A Swift binding framework
Swift Objective-C Ruby
Latest commit 601cb73 Jan 19, 2017 @srdanrasic srdanrasic committed on GitHub Merge pull request #369 from tonyarnold/feature/add-nspopupbutton-dyn…

Expose NSPopUpButton’s reactive extensions as DynamicSubjects


Bond, Swift Bond

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Bond is a Swift binding framework that takes binding concepts to a whole new level. It's simple, powerful, type-safe and multi-paradigm - just like Swift.

Bond is built on top of ReactiveKit and bridges the gap between the reactive and imperative paradigms. You can use it as a standalone framework to simplify your state changes with bindings and reactive data sources, but you can also use it with ReactiveKit to complement your reactive data flows with bindings and reactive delegates and data sources.

Note: This document describes Bond v6. For changes check out the migration section!

What can it do?

Let's say you would like to act on a text change event of a UITextField. Well, you could setup 'target-action' mechanism between your object and go through all that target-action selector registration pain, or you could simply use Bond and do this:

textField.reactive.text.observeNext { text in

Now, instead of printing what the user has typed, you can bind it to a UILabel:

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

Because binding to a label text property is so common, you can even do:

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

That one line establishes a binding between text field's text property and label's text property. In effect, whenever user makes a change to the text field, that change will be automatically propagated to the label.

More often than not, direct binding is not enough. Usually you need to transform input is some way, like prepending a greeting to a name. As Bond is backed by ReactiveKit it has full confidence in functional paradigm.

  .map { "Hi " + $0 }
  .bind(to: label)

Whenever a change occurs in the text field, new value will be transformed by the closure and propagated to the label.

Notice how we've used reactive.text property of the UITextField. It's an observable representation of the text property provided by Bond framework. There are many other extensions like that one for various UIKit components. They are all placed within .reactive proxy. Just start typing .reactive. any UIKit object and you'll get the list of available extensions.

For example, to observe button events do:

  .observeNext { e in
    print("Button tapped.")

Handling touchUpInside event is used so frequently that Bond comes with the extension just for that event:

  .observe {
    print("Button tapped.")

You can use any ReactiveKit operators to transform or combine signals. Following snippet depicts how values of two text fields can be reduced to a boolean value and applied to button's enabled property.

combineLatest(emailField.reactive.text, passField.reactive.text) { email, pass in
    return email.length > 0 && pass.length > 0
  .bind(to: button.reactive.enabled)

Whenever user types something into any of these text fields, expression will be evaluated and button state updated.

Bond's power is not, however, in coupling various UI components, but in the binding of a Model (or a ViewModel) to a View and vice-versa. It's great for MVVM paradigm. Here is how one could bind user's number of followers property of the model to the label.

  .map { "\($0)" }
  .bind(to: label)

Point here is not in the simplicity of a value assignment to the text property of a label, but in the creation of a binding which automatically updates label text property whenever number of followers change.

Bond also supports two way bindings. Here is an example of how you could keep username text field and username property of your view model in sync (whenever any of them change, other one will be updated too):

  .bidirectionalBind(to: usernameTextField.reactive.text)

Bond is also great for observing various different events and asynchronous tasks. For example, you could observe a notification just like this:

  .observeNext { notification in
    print("Got \(notification)")
  .dispose(in: reactive.bag)

Let me give you one last example. Say you have an array of repositories you would like to display in a collection view. For each repository you have a name and its owner's profile photo. Of course, photo is not immediately available as it has to be downloaded, but once you get it, you want it to appear in collection view's cell. Additionally, when user does 'pull down to refresh' and your array gets new repositories, you want those in collection view too.

So how do you proceed? Well, instead of implementing a data source object, observing photo downloads with KVO and manually updating the collection view with new items, with Bond you can do all that in just few lines:

repositories.bind(to: collectionView) { array, indexPath, collectionView in
  let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! RepositoryCell
  let repository = array[indexPath.item]

    .bind(to: cell.nameLabel)
    .dispose(in: cell.onReuseBag)

    .bind(to: cell.avatarImageView)
    .dispose(in: cell.onReuseBag)

  return cell

Yes, that's right!


Observable wraps mutable state into an object that enables observation of that state. Whenever the state changes, an observer can be notified.

To create the observable, just initialize it with the initial value.

let name = Observable("Jim")

nil is valid value for observables that wrap optional type.

Observables are signals just like signals of Signal type from ReactiveKit framework. To learn more about signals, consult ReactiveKit documentation. Observables 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 observable so that snippet will print "Hi Jim!".

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

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

Observables, like signals, can be bound to views:

name.bind(to: nameLabel)

Observable is just a typealias for ReactiveKit Property type. You can use that name if it suits you better.


Binding is a connection between a Signal/Observable that produces events and a Bond that observers events and performs certain actions (e.g. updates UI).

The producing side of bindings are signals that are defined in ReactiveKit framework on top of which Bond is built. To learn more about signals, consult ReactiveKit documentation.

The consuming side of bindings is represented by the Bond type. It's a simple struct that performs an action on a given target whenever the bound signal fires an event.

public struct Bond<Element>: BindableProtocol {
  public init<Target: Deallocatable>(target: Target, setter: @escaping (Target, Element) -> Void)

The only requirement is that the target must be "deallocatable", in other words that it provides a Signal of its own deallocation.

public protocol Deallocatable: class {
  var deallocated: Signal<Void, NoError> { get }

All NSObject subclasses conform to that protocol out of the box. Let's see how we could implement a Bond for text property of a label. It's recommended to implement reactive extensions on ReactiveExtensions proxy protocol. That way you encapsulate extensions within the .reactive property.

extension ReactiveExtensions where Base: UILabel {
  var myTextBond: Bond<String?> {
    return bond { label, text in
      label.text = text

That's it! To bind any string signal, just use bind(to:) method on that bond.

let name: Signal<String, NoError> = ...
name.bind(to: nameLabel.reactive.myTextBond)

Bonds will automatically ensure that the target object is updated on the main thread (queue). That means that the signal can generate events on a background thread without you worrying how the UI will be updated - it will always happen on the main thread.

Note that you can bind only non-failable signals, i.e. signals with NoError error type. Only those kind of signals are safe to represent the data that UI displays.

Bindings will automatically dispose themselves (i.e. cancel source signals) when the binding target gets deallocated. For example, if we do

blurredImage().bind(to: imageView)

then the image processing will be automatically cancelled when the image view gets deallocated. Isn't that cool!

Inline Bindings

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

let presentUserProfile: Signal<User, NoError> = ...

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: reactive.bag)

But that's 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 Bond provides a better way. We can create 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 a signal to the target responsible for performing side effects (in our example, to the object responsible for presenting a profile view controller). The closure you provide will be called whenever the signal emits an event with both the target and the sent element as arguments.

Reactive Delegates

Bond provides NSObject extensions that makes it easy to convert delegate pattern into signals.

First make an extension on ReactiveExtensions (where Base is defined as your type - UITableView in the following example), that provides a reactive delegate proxy:

extension ReactiveExtensions where Base: UITableView {
  public var delegate: ProtocolProxy {
    return base.protocolProxy(for: UITableViewDelegate.self, setter: NSSelectorFromString("setDelegate:"))

Note: reactive.delegate is already provided by Bond. This is an example of the implementation.

You can then convert methods of that protocol into signals:

extension UITableView {
  var selectedRow: Signal<Int, NoError> {
    return reactive.delegate.signal(for: #selector(UITableViewDelegate.tableView(_:didSelectRowAtIndexPath:))) { (subject: PublishSubject<Int, NoError>, _: UITableView, indexPath: NSIndexPath) in

Method signal(for:) takes two parameters: a selector to convert to a signal and a mapping closure that maps selector method arguments into a signal.

Now you can do:

tableView.selectedRow.observeNext { row in
  print("Tapped row at index \(row).")
}.dispose(in: reactive.bag)

Note: Protocol proxy takes up delegate slot of the object so if you also need to implement delegate methods manually, don't set tableView.delegate = x, rather set tableView.reactive.delegate.forwardTo = x.

Protocol methods that return values are usually used to query data. Such methods can be set up to be fed from a property type. For example:

let numberOfItems = Property(12)

  property: numberOfItems,
  to: #selector(UITableViewDataSource.tableView(_:numberOfRowsInSection:)),
  map: { (value: Int, _: UITableView, _: Int) -> Int in value }

Method feed takes three parameters: a property to feed from, a selector, and a mapping closure that maps from the property value and selector method arguments to the selector method return value.

You should not set more that one feed property per selector.

Note that in the mapping closures of both signal(for:) and feed methods you must be explicit about argument and return types. Also, you must use ObjC types as this is ObjC API. For example, use NSString instead of String.

Reactive Data Sources

Bond provides a way to make reactive data sources and allows such sources to be easily bound to table or collection views.

Any signal that emits elements of the following type can be bound to a table or collection view

public struct DataSourceEvent<DataSource: DataSourceProtocol>: DataSourceEventProtocol {
  public let kind: DataSourceEventKind
  public let dataSource: DataSource

where the data source is any object conforming to DataSource protocol

public protocol DataSourceProtocol {
  func numberOfSections() -> Int
  func numberOfItems(inSection section: Int) -> Int

and kind is a case of the enumeration DataSourceEventKind:

public enum DataSourceEventKind {
  case reload

  case insertRows([IndexPath])
  case deleteRows([IndexPath])
  case reloadRows([IndexPath])
  case moveRow(IndexPath, IndexPath)

  case insertSections(IndexSet)
  case deleteSections(IndexSet)
  case reloadSections(IndexSet)
  case moveSection(Int, Int)

  case beginUpdates
  case endUpdates

If you have a signal that emits an array of elements you can bind it to a table view.

let places = SafeSignal.just(["London", "Berlin", "Copenhagen"])

places.bind(to: tableView) { places, indexPath, tableView in
  let cell = tableView.dequeueCell(withIdentifier: "Cell", for: indexPath) as! PlaceCell
  cell.place = places[indexPath.row]
  return cell

Whenever the signal emits new array, it will be mapped to a .reload event and cause the table view to update. To get fine-grained changes, you should use better data source.

ObservableArray / MutableObservableArray

When working with arrays, it's usually not enough to know only that the array has changed, but how exactly did it change. New elements could have been inserted into the array and old ones deleted or updated. Bond provides mechanisms for observing such fine-grained changes.

Creating a Signal/Observable/Property of arrays enables observation of the change of the array as whole, but to observe fine-grained changes Bond provides you with the ObservableArray type. Just like the Observable, it is a type conforming to SignalProtocol, but instead of sending events that match the wrapped value type, it sends events of the ObservableArrayChange type that actually conforms to DataSourceEventProtocol. Such event contains both the array itself (the data source) and the change that was just applied to the array (like element insertion or deletion).

To create observable array, just initialize it with the initial array.

let names = MutableObservableArray(["Steve", "Tim"])

When observing observable array, events you receive will contain detailed description of changes that happened.

names.observeNext { e in
  print("array: \(e.source), change: \(e.change)")

You work with the observable array like you'd work with the array it encapsulates.

names.append("John") // prints: array ["Steve", "Tim", "John"], change: .inserts([2])
names.removeLast()   // prints: array ["Steve", "Tim"], change: .deletes([2])
names[1] = "Mark"    // prints: array ["Steve", "Mark"], change: .updates([1])

Observable array can be mapped or filtered. For example, if we map our array

names.map { $0.characters.count }.observeNext { event in
  print("array: \(e.source), change: \(e.change)")

then modifying it

names.append("Tony") // prints: array [5, 3, 4], change: .inserts([2])

gives us fine-grained notification of mapped array changes.

Mapping and filtering arrays operates on an array signal. To get the result back as an observable array, just bind it to an instance of ObservableArray.

let nameLengths = ObservableArray<Int>()
names.map { $0.characters.count }.bind(to: nameLengths)

Such features enable us to build powerful UI bindings. Observable array can be bound to UITableView or UICollectionView. Just provide a closure that creates cells to the bind(to:) method.

let posts: ObservableArray <[Post]> = ...

posts.bind(to: tableView) { posts, indexPath, tableView in
  let cell = tableView.dequeueCell(withIdentifier: "PostCell", for: indexPath) as! PostCell
  cell.post = posts[indexPath.row]
  return cell

Subsequent changes done to the posts array will then be automatically reflected in the table view.

Array diff

When you need to replace an array with another array, but need an event that contains fine-grained changes (for example to update table/collection view with nice animations), you can use method replace(with:performDiff:). Let's say you have

let numbers: MutableObservableArray([1, 2, 3])

and you do

numbers.replace(with: [0, 1, 3, 4], performDiff: true)

then the row at index path 1 would be deleted and new rows would be inserted at index paths 0 and 3. The view would automatically animate only the changes from the merge. Helpful, isn't it.

Observable2DArray / MutableObservable2DArray

Array is often not enough. Usually our data is grouped into sections. To enable such use case, Bond provides two-dimensional arrays that can be observed and bound to table or collection views.

Let's explain this type by example. First we'll need some sections. A section represents a group of items. Those items, i.e. section can have a metadata associated with it. In iOS it's useful to display section header and footer titles to the user so let's define that as our metadata:

typealias SectionMetadata = (header: String, footer: String)

If you need only, for example, header title, then you don't need to define separate type. Just use String instead of SectionMetadata in examples that follow.

Now that we have defined our metadata type, we can create a section:

let cities = Observable2DArraySection<SectionMetadata, String>(
  metadata: (header: "Cities", footer: "That's it"),
  items: ["Paris", "Berlin"]

Section is defined with Observable2DArraySection type. It's generic over its metadata type and type of the items it can contain. To create a section we passed section metadata and section items.

We can now create an observable 2D array. Let's create mutable variant so we can later modify it.

let array = MutableObservable2DArray([cities])

You just pass it an array of sections. Such array can be bound to a table or collection view. You can bind it the same way as you would bind ObservableArray. However, if you want to display header and/or footer titles, you'll need to define TableViewBond object.

struct MyBond: TableViewBond {

  func cellForRow(at indexPath: IndexPath, tableView: UITableView, dataSource: Observable2DArray<SectionMetadata, String>) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "Cell", for: indexPath)
    cell.textLabel?.text = array[indexPath]
    return cell

  func titleForHeader(in section: Int, dataSource: Observable2DArray<SectionMetadata, String>) -> String? {
    return dataSource[section].metadata.header

  func titleForFooter(in section: Int, dataSource: Observable2DArray<SectionMetadata, String>) -> String? {
    return dataSource[section].metadata.footer

Only the method cellForRow:at:tableView: is required. Other two are optional and are used when we want to show header and/or footer titles.

Method cellForRow:at:tableView: describes how cells are instantiated (dequeued) and filled with data. Method titleForHeader/Footer just reads section metadata from the data source object and returns it.

If you don't need to display header and/or footer titles, you don't need to create TableViewBond type. Just bind Observable2DArray as you would bind ObservableArray as described in the previous section of this document.

Now that we have a table view bond type, you can bind our array to the table view:

array.bind(to: tableView, using: MyBond())

We just pass it an instance of table view bond type.

And that's it. If you run that code you'll see a table view with one section that has header and footer and two items.

If you now modify the array like

array.appendItem("Copenhagen", toSection: 0)

the new item will automatically be inserted and animated into the table view.

You can also, for example; add another section:

let countries = Observable2DArraySection<SectionMetadata, String>(metadata: ("Countries", "No more..."), items: ["France", "Croatia"])

There are many other methods. Just look at the code reference or source.


  • iOS 8.0+ / macOS 10.9+ / tvOS 9.0+
  • Xcode 8


  • If you'd like to ask a general question, use Stack Overflow.
  • If you'd like to ask a quick question or chat about the project, try Gitter.
  • If you found a bug, open an issue.
  • If you have a feature request, open an issue.
  • If you want to contribute, submit a pull request (include unit tests).



  1. Add the following to your Cartfile:
    github "ReactiveKit/Bond" "master"
  2. Run carthage update
  3. Add the framework as described in Carthage Readme


  1. Add the following to your Podfile:
    pod 'Bond', '~> 6.0-beta'
  2. Run pod install.


Migration from v5.x to v6.x

  • Extensions are moved into reactive proxy. Replace occurrences of bnd_ with reactive.. For example label.bnd_text becomes label.reactive.text. The simplest way is to do Search and Replace in Xcode across the project.
  • Bond<Target, Element> becomes Bond<Element>.
  • DynamicSubject<Target, Element> becomes DynamicSubject<Element>.
  • disposeIn() is deprecated in favour of dispose(in:).

Migration from v4.x to v5.x

There are some big changes in Bond v5! Bond is now backed by ReactiveKit framework. All reactive types have been moved down to ReactiveKit. Bond builds its infrastructure on top of ReactiveKit types, primarily on top of Signal that serves the purpose of EventProducer.

Bindings have been improved and simplified. It gives them better performances and additional uses. ObservableArray has been reimplemented and significantly simplified and optimised. New features are introduced: reactive delegates and reactive data sources.

What that means for you? Well, nothing has changed conceptually so your migration should be easy. Following is a list of changes:

  • EventProducer is removed. Use Signal from ReactiveKit for reactive programming.
  • Operator deliverOn is renamed to observeOn.
  • Method bindTo is renamed to bind(to:).
  • Method observe is renamed to observeNext.
  • ObservableArray is reimplemented. Mapping and filtering it is not supported any more.
  • ObservableArray is now immutable. Use MutableObservableArray instead.
  • Table view and collection view binding closure now has the data source as first argument and the index path as second argument.
  • KVO can now be established using the method dynamic(keyPath:ofType:) on any NSObject subclass.
  • Queue is removed. Use DispatchQueue instead.


The MIT License (MIT)

Copyright (c) 2015-2017 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.