal operators instead of blocking work.

RACScheduler is also somewhat similar to NSOperationQueue, but schedulers do not allow tasks to be reordered or depend on one another.

Value types

RAC offers a few miscellaneous classes for conveniently representing values in a stream:

RACTuple is a small, constant-sized collection that can contain nil (represented by RACTupleNil). It is generally used to represent the combined values of multiple streams.
RACUnit is a singleton "empty" value. It is used as a value in a stream for those times when more meaningful data doesn't exist.
RACEvent represents any signal event as a single value. It is primarily used by the -materialize method of RACSignal.

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