Two way binding in RxSwift

Question

I read the two way binding operator in sample code of RxSwift.

func <-> <T>(property: ControlProperty<T>, variable: Variable<T>) -> Disposable {
    let bindToUIDisposable = variable.asObservable()
        .bindTo(property)
    let bindToVariable = property
        .subscribe(onNext: { n in
            variable.value = n
        }, onCompleted:  {
            bindToUIDisposable.dispose()
        })

    return StableCompositeDisposable.create(bindToUIDisposable, bindToVariable)
}

When property changed, it will notify variable, and set the variable's value, while the variable's value is set, it will notify the property. I think it will lead to endless loop...

Answer 1

I believe you can just use bindTo 🙂. Here are implementations for ControlProperty <-> Variable and Variable <-> Variable:

infix operator <-> { precedence 130 associativity left }

func <-><T: Comparable>(property: ControlProperty<T>, variable: Variable<T>) -> Disposable {
    let variableToProperty = variable.asObservable()
        .distinctUntilChanged()
        .bindTo(property)

    let propertyToVariable = property
        .distinctUntilChanged()
        .bindTo(variable)

    return StableCompositeDisposable.create(variableToProperty, propertyToVariable)
}

func <-><T: Comparable>(left: Variable<T>, right: Variable<T>) -> Disposable {
    let leftToRight = left.asObservable()
        .distinctUntilChanged()
        .bindTo(right)

    let rightToLeft = right.asObservable()
        .distinctUntilChanged()
        .bindTo(left)

    return StableCompositeDisposable.create(leftToRight, rightToLeft)
}

Examples of ControlProperty <-> Variable (such as UITextField and UITextView) are in the RxSwiftPlayer project

// Example of Variable <-> Variable

let disposeBag = DisposeBag()
let var1 = Variable(1)
let var2 = Variable(2)

(var1 <-> var2).addDisposableTo(disposeBag)

var1.value = 10
print(var2.value) // 10

var2.value = 20
print(var1.value) // 20

Answer 2

Thanks for raising the question, I spent some time digging around the ControlProperty implementation (note I've added a .debug() call to trace the values generated for control property).

public struct ControlProperty<PropertyType> : ControlPropertyType {
    public typealias E = PropertyType

    let _values: Observable<PropertyType>
    let _valueSink: AnyObserver<PropertyType>

    public init<V: ObservableType, S: ObserverType where E == V.E, E == S.E>(values: V, valueSink: S) {
        _values = values.debug("Control property values").subscribeOn(ConcurrentMainScheduler.instance)
        _valueSink = valueSink.asObserver()
    }

    public func on(event: Event<E>) {
        switch event {
        case .Error(let error):
            bindingErrorToInterface(error)
        case .Next:
            _valueSink.on(event)
        case .Completed:
            _valueSink.on(event)
        }
    }
}

My test setup was as following, I've removed all views positioning here to make it shorter:

import UIKit
import RxSwift
import RxCocoa
class ViewController: UIViewController {
    let variable = Variable<Bool>(false);
    let bag = DisposeBag();

    override func loadView() {
        super.loadView()

        let aSwitch = UISwitch();
        view.addSubview(aSwitch)

        (aSwitch.rx_value <-> variable).addDisposableTo(bag);

        let button = UIButton();
        button.rx_tap.subscribeNext { [weak self] in
            self?.variable.value = true;
        }.addDisposableTo(bag)
        view.addSubview(button);
    }
 }

infix operator <-> {
}

func <-> <T>(property: ControlProperty<T>, variable: Variable<T>) -> Disposable{
    let bindToUIDisposable = variable.asObservable().debug("Variable values in bind")
    .bindTo(property)

    let bindToVariable = property
        .debug("Property values in bind")
        .subscribe(onNext: { n in
            variable.value = n
            }, onCompleted:  {
                 bindToUIDisposable.dispose()
        })

    return StableCompositeDisposable.create(bindToUIDisposable, bindToVariable)
 }

Now to the results. First we try tapping the button, which should set the variable to true. This triggers on(event: Event<E>) on ControlProperty and sets the switch value to true.

2016-05-28 12:24:33.229: Variable values in bind -> Event Next(true)

// value flow
value assigned to Variable -> 
Variable emits event -> 
ControlProperty receives event -> 
value assigned to underlying control property (e.g. `on` for `UISwitch`)

Next lets trigger the switch itself. So as we can see, the control generated an event as a result of UIControlEventValueChanged which was passed through _values on ControlProperty, and then its value got assigned to Variable value as in example above. But there's no loop, since update to the Variable value doesn't trigger a control event on the switch.

2016-05-28 12:29:01.957: Control property values -> Event Next(false)
2016-05-28 12:29:01.957: Property values in bind -> Event Next(false)
2016-05-28 12:29:01.958: Variable values in bind -> Event Next(false)

// value flow
trigger the state of control (e.g. `UISwitch`) -> 
ControlProperty emits event -> 
value assigned to Variable -> 
Variable emits event -> 
ControlProperty receives event -> 
value assigned to underlying control property (e.g. `on` for `UISwitch`)

So a simple explanation would be:

• a value from a control is emitted once some kind of UIControlEvent is triggered
• when a value is assigned directly to the control property, the control doesn't trigger a change event so there's no loop.

Hope it helps, sorry for a bit messy explanation - I've found it out by experiment)

Answer 3

You type anything it will be clear after 5 seconds. This was taken from above answer

import UIKit
import RxSwift
import RxCocoa

class UserViewModel {
    let username = BehaviorSubject<String?>(value: "")
}

class ViewController: UIViewController {

    @IBOutlet weak var email: UITextField!

    var userViewModel = UserViewModel()
    let bag = DisposeBag()

    override func viewDidLoad() {
        super.viewDidLoad()

        userViewModel.username.asObservable().subscribe { print($0) }.disposed(by: bag)
        (email.rx.text <-> userViewModel.username).disposed(by: bag)

        // clear value of the username.
        DispatchQueue.main.asyncAfter(deadline: .now()+5) {
            self.userViewModel.username.onNext(nil)
        }
    }
}

infix operator <->

@discardableResult func <-><T>(property: ControlProperty<T>, variable: BehaviorSubject<T>) -> Disposable {
    let variableToProperty = variable.asObservable()
        .bind(to: property)

    let propertyToVariable = property
        .subscribe(
            onNext: { variable.onNext($0) },
            onCompleted: { variableToProperty.dispose() }
    )

    return Disposables.create(variableToProperty, propertyToVariable)
}

Answer 4

There is no obstacle to bind BehaviourRelay back to control property. You just need to filter events with the same value (to prevent infinite loop).

For example, in my case, I need to bind email to text field. But I want to remove whitespaces during email input. Here is an example how I achieved it:

emailTextField.rx.text
  .map { $0?.trimmingCharacters(in: CharacterSet.whitespaces) } // remove whitespaces from input
  .bind(to: viewModel.email)
  .disposed(by: disposeBag)

// Just filter all events with no actual value change to prevent infinite loop
viewModel.email
  .filter { $0 != self.emailTextField.text } // if it removed whitespaces in mapping, values will not match and text in text field will be updated
  .bind(to: emailTextField.rx.text)
  .disposed(by: disposeBag)