ReactiveCocoa

ReactiveCocoa (RAC) is a Cocoa framework inspired by Functional Reactive Programming. It provides APIs for composing and transforming streams of values over time.

Introduction
Example: online search
Objective-C and Swift
How does ReactiveCocoa relate to Rx?
Getting started

If you’re already familiar with functional reactive programming or what ReactiveCocoa is about, check out the Documentation folder for more in-depth information about how it all works. Then, dive straight into our documentation comments for learning more about individual APIs.

If you have a question, please see if any discussions in our GitHub issues or Stack Overflow have already answered it. If not, please feel free to file your own!

Many thanks to Rheinfabrik for generously sponsoring the development of ReactiveCocoa 3!

Introduction

ReactiveCocoa is inspired by functional reactive programming. Rather than using mutable variables which are replaced and modified in-place, RAC offers “event streams,” represented by the Signal and SignalProducer types, that send values over time.

Event streams unify all of Cocoa’s common patterns for asynchrony and event handling, including:

Delegate methods
Callback blocks
NSNotifications
Control actions and responder chain events
Futures and promises
Key-value observing (KVO)

Because all of these different mechanisms can be represented in the same way, it’s easy to declaratively chain and combine them together, with less spaghetti code and state to bridge the gap.

For more information about the concepts in ReactiveCocoa, see the Framework Overview.

Example: online search

Let’s say you have a text field, and whenever the user types something into it, you want to make a network request which searches for that query.

Observing text edits

The first step is to observe edits to the text field, using a RAC extension to UITextField specifically for this purpose:

let searchStrings = textField.rac_textSignal().toSignalProducer()
    |> map { text in text as! String }

This gives us a signal producer which sends values of type String. (The cast is currently necessary to bridge this extension method from Objective-C.)

Making network requests

With each string, we want to execute a network request. Luckily, RAC offers an NSURLSession extension for doing exactly that:

let searchResults = searchStrings
    |> flatMap(.Latest) { query in
        let URLRequest = self.searchRequestWithEscapedQuery(query)
        return NSURLSession.sharedSession().rac_dataWithRequest(URLRequest)
    }
    |> map { data, URLResponse in
        let string = String(data: data, encoding: NSUTF8StringEncoding)!
        return parseJSONResultsFromString(string)
    }
    |> observeOn(UIScheduler())

This has transformed our producer of Strings into a producer of Arrays containing the search results, which will be forwarded on the main thread (thanks to the UIScheduler).

Additionally, flatMap(.Latest) here ensures that only one search—the latest—is allowed to be running. If the user types another character while the network request is still in flight, it will be cancelled before starting a new one. Just think of how much code that would take to do by hand!

Receiving the results

This won’t actually execute yet, because producers must be started in order to receive the results (which prevents doing work when the results are never used). That’s easy enough:

searchResults.start(next: { results in
    println("Search results: \(results)")
})

Here, we watch for the Next event, which contains our results, and just log them to the console. This could easily do something else instead, like update a table view or a label on screen.

Handling errors

In this example so far, any network error will generate an Error event, which will terminate the event stream. Unfortunately, this means that future queries won’t even be attempted.

To remedy this, we need to decide what to do with errors that occur. The quickest solution would be to log them, then ignore them:

    |> flatMap(.Latest) { query in
        let URLRequest = self.searchRequestWithEscapedQuery(query)

        return NSURLSession.sharedSession().rac_dataWithRequest(URLRequest)
            |> catch { error in
                println("Network error occurred: \(error)")
                return SignalProducer.empty
            }
    }

By replacing errors with the empty event stream, we’re able to effectively ignore them.

However, it’s probably more appropriate to retry at least a couple of times before giving up. Conveniently, there’s a retry operator to do exactly that!

Our improved searchResults producer might look like this:

let searchResults = searchStrings
    |> flatMap(.Latest) { query in
        let URLRequest = self.searchRequestWithEscapedQuery(query)

        return NSURLSession.sharedSession().rac_dataWithRequest(URLRequest)
            |> retry(2)
            |> catch { error in
                println("Network error occurred: \(error)")
                return SignalProducer.empty
            }
    }
    |> map { data, URLResponse in
        let string = String(data: data, encoding: NSUTF8StringEncoding)!
        return parseJSONResultsFromString(string)
    }
    |> observeOn(UIScheduler())

Throttling requests

Now, let’s say you only want to actually perform the search when the user pauses typing, to minimize traffic.

ReactiveCocoa has a declarative throttle operator that we can apply to our search strings:

let searchStrings = textField.rac_textSignal().toSignalProducer()
    |> map { text in text as! String }
    |> throttle(0.5, onScheduler: QueueScheduler.mainQueueScheduler)

This prevents values from being sent less than 0.5 seconds apart, so the user must stop editing for at least that long before we’ll use their search string.

To do this manually would require significant state, and end up much harder to read! With ReactiveCocoa, we can use just one operator to incorporate time into our event stream.

Objective-C and Swift

Although ReactiveCocoa was started as an Objective-C framework, as of version 3.0, all major feature development is concentrated on the Swift API.

RAC’s Objective-C API and Swift API are entirely separate, but there is a bridge to convert between the two. This is mostly meant as a compatibility layer for older ReactiveCocoa projects, or to use Cocoa extensions which haven’t been added to the Swift API yet.

The Objective-C API will continue to exist and be supported for the foreseeable future, but it won’t receive many improvements. For more information about using this API, please consult our legacy documentation.

We highly recommend that all new projects use the Swift API.

How does ReactiveCocoa relate to Rx?

ReactiveCocoa was originally inspired, and therefore heavily influenced, by Microsoft’s Reactive Extensions (Rx) library. There are many ports of Rx, including RxSwift, but ReactiveCocoa is intentionally not a direct port.

Where RAC differs from Rx, it is usually to:

Create a simpler API
Address common sources of confusion
More closely match Cocoa conventions

The following are some of the concrete differences, along with their rationales.

Naming

In most versions of Rx, Streams over time are known as Observables, which parallels the Enumerable type in .NET. Additionally, most operations in Rx.NET borrow names from LINQ, which uses terms reminiscient of relational databases, like Select and Where.

RAC is focused on matching Swift naming first and foremost, with terms like map and filter instead. Other naming differences are typically inspired by significantly better alternatives from Haskell or Elm (which is the primary source for the “signal” terminology).

Signals and Signal Producers (“hot” and “cold” observables)

One of the most confusing aspects of Rx is that of “hot”, “cold”, and “warm” observables (event streams).

In short, given just a method or function declaration like this, in C#:

IObservable<string> Search(string query)

… it is impossible to tell whether subscribing to (observing) that IObservable will involve side effects. If it does involve side effects, it’s also impossible to tell whether each subscription has a side effect, or if only the first one does.

This example is contrived, but it demonstrates a real, pervasive problem that makes it extremely hard to understand Rx code (and pre-3.0 ReactiveCocoa code) at a glance.

ReactiveCocoa 3.0 has solved this problem by distinguishing side effects with the separate Signal and SignalProducer types. Although this means there’s another type to learn about, it improves code clarity and helps communicates intent much better.

In other words, ReactiveCocoa’s changes here are simple, not easy.

Typed errors

When signals and signal producers are allowed to error in ReactiveCocoa, the kind of error must be specified in the type system. For example, Signal<Int, NSError> is a signal of integer values that may send an error of type NSError.

More importantly, RAC allows the special type NoError to be used instead, which statically guarantees that an event stream is not allowed to send an error. This eliminates many bugs caused by unexpected error events.

In Rx systems with types, event streams only specify the type of their values—not the type of their errors—so this sort of guarantee is impossible.

UI programming

Rx is basically agnostic as to how it’s used. Although UI programming with Rx is very common, it has few features tailored to that particular case.

RAC takes a lot of inspiration from ReactiveUI, including the basis for Actions.

Unlike ReactiveUI, which unfortunately cannot directly change Rx to make it more friendly for UI programming, ReactiveCocoa has been improved many times specifically for this purpose—even when it means diverging further from Rx.

Getting started

ReactiveCocoa supports OS X 10.9+ and iOS 8.0+.

To add RAC to your application:

Add the ReactiveCocoa repository as a submodule of your application’s repository.
Run script/bootstrap from within the ReactiveCocoa folder.
Drag and drop ReactiveCocoa.xcodeproj, Carthage/Checkouts/Box/Box.xcodeproj and Carthage/Checkouts/Result/Result.xcodeproj into your application’s Xcode project or workspace.
On the “General” tab of your application target’s settings, add ReactiveCocoa.framework, Box.framework and Result.framework to the “Embedded Binaries” section.
If your application target does not contain Swift code at all, you should also set the EMBEDDED_CONTENT_CONTAINS_SWIFT build setting to “Yes”.

Or, if you’re using Carthage, simply add ReactiveCocoa to your Cartfile:

github "ReactiveCocoa/ReactiveCocoa"

If you would prefer to use CocoaPods, there are some unofficial podspecs that have been generously contributed by third parties.

Once you’ve set up your project, check out the Framework Overview for a tour of ReactiveCocoa’s concepts, and the Basic Operators for some introductory examples of using it.

wpstarnice/ReactiveCocoa