Bindings

Bindings

Unidirectional binding operators and reactive extensions for Cocoa classes.

The operators

Bindings provides two operators: <~, the input binding operator, and ~>, the output binding operator.

Input bindings update the state of your UI

import Bindings
import UIKitBindings

nameLabel.reactive.text <~ viewModel.fullName

Output bindings respond to changes

nameField.reactive.edited ~> viewModel.setName
UIApplication.reactive.didBecomeActiveNotification ~> viewModel.refresh

Common bindings for system classes

In addition to the core operator definitions, Bindings also provides conveniences for using bindings with many system classes. Take a look at the following modules to see what's provided (and consider [contributing][CONTRIBUTING]!):

• UIKitBindings for apps using UIKit (iOS, iPadOS, Catalyst and more)
• More to come...

Reactive extensions

It's convenient to group reactive extensions into their own namespace, especially when extending types you don't own with reactive APIs.

Use the ReactiveExtensionProvider protocol to define the .reactive property on your types:

extension MyViewModel: ReactiveExtensionProvider {}

Then add reactive extensions via the Reactive type:

extension UserDefaults {
  var username: String? {
    get { string(forKey: "username") }
    set { set(newValue, forKey: "username") }
  }
}

extension Reactive where Base: UserDefaults {
  var username: NSObject.KeyValueObservingPublisher<Base, String?> {
    base.publisher(for: \.username)
  }
}

UserDefaults.standard.reactive.username.sink { username in
  print("New username: \(username)")
}

Bindings last for the lifetime of their owner

In Combine, the AnyCancellable class controls the lifetime of a subscription. Subscriptions can be automatically cancelled by using the store(in:) method:

import UIKit

class MyViewController: UIViewController {
  @IBOutlet private var usernameLabel: UILabel!
  private var subscriptions: [AnyCancellable] = []

  override func viewDidLoad() {
    super.viewDidLoad()

    UserDefaults.standard.reactive.username
      .sink { [usernameLabel] in usernameLabel?.text = $0 }
      .store(in: &subscriptions)
  }
}

Bindings encapsulates this pattern with the BindingOwner protocol, which automatically defines a subscriptions collection on all conforming classes. The BindingSink subscriber then automatically disposes of the binding subscription when its owner goes out of scope:

extension Reactive where Base: UILabel {
  var text: BindingSink<Base, String?> {
    BindingSink(owner: base) { label, newValue in
      label.text = newValue
    }
  }
}

// automatically cancelled when `usernameLabel` goes out of scope
usernameLabel.reactive.text <~ UserDefaults.standard.reactive.username