A Swift Reactive Programming Kit
Latest commit fcf4a94 Oct 13, 2018

README.md

ReactiveKit

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ReactiveKit is a lightweight Swift framework for reactive and functional reactive programming. With just over 2000 lines of code it enables you to get into reactive world today.

The framework is compatible with all Apple platforms and Linux. If you are developing an iOS or macOS app, make sure to also check out Bond framework that provides UIKit and AppKit bindings, reactive delegates and data sources.

This document will introduce the framework by going through its implementation. By the end you should be equipped with a pretty good understanding of how is the framework implemented and what are the best ways to use it.

Summary

Introduction

Consider how text of a text field changes as user enters his name. Each entered letter gives us a new state.

---[J]---[Ji]---[Jim]--->

We can think of these state changes as a sequence of events. It is quite similar to an array or a lists, but with the difference that events are generated over time as opposed to having them all in memory at once.

The idea behind reactive programming is that everything can be represented as a sequence. Let us consider another example - a network request.

---[Response]--->

Outcome of a network request is a response. Although we have only one response, we can still think of it as a sequence. An array of one element is still an array.

Arrays are finite so they have a property that we call size. It is a measure of how much memory the array occupies. When we talk about sequences over time, we do not know how many events will they generate during their lifetime. We do not know how many letters will the user enter. However, we would still like to know when the sequence is done generating the events.

To get that information, we can introduce a special kind of event - a completion event. It is an event that marks the end of a sequence. No event shall follow the completion event.

We will denote completion event visually with a vertical bar.

---[J]---[Ji]---[Jim]---|--->

Completion event is important because it tells us that whatever was going on is now over. We can finalize the work at that point and dispose any resources that might have been used in processing the sequence.

Unfortunately, the universe is not governed by the order, rather by the chaos. Unexpected things happen and we have to anticipate that. For example, a network request can fail so instead of a response, we can receive an error.

---!Error!--->

In order to represent errors in our sequences, we will introduce yet another kind of event. We will call it a failure event. Failure event will be generated when something unexpected happens. Just like the completion event, failure event will also represent the end of a sequence. No event shall follow the failure event.

Let us see how the event is defined in ReactiveKit.

/// An event of a sequence.
public enum Event<Element, Error: Swift.Error> {

  /// An event that carries next element.
  case next(Element)

  /// An event that represents failure. Carries an error.
  case failed(Error)

  /// An event that marks the completion of a sequence.
  case completed
}

It is just an enumeration of the three kinds of events we have. Sequences will usually have zero or more .next events followed by either a .completed or a .failed event.

What about sequences? In ReactiveKit they are called signals. Here is the protocol that defines them.

/// Represents a sequence of events.
public protocol SignalProtocol {

  /// The type of elements generated by the signal.
  associatedtype Element

  /// The type of error that can terminate the signal.
  associatedtype Error: Swift.Error

  /// Register the given observer.
  /// - Parameter observer: A function that will receive events.
  /// - Returns: A disposable that can be used to cancel the observation.
  public func observe(with observer: @escaping Observer<Element, Error>) -> Disposable
}

A signal represents the sequence of events. The most important thing you can do on the sequence is observe the events it generates. Events are received by the observer. An observer is nothing more than a function that accepts an event.

/// Represents a type that receives events.
public typealias Observer<Element, Error: Swift.Error> = (Event<Element, Error>) -> Void

Signals

We have seen the protocol that defines signals, but what about the implementation? Let us implement a basic signal type!

public struct Signal<Element, Error: Swift.Error>: SignalProtocol {

  private let producer: (Observer<Element, Error>) -> Void

  public init(producer: @escaping (Observer<Element, Error>) -> Void) {
    self.producer = producer
  }

  public func observe(with observer: @escaping Observer<Element, Error>) {
    producer(observer)
  }
}

We have defined our signal as a struct of one property - a producer. As you can see, producer is just a function that takes the observer as an argument. When we start observing the signal, what we do is basically execute the producer with the given observer. That is how simple signals are!

Signal in ReactiveKit is implemented almost like what we have shown here. It has few additions that give us some guarantees that we will talk about later.

Let us create an instance of the signal that sends first three positive integers to the observer and then completes.

Visually that would look like:

---[1]---[2]---[3]---|--->

While in the code, we would do:

let counter = Signal<Int, NoError> { observer in

  // send first three positive integers
  observer(.next(1))
  observer(.next(2))
  observer(.next(3))

  // send completed event
  observer(.completed)
}

Since the observer is just a function that receives events, we just execute it with the event whenever we want to send a new one. We always finalize the sequence by sending either .completed or .failed event so that the receiver knows when the signal is over with event production.

ReactiveKit wraps the observer into a struct with various helper methods to make it easier to send events. Here is a protocol that defines it.

/// Represents a type that receives events.
public protocol ObserverProtocol {

  /// Type of elements being received.
  associatedtype Element

  /// Type of error that can be received.
  associatedtype Error: Swift.Error

  /// Send the event to the observer.
  func on(_ event: Event<Element, Error>)
}

Our observer we introduced earlier is basically the on(_:) method. ReactiveKit also provides this extensions on the observer:

public extension ObserverProtocol {

  /// Convenience method to send `.next` event.
  public func next(_ element: Element) {
    on(.next(element))
  }

  /// Convenience method to send `.failed` event.
  public func failed(_ error: Error) {
    on(.failed(error))
   }

  /// Convenience method to send `.completed` event.
  public func completed() {
    on(.completed)
  }

  /// Convenience method to send `.next` event followed by a `.completed` event.
  public func completed(with element: Element) {
    next(element)
    completed()
  }
}

So with ReactiveKit we can implement previous example like this:

let counter = Signal<Int, NoError> { observer in

  // send first three positive integers
  observer.next(1)
  observer.next(2)
  observer.next(3)

  // send completed event
  observer.completed()
}

What happens when we observe such signal? Remember, the observer is a function that receives events so we can just pass a closure to our observe method.

counter.observe(with: { event in
  print(event)
})

Of course, we will get our three events printed out.

next(1)
next(2)
next(3)
completed

Wrapping asynchronous calls into signals

We can easily wrap asynchronous calls into signals because of the way we implemented our Signal type. Let us say that we have an asynchronous function that fetches the user.

func getUser(completion: (Result<User, ClientError>) -> Void) -> URLSessionTask

The function communicates fetch result through a completion closure and a Result type whose instance will contain either a user or an error. To wrap this into a signal, all we need to do is call that function within our signal initializer's producer closure and send relevant events as they happen.

func getUser() -> Signal<User, ClientError> {
  return Signal { observer in
    getUser(completion: { result in
      switch result {
      case .success(let user):
        observer.next(user)
        observer.completed()
      case .failure(let error):
        observer.failed(error)
    })
    // return disposable, continue reading
  }
}

If we now observe this signal, we will get either a user and a completion event

---[User]---|--->

or an error

---!ClientError!--->

In code, getting the user would look like:

let user = getUser()

user.observe { event in
  print(event) // prints ".next(user), .completed" in case of successful response
}

Let me ask you one important question here. When is the request to get the user executed, i.e. when is the asynchronous function getUser(completion:) called? Think about it.

We call getUser(completion:) within our producer closure that we pass to the signal initializer. That closure however is not executed when the signal is created. That means that the code let user = getUser() does not trigger the request. It merely creates a signal that knows how to execute the request.

Request is made when we call the observe(with:) method because that is the point when our producer closure gets executed. It also means that if we call the observe(with:) method more than once, we will call the producer more than once, so we will execute the request more than once. This is a very powerful aspect of signals and we will get back to it later when we will talk about sharing sequences of events. For now just remember that each call to observe(with:) means that events get produced all over again.

Disposing signals

Our example function getUser(completion:) returns a URLSessionTask object. We often do not think about it, but HTTP requests can be cancelled. When the screen gets dismissed, we should probably cancel any ongoing requests. A way to do that is to call cancel() on URLSessionTask that we used to make the request. How do we handle that with signals?

If you have been reading the code examples carefully, you have probably noticed that we did not correctly conform our Signal to SignalProtocol. The protocol specifies that the observe(with:) method returns something called Disposable. A disposable is an object that can cancel the signal observation and any underlying tasks.

Let me give you the definition of a disposable from ReactiveKit.

public protocol Disposable {

  /// Cancel the signal observation and any underlying tasks.
  func dispose()

  /// Returns `true` if already disposed.
  var isDisposed: Bool { get }
}

It has a method to cancel the observation and a property that can tell us if it has been disposed or not. Cancelling the observation is also referred to as disposing the signal.

There are various implementations of Disposable, but let us focus on the one that is most commonly used in signal creation. When the signal gets disposed, we often want to perform some action to clean up the resources or stop underlying tasks. What a better way to do that then to execute a closure when the the signal gets disposed. Let us implement a disposable that executes a given closure when it gets disposed. We will call it BlockDisposable.

public final class BlockDisposable: Disposable {

  private var handler: (() -> Void)?

  public var isDisposed: Bool {
    return handler == nil
  }

  public init(_ handler: @escaping () -> Void) {
    self.handler = handler
  }

  public func dispose() {
    handler?()
    handler = nil
  }
}

Simple enough. It just executes the given closure when the dispose() method is called. How do we use such disposable? Well, we will need to improve our signal implementation.

Who should create the disposable? Since the disposable represents a way to communicate the signal cancellation, it is obviously the one who created the signal that should also provide a disposable that can cancel the signal. To do that we will refactor the signal producer to return a disposable. Additionally, we will return that disposable from the observe(with:) method so that whoever will be observing the signal can cancel the observation.

public struct Signal<Element, Error: Swift.Error>: SignalProtocol {

  private let producer: (Observer<Element, Error>) -> Disposable

  public init(producer: @escaping (Observer<Element, Error>) -> Disposable) {
    self.producer = producer
  }

  public func observe(with observer: @escaping Observer<Element, Error>) -> Disposable {
    return producer(observer)
  }
}

This means that when we are creating a signal, we also have to provide a disposable. Let us refactor our asynchronous function wrapper signal to provide a disposable.

func getUser() -> Signal<User, ClientError> {
  return Signal { observer in
    let task = getUser(completion: { result in
      switch result {
      case .success(let user):
        observer.next(user)
        observer.completed()
      case .failure(let error):
        observer.failed(error)
    })

    return BlockDisposable {
      task.cancel()
    }
  }
}

We just return an instance of BlockDisposable that cancels the task when it gets disposed. We can then get that disposable when observing the signal.

let disposable = getUser().observe { event in
  print(event)
}

When we are no longer interested in signal events, we can just dispose the disposable. It will cancel the observation and cancel network task.

disposable.dispose()

In actual implementation of Signal in ReactiveKit there are additional mechanisms that prevent events from being sent when the signal is disposed so there is a guarantee that no events will be received after the signal is disposed. Any events sent from the producer after the signal is disposed are ignored.

In ReactiveKit, signals are automatically disposed when they terminate with either a .completed or .failed event.

Transforming signals

This is all so good, but why should we do it? What are the benefits? Here comes the most interesting aspect of reactive programming - signal operators.

Operators are functions (i.e. methods) that transform one or more signals into other signals. One of the basic operations on signals is filtering. Say that we have a signal of city names, but we want only the names starting with letter P.

filter(
---[Berlin]---[Paris]---[London]---[Porto]---|--->
)

--------------[Paris]--------------[Porto]---|--->

How could we implement such operator? Very easily.

extension SignalProtocol {

  /// Emit only elements that pass `isIncluded` test.
  public func filter(_ isIncluded: @escaping (Element) -> Bool) -> Signal<Element, Error> {
    return Signal { observer in
      return self.observe { event in
        switch event {
        case .next(let element):
          if isIncluded(element) {
            observer.next(element)
          }
        default:
          observer(event)
        }
      }
    }
  }
}

We have written an extension method on the SignalProtocol in which we create a signal. In the created signal's producer we observe self - the signal we are filtering - and propagate .next events that pass the test. We also propagate completion and failure events in the default case.

We use the operator by calling it on a signal.

cities.filter { $0.hasPrefix("P") }.observe { event in
  print(event) // prints .next("Paris"), .next("Porto"), .completed
}

There are many operators on signals. ReactiveKit is basically a collection of signal operators. Let us see another common one.

When observing signals we often do not care about terminal events, all we care about is the elements in .next events. We could write an operator that gives us just that.

extension SignalProtocol {

  /// Register an observer that will receive elements from `.next` events of the signal.
  public func observeNext(with observer: @escaping (Element) -> Void) -> Disposable {
    return observe { event in
      if case .next(let element) = event {
        observer(element)
      }
    }
  }
}

It should be pretty straightforward - just propagate the elements from .next event and ignore everything else. Now we can do:

cities.filter { $0.hasPrefix("P") }.observeNext { name in
  print(name) // prints "Paris", "Porto"
}

ReactiveKit also provides observeFailed and observeCompleted operators when you are interested only in those events.

Writing operators on signals is as simple as writing an extension method. When you need something that is not provided by the framework, just write it by yourself! ReactiveKit is written to be simple to understand. Whenever you are stuck, just look into the implementation.

More about errors

We have seen that a signal can terminate with an error. In our getUser example, when the network request fails we send .failed event. For that reason, our Signal type is generic both over the elements it sends and the errors it can fail with. There are, however, situations when signals are guaranteed not to fail, i.e. when they can never send an error. How do we define that?

ReactiveKit provides following type:

/// An error type that cannot be instantiated. Used to make signals non-failable.
public enum NoError: Error {
}

An enum with no cases that conforms to Swift.Error protocol. Since it has no cases, we can never make an instance of it. We will use this trick to get the compile-time guarantee that a signal will not fail.

For example, if we try

let signal = Signal<Int, NoError> { observer in
  ...
  observer.failed(/* What do I send here? */)
  ...
}

we will hit the wall because we cannot create an instance of NoError so we cannot send .failed event. This is a very powerful and important feature because whenever you see a signal whose errors are specialized to NoError type you can safely assume that signal will not fail - because it cannot.

Signals that do not fail are very common so ReactiveKit defines a typealias to make their usage simple and consistent.

public typealias SafeSignal<Element> = Signal<Element, NoError>

You are recommend to use SafeSignal whenever you have a signal that cannot fail.

Bindings work only with safe (non-failable) signals.

Creating simple signals

You will often need a signal that emits just one element and then completes. To make it, use static method just.

let signal = SafeSignal.just(5)

That will give you following signal:

---5-|--->

If you need a signal that fires multiple elements and then completes, you can convert any Sequence to a signal with static method sequence.

let signal = SafeSignal.sequence([1, 2, 3])
---1-2-3-|--->

To create a signal that just completes without sending any elements, do

let signal = SafeSignal<Int>.completed()
---|--->

To create a signal that just fails, do

let signal = Signal<Int, MyError>.failed(MyError.someError)
---!someError!--->

You can also create a signal that never sends any events (i.e. a signal that never terminates).

let signal = SafeSignal.never()
------>

Sometimes you will need a signal that sends specific element after certain amount of time passes:

let signal = SafeSignal.timer(element: 5, time: 60)
---/60 seconds/---5-|-->

Finally, when you need a signal that sends an integer every interval seconds, do

let signal = SafeSignal.interval(5)
---0---1---2---3---...>

Disposing in a bag

Handling disposables can be cumbersome when doing multiple observation. To simplify it, ReactiveKit provides a type called DisposeBag. It is a container into which you can put your disposables. The bag will dispose all disposables that were put into it when it gets deallocated.

class Example {

  let bag = DisposeBag()

  init() {
    ...
    someSignal
      .observe { ... }
      .dispose(in: bag)

    anotherSignal
      .observe { ... }
      .dispose(in: bag)
    ...
  }
}

In the example, instead of handling the disposables, we just put them into a bag by calling dispose(in:) method on the disposable. Disposables will then get disposed automatically when the bag gets deallocated. Note that you can also call dispose() on the bag to dispose its contents at will.

ReactiveKit provides a bag on NSObject and its subclasses out of the box. If you are doing iOS or Mac development you will get a free bag on your view controllers and other UIKit objects since all of them are NSObject subclasses.

extension NSObject {
  public var bag: DisposeBag { get }
}

If you are like me and do not want to worry about disposing, check out bindings.

Threading

By default observers receive events on the thread or the queue where the event is sent from.

For example, if we have a signal that is created like

let someImage = SafeSignal<UIImage> { observer in
  ...
  DispatchQueue.global(qos: .background).async {
    observer.next(someImage)
  }
  ...
}

and if we use it to update the image view

someImage
  .observeNext { image in
    imageView.image = image // called on background queue
  }
  .dispose(in: bag)

we will end up with a weird behaviour. We will be setting image from the background queue on an instance of UIImageView that is not thread safe - just like the rest of UIKit.

We could set the image in another async dispatch to main queue, but there is a better way. Just use the operator observeOn with the queue you want the observer to be called on.

someImage
  .observeOn(.main)
  .observeNext { image in
    imageView.image = image // called on main queue
  }
  .dispose(in: bag)

There is also another side to this. You might have a signal that does some slow synchronous work on whatever thread or queue it is observed on.

let someData = SafeSignal<Data> { observer in
  ...
  let data = // synchronously load large file
  observer.next(data)
  ...
}

We, however, do not want observing that signal to block the UI.

someData
  .observeNext { data in // blocks current thread
    display(data)
  }
  .dispose(in: bag)

We would like to do the loading on another queue. We could dispatch async the loading, but what if we cannot change the signal producer closure because it is defined in a framework or there is another reason we cannot change it. That is when the operator executeOn saves the day.

someData
  .executeOn(.global(qos: .background))
  .observeOn(.main)
  .observeNext { data in // does not block current thread
    display(data)
  }
  .dispose(in: bag)

By applying executeOn we define where the signal producer gets executed. We usually use it in a combination with observeOn to define where the observer receives events.

Note that there are also operators observeIn and executeIn. Those operators are similar to the ones we described with the difference that they work with execution contexts instead of with dispatch queues. Execution context is a simple abstraction over a thread or a queue. You can see how it is implemented here.

Bindings

Bindings are observations with perks. Most of the time you should be able to replace observation with a binding. Consider the following example. Say we have a signal of users

let presentUserProfile: SafeSignal<User> = ...

and we would like to present a profile screen when a user is sent on the signal. Usually we would do something like:

presentUserProfile.observeOn(.main).observeNext { [weak self] user in
  let profileViewController = ProfileViewController(user: user)
  self?.present(profileViewController, animated: true)
}.dispose(in: bag)

But that is ugly! We have to dispatch everything to the main queue, be cautious not to create a retain cycle and ensure that the disposable we get from the observation is handled.

Thankfully there is a better way. We can create an inline binding instead of the observation. Just do the following

presentUserProfile.bind(to: self) { me, user in
  let profileViewController = ProfileViewController(user: user)
  me.present(profileViewController, animated: true)
}

and stop worrying about threading, retain cycles and disposing because bindings take care of all that automatically. Just bind the signal to the target responsible for performing side effects (in our example, to the view controller responsible for presenting a profile view controller). The closure you provide will be called whenever the signal emits an element with both the target and the sent element as arguments.

Binding targets

You can bind to targets that conform both to Deallocatable and BindingExecutionContextProvider protocols.

You can actually bind to targets that conform only to Deallocatable protocol, but then you have to pass the execution context in which to update the target by calling bind(to:context:setter).

Objects that conform to Deallocatable provide a signal that can tell us when the object gets deallocated.

public protocol Deallocatable: class {

  /// A signal that fires `completed` event when the receiver is deallocated.
  var deallocated: SafeSignal<Void> { get }
}

ReactiveKit provides conformance to this protocol for NSObject and its subclasses out of the box.

How do you conform to Deallocatable? The simplest way is conforming to DisposeBagProvider instead.

/// A type that provides a dispose bag.
/// `DisposeBagProvider` conforms to `Deallocatable` out of the box.
public protocol DisposeBagProvider: Deallocatable {

  /// A `DisposeBag` that can be used to dispose observations and bindings.
  var bag: DisposeBag { get }
}

extension DisposeBagProvider {

  public var deallocated: SafeSignal<Void> {
    return bag.deallocated
  }
}

As you can see, DisposeBagProvider inherits Deallocatable and implements it by taking the deallocated signal from the bag. So all that you need to do is provide a bag property on your type.

BindingExecutionContextProvider protocol provides the execution context in which the object should be updated. Execution context is just a wrapper over a dispatch queue or a thread. You can see how it is implemented here.

public protocol BindingExecutionContextProvider {

  /// An execution context used to deliver binding events.
  var bindingExecutionContext: ExecutionContext { get }
}

Bond framework provides BindingExecutionContextProvider conformance to various UIKit objects so they can be seamlessly bound to while ensuring the main thread.

You can conform to this protocol by providing execution context.

extension MyViewModel: BindingExecutionContextProvider {

  public var bindingExecutionContext: ExecutionContext {
    return .immediateOnMain
  }
}

ExecutionContext.immediateOnMain executes synchronously if the current thread is main, otherwise it makes asynchronous dispatch to main queue. If you want to bind on background queue, you can return .global(qos: .background) instead.

Note that updating UIKit or AppKit objects must always happen from the main thread or queue.

Now we can peek into the binding implementation.

extension SignalProtocol where Error == NoError {

  @discardableResult
  public func bind<Target: Deallocatable>(to target: Target, setter: @escaping (Target, Element) -> Void) -> Disposable
  where Target: BindingExecutionContextProvider
  {
    return take(until: target.deallocated)
      .observeIn(target.bindingExecutionContext)
      .observeNext { [weak target] element in
        if let target = target {
          setter(target, element)
        }
      }
  }
}

First of all, notice @discardableResult annotation. It is there because we can safely ignore the returned disposable. The binding will automatically be disposed when the target gets deallocated. That is ensured by the take(until:) operator. It propagates events from self until the given signal completes - in our case until target.deallocated signal completes. We then just observe in the right context and on each next element update the target using the provided setter closure.

Note also that bindings are implemented only on non-failable signals.

Binding to a property

Given a string signal name, we know that we can bind it to a label by doing

name.bind(to: label) { label, name in
  label.text = name
}

but would it not be great if we could make it a one-liner? With Swift 4 key paths we can! Just do

name.bind(to: label, keyPath: \.text)

where the target is the same target as in previous example and keyPath is a key path to the property that should be updated with each new element sent on the signal!

If you opt-in for a Bond framework, things get even simpler:

name.bind(to: label.reactive.text)

Bond provides a type called Bond that acts as a binding target that we can use to make reactive extensions for various properties. Check out its documentation for more info.

Sharing sequences of events

Whenever we observe a signal, we execute its producer. Consider the following signal:

let user = Signal { observer in
  print("Fetching user...")
  ...
}

If we now do

user.observe { ... } // prints: Fetching user...
user.observe { ... } // prints: Fetching user...

the producer will be called twice and the user will be fetched twice. Same behaviour might sneak by unnoticed in the code like:

user.map { $0.name }.observe { ... } // prints: Fetching user...
user.map { $0.email }.observe { ... } // prints: Fetching user...

You can think of each signal observation as a process of its own. Often this behaviour is exactly what we need, but sometimes we can optimize our code by sharing one sequence to multiple observers. To achieve that, all we need to do is apply the operator shareReplay(limit:).

let user = user.shareReplay(limit: 1)

user.map { $0.name }.observe { ... } // prints: Fetching user...
user.map { $0.email }.observe { ... } // Does not print anything, but still gets the user :)

Argument limit specifies how many elements (.next events) should be replayed to the observer. Terminal events are always replayed. One element is often all we need. Operator shareReplay(limit:) is a combination of two operators. In order to understand it, we will introduce two interesting concepts: subjects and connectable signals.

Subjects

At the beginning of the document, we defined signal with the SignalProtocol protocol. We then implemented a concrete Signal type that conformed to that protocol by executing the producer closure for each observation. Producer would sends events to the observer given to the method observe(with:).

Could we have implemented signal differently? Let us try making another kind of a signal - one that is also an observer. We will call it Subject. What follows is the simplified implementation of Subject provided by ReactiveKit.

open class Subject<Element, Error: Swift.Error>: SignalProtocol, ObserverProtocol {

  private var observers: [Observer<Element, Error>] = []

  open func on(_ event: Event<Element, Error>) {
    observers.forEach { $0(event) }
  }

  open func observe(with observer: @escaping Observer<Element, Error>) -> Disposable {
    observers.append(observer)
    return /* a disposable that removes the observer from the array */
  }
}

Our new kind of signal, subject, is an observer itself that holds an array of its own observers. When the subject receives an event (when method on(_:) is called), the event is just propagated to all registered observers. Observing this subject means adding the given observer into the array of observers.

How do we use such subject?

let name = Subject<String, NoError>()

name.observeNext { name in print("Hi \(name)!") }

name.on(.next("Jim")) // prints: Hi Jim!

// ReactiveKit provides few extension toon the ObserverProtocol so we can also do:
name.next("Kathryn") // prints: Hi Kathryn!

name.completed()

Note: When using ReactiveKit you should actually use PublishSubject instead. It has the same behaviour and interface as Subject we defined here - just a different name in order to be consistent with ReactiveX API.

As you can see, we do not have a producer closure, rather we send events to the subject itself. The subject then propagates those events to its own observers.

Subjects are useful when we need to convert actions from imperative world into signals in reactive world. For example, say we needed view controller appearance events as a signal. We can make a subject property and then send events to it from viewDidAppear override. Such subject would then represent a signal of view controller appearance events.

class MyViewController: UIViewController {

  fileprivate let _viewDidAppear = PublishSubject<Void, NoError>()

  override viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    _viewDidAppear.next()
  }
}

We could have exposed subject publicly, but then anyone would be able to send events on it. Better approach is to make it fileprivate as we did and then expose it publicly as a signal. It is recommended to put all reactive extensions into an extension of ReactiveExtensions type provided by ReactiveKit. Here is how you do it:

extension ReactiveExtensions where Base: MyViewController {

  var viewDidAppear: SafeSignal<Void> {
    return base._viewDidAppear.toSignal() // convert Subject to Signal
  }
}

We can then use our signal like:

myViewController.reactive.viewDidAppear.observeNext {
  print("view did appear")
}

Subjects represent kind of signals that are called hot signals. They are called hot because they "send" events regardless if there are observer registered or no. On the other hand, Signal type represents kind of signals that are called cold signal. Signals of that kind do not produce events until we give them an observer that will receive events.

As you could have inferred from the implementation, observing a subject gives us only the events that are sent after the observer is registered. Any events that might have been sent before the observer became registered will not be received by the observer. Is there a way to solve this? Well, we could buffer the received events and then replay them to new observers. Let us do that in a subclass.

public final class ReplaySubject<Element, Error: Swift.Error>: Subject<Element, Error> {

  private var buffer: [Event<Element, Error>] = []

  public override func on(_ event: Event<Element, Error>) {
    events.append(event)
    super.on(event)
  }

  public func observe(with observer: @escaping Observer<Element, Error>) -> Disposable {
    buffer.forEach { observer($0) }
    return super.observe(with: observer)
  }
}

Again, this is simplified version of the ReplaySubject provided by ReactiveKit, but it has everything needed to explain the concept. Whenever an event is received, we put it in the buffer. When the observer gets registered, we then replay all events that we have in the buffer. Any future events will be propagated just like in Subject.

Note: ReplaySubject provided by ReactiveKit supports limiting the buffer to certain size so it does not grow forever. Usually it will be enough to limit it to just one event by instantiating it with ReplaySubject(bufferSize: 1). Buffer always keeps the latest event and removes older ones.

At this point you might have the idea how to achieve the behaviour of the shareReplay operator. We could observe original signal with the replay subject and then observe that subject multiple times. But in order to implement that as an operator and make it opaque to the user, we need to learn about connectable signals.

Connectable signals

We have seen two kinds of signals so far. A Signal that produces events only if the observer is registered and a Subject that produces events regardless if there are any observers registered. A connectable signal will be the third kind of a signal we will implement. This one will start producing events when we call connect() on it. Let us define a protocol first.

/// Represents a signal that is started by calling `connect` on it.
public protocol ConnectableSignalProtocol: SignalProtocol {

  /// Start the signal.
  func connect() -> Disposable
}

We will build a connectable signal as a wrapper over any other kind of a signal. We will leverage subjects for the implementation.

public final class ConnectableSignal<Source: SignalProtocol>: ConnectableSignalProtocol {

  private let source: Source
  private let subject: Subject<Source.Element, Source.Error>

  public init(source: Source, subject: Subject<Source.Element, Source.Error>) {
    self.source = source
    self.subject = subject
  }

  public func connect() -> Disposable {
    return source.observe(with: subject)
  }

  public func observe(with observer: @escaping Observer<Source.Element, Source.Error>) -> Disposable {
    return subject.observe(with: observer)
  }
}

We need two things here: a source signal that we are wrapping into a connectable one and a subject that will propagate events from the source to the connectable signal's observers. We will require them in the initializer and save them as properties.

Observing the connectable signal actually means observing the underlying subject. Starting the signal is now trivial - all we need to do is start observing the source signal with the subject (remember - subject is also an observer). That will make events flow from the source into the observers registered to the subject.

We now have all parts to implement shareReplay(limit:). Let us start with replay(limit:).

extension SignalProtocol {

  /// Ensure that all observers see the same sequence of elements. Connectable.
  public func replay(_ limit: Int = Int.max) -> ConnectableSignal<Self> {
    return ConnectableSignal(source: self, subject: ReplaySubject(bufferSize: limit))
  }
}

Trivial enough. Creating a ConnectableSignal with ReplaySubject ensures that all observers get the same sequence of events and that the source signal is observed only once. The only problem is that the returned signal is a connectable signal so we have to call connect() on it in order to start events.

We somehow need to convert connectable signal into a non-connectable one. In order to do that, we need to call connect at the right time and dispose at the right time. What are the right times? It is only reasonable - right time to connect is on the first observation and right time to dispose is when the last observation is disposed.

In order to do this, we will keep a reference count. With each new observer, the count goes up, while on each disposal it goes down. We will connect when count goes from 0 to 1 and dispose when count goes from 1 to 0.

public extension ConnectableSignalProtocol {

  /// Convert connectable signal into the ordinary signal by calling `connect`
  /// on the first observation and calling dispose when number of observers goes down to zero.
  public func refCount() -> Signal<Element, Error> {
    // check out: https://github.com/ReactiveKit/ReactiveKit/blob/e781e1d0ce398259ca38cc0d5d0ed6b56d8eab39/Sources/Connectable.swift#L68-L85
   }
}

Implementing shareReplay operator

Now that we know about subjects and connectable signals, we can implement the operator shareReplay(limit:). It is quite simple:

/// Ensure that all observers see the same sequence of elements.
public func shareReplay(limit: Int = Int.max) -> Signal<Element, Error> {
  return replay(limit).refCount()
}

Handling signal errors

You might ignore them and delay, but at one point you will need to handle the errors that the signal can fail with.

If the signal has the potential of recovering by retrying the original producer, you can use retry operator.

let image /*: Signal<UIImage, NetworkError> */ = getImage().retry(3)

Imagine how many number of lines would that take in imperative paradigm :)

Operator retry will only work sometimes and it will fail eventually. The result of applying the operator is still a failable signal.

How do we convert failable signal into non-failable (safe) signal? We have to handle the error somehow. One way is to recover with a default element.

let image /*: SafeSignal<UIImage> */ = getImage().recover(with: .placeholder)

Now we get SafeSignal because the transformed signal will never fail. Any .failed event that might occur on original signal will just be replaced with .next event containing the default element (placeholder image in our example).

Alternative way to get safe signal is to ignore - suppress - the error. You would do this if you really do not care about the error and nothing bad will happen if you ignore it.

let image /*: SafeSignal<UIImage> */ = getImage().suppressError(logging: true)

It is always a good idea to log the error.

If you need to do alternative logic in case of an error, you would flat map it onto some other signal.

let image = getImage().flatMapError { error in
  return getAlternativeImage()
}

Property

Property wraps mutable state into an object that enables observation of that state. Whenever the state changes, an observer will be notified. Just like the PublishSubject, it represents a bridge into the imperative world.

To create a property, just initialize it with the initial value.

let name = Property("Jim")

nil is valid value for properties that wrap optional type.

Properties are signals just like signals of Signal type. They can be transformed into another signals, observed and bound in the same manner as signals can be.

For example, you can register an observer with observe or observeNext methods.

name.observeNext { value in
  print("Hi \(value)!")
}

When you register an observer, it will be immediately invoked with the current value of the property so the snippet will print "Hi Jim!".

To change value of the property afterwards, just set the value property.

name.value = "Jim Kirk" // Prints: Hi Jim Kirk!

Loading signals

Signals usually represent asynchronous actions, network calls for example. Any good app will display some kind of a loading indicator to the user while the call is in progress and an error dialog when the call fails, probably with an option to retry. To facilitate those use cases, ReactiveKit provides LoadingSignal and LoadingProperty types.

An action or a work can be in one of the three states: loading, loaded, loading failed. RectiveKit defines those states with the enum LoadingState:

/// Represents loading state of an asynchronous action.
public enum LoadingState<LoadingValue, LoadingError: Error>: LoadingStateProtocol {

  /// Value is loading.
  case loading

  /// Value is loaded.
  case loaded(LoadingValue)

  /// Value loading failed with the given error.
  case failed(LoadingError)
}

A signal with elements of LoadingState type is typealiased as LoadingSignal:

public typealias LoadingSignal<LoadingValue, LoadingError: Error> = SafeSignal<LoadingState<LoadingValue, LoadingError>>

Notice that loading signal is a safe signal. Signal itself can never fail, but errors can be emitted as .failed loading state. This means that the error does not terminate the signal - new events can be received after the error.

How does one convert regular signals into loading signals? It is as simple as applying toLoadingSignal operator. Say that we have a signal that represents some resource fetching operation:

let fetchImage: Signal<UIImage, ApplicationError> = ...

We can then convert that signal into a loading signal by applying toLoadingSignal operator.

fetchImage
    .toLoadingSignal()
    .observeNext { loadingState in
        switch loadingState {
        case .loading:
            // display loading indicator
        case .loaded(let image):
            // hide loading indicator
            // display image
        case .failed(let error):
            // hide loading indicator
            // display error message
        }
    }

Observing next element now gives us the loading state of the signal. We will receive .loading state as soon as we start the observation. When the resource loading completes, we will receive either the resource in the .loaded state or the error in the .failed state.

Consuming loading state

Loading signal looks great, but it is not fun to manually update the loading state of each view we are loading the data for. Thankfully there is a better way - LoadingStateListener protocol:

/// A consumer of loading state.
public protocol LoadingStateListener: class {

    /// Consume observed loading state.
    func setLoadingState<LoadingValue, LoadingError>(_ state: ObservedLoadingState<LoadingValue, LoadingError>)
}

This protocol could be implemented by anything that updates its appearance based on the loading state of the data it displays. On iOS, a good candidate would be UIViewController or UIView. For example:

extension UIViewController: LoadingStateListener {

    public func setLoadingState<LoadingValue, LoadingError>(_ state: ObservedLoadingState<LoadingValue, LoadingError>) {
        switch state {
        case .loading:
            // display loading indicator
        case .reloading:
            // display reloading indicator
        case .loaded(let value):
            // hide loading indicator
            // display value
        case .failed(let error):
            // hide loading indicator
            // display error
        }
    }
}

Notice that LoadingStateListener gets ObservedLoadingState instead of LoadingState. The difference between the two is that the former has one additional state: .reloading. ReactiveKit will automatically convert subsequent .loading states into .reloading states so that you can potentially act differently in those two cases.

Now that we have a loading state listener, we can convert any loading signal into a regular safe signal by consuming its loading state by the listener:

fetchImage
    .toLoadingSignal()
    .consumeLoadingState(by: viewController)
    .bind(to: viewController.imageView) { imageView, image in
        imageView.image = image
    }

Exciting! Operator consumeLoadingState takes the loading state listener and updates it each time a state is produced by the loading signal. It returns a safe signal of loading values, i.e. it unwraps the underlying value from the .loaded state. In our example that would be SafeSignal<UIImage> which we can then bind to our image view and update its content.

Transforming loading signals

ReactiveKit provides a number of operators specific to loading signals like value, mapValue, mapLoadingError, dematerializeLoadingState and flatMapValue. You can, however, apply regular signal operators to loading signals that operate on their values. To do that, use liftValue operator. For example, to skip first three values and delay them for a second, do the following:

aLoadingSignal.liftValue {
    $0.skip(first: 3).delay(interval: 1)
}

liftValue accepts a closure that is given a regular signal that you can then transform using regular signal operators.

Loading property

We often need a way to store a result of an asynchronous operation and way to refresh (reload) it. To do that we can use LoadingProperty type. It is similar to the regular Property, but instead of initializing it with a value, we initialize it with a closure that provides a loading signal - a closure that can do some work. LoadingProperty can then be used as any other LoadingSignal. It will load its value, i.e. perform the work, when we observe (or bind) it for the first time. It also provides a way to reload the value by performing the work again.

Here is an example of how we could use LoadingProperty to implement a simple user service:

class UserService {

    let user: LoadingProperty<User, ApplicationError>

    init(_ api: API) {
        
        user = LoadingProperty {
            api.fetchUser()
        }
    }

    func refresh() -> LoadingSignal<User, ApplicationError> {
        return user.reload()
    }
}

Other common patterns

Performing an action on .next event

Say that you have a button that (re)loads a photo in your app. How to implement that in reactive world? First we will need a signal that represents buttons taps. With Bond framework you can get that signal just like this:

let reload /*: SafeSignal<Void> */ = button.reactive.tap

The signal will send .next event whenever the button is tapped. We would like to load the photo on each such event. In order to do so, we will flat map the reload signal into photo requests.

let photo = reload.flatMapLatest { _ in
  return apiClient().loadPhoto() // returns Signal<UIImage, NetworkError>
}

photo will be of whatever type the inner signal was - in our case Signal<UIImage, NetworkError>. We can then bind that to the image view:

photo
  .suppressError(logging: true)  // we can bind only safe signals
  .bind(to: imageView.reactive.image) // using Bond framework

What will happen is that whenever the button is tapped a new photo request will be made and the image view's image will be updated.

There are two other operators that flat map signals: flatMapConcat and flatMapMerge. The difference between the three is in the way they handle propagation of events from the inner signals in case when there are more than one inner signals active. For example, say that user taps reload button before the previous request is finished. What happens?

  • flatMapLatest will dispose previous signal and start the new one.
  • flatMapConcat will start new signal, but it will not propagate its events until the previous signal completes.
  • flatMapMerge will start new signal, but it will propagate events from all signals as they come - regardless what signal started first.

Combining multiple signals

Say you had a username and password signals and you would like a signal that tells you if they are both entered so that you can enable a login button. You can use combineLatest operator to achieve that.

let username = usernameLabel.reactive.text
let password = passwordLabel.reactive.text

let canLogIn = combineLatest(username, password) { username, password in
  return !username.isEmpty && !password.isEmpty
}

canLogIn.bind(to: loginButton.reactive.isEnabled)

All you have to provide to the operator is the signals and a closure that maps the latest elements from those signals to a new element.

Reactive extensions are provided by Bond framework.

Requirements

  • iOS 8.0+ / macOS 10.9+ / tvOS 9.0+ / watchOS 2.0+
  • Xcode 9

or

  • Linux + Swift 4.0

Installation

Bond framework is optional, but recommended for Cocoa / Cocoa touch development.

Note: Since v3.7, ReactiveKit is using Swift 4 syntax that compiles under Xcode 9. If you are still using Xcode 8, please do not update to v3.7 and stay on the latest v3.6.x version.

Carthage

github "DeclarativeHub/ReactiveKit"
github "DeclarativeHub/Bond"

CocoaPods

pod 'ReactiveKit'
pod 'Bond'

Swift Package Manager

// swift-tools-version:4.0

import PackageDescription

let package = Package(
  name: "MyApp",
  dependencies: [
    .package(url: "https://github.com/DeclarativeHub/ReactiveKit.git", from: "3.9.0")
  ],
  targets: [
    .target(name: "MyApp", dependencies: ["ReactiveKit"])
  ]
)

Communication

  • If you'd like to ask a general question, open an issue.
  • If you have found a bug, open an issue or do a pull request with the fix.
  • If you have a feature request, open an issue.
  • If you want to contribute, submit a pull request (include unit tests).

Additional Documentation

License

The MIT License (MIT)

Copyright (c) 2015-2018 Srdan Rasic (@srdanrasic)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.