问题
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...
回答1:
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
UIControlEventis 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)
回答2:
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
回答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)
}
回答4:
@dengApro's answer is very close.
The source code in UITextField+Rx.swift:
/// Reactive wrapper for `text` property.
public var value: ControlProperty<String?> {
return base.rx.controlPropertyWithDefaultEvents(
getter: { textField in
textField.text
},
setter: { textField, value in
// This check is important because setting text value always clears control state
// including marked text selection which is imporant for proper input
// when IME input method is used.
if textField.text != value {
textField.text = value
}
}
)
}
Assigning textField a value could not be subscribed, because ofcontrolPropertyWithDefaultEvents
The source code in UIControl+Rx.swift:
/// This is a separate method to better communicate to public consumers that
/// an `editingEvent` needs to fire for control property to be updated.
internal func controlPropertyWithDefaultEvents<T>(
editingEvents: UIControl.Event = [.allEditingEvents, .valueChanged],
getter: @escaping (Base) -> T,
setter: @escaping (Base, T) -> Void
) -> ControlProperty<T> {
return controlProperty(
editingEvents: editingEvents,
getter: getter,
setter: setter
)
}
So just the two events UIControl.Event = [.allEditingEvents, .valueChanged] can be observable,
Variable changed, Variable bind to ControlProperty, ControlProperty changed not because of [.allEditingEvents, .valueChanged], then done.
ControlProperty changed, ControlProperty bind to Variable, Variable changed and bind to ControlProperty,ControlProperty seted not because of [.allEditingEvents, .valueChanged], then done.
In the source code of controlProperty, will establish the UIControl target - action.
[.allEditingEvents, .valueChanged] contains of editingDidBegin, editingChanged, editingDidEnd, editingDidEndOnExit, valueChanged,
So assigning to textField.text directly will trigger no event.
回答5:
The source code in UITextField+Rx.swift:
/// Reactive wrapper for `text` property.
public var value: ControlProperty<String?> {
return base.rx.controlPropertyWithDefaultEvents(
getter: { textField in
textField.text
},
setter: { textField, value in
// This check is important because setting text value always clears control state
// including marked text selection which is imporant for proper input
// when IME input method is used.
if textField.text != value {
textField.text = value
}
}
)
}
The magic is in the setter:
if textField.text != value {
textField.text = value
}
So a ControlProperty is two way binding to a Variable,
The ControlProperty will not always change, because the if judgement in setter method.
I checked in RxSwift 5.0.1
回答6:
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)
来源：https://stackoverflow.com/questions/37496074/two-way-binding-in-rxswift