将 SwiftUI 按钮绑定到 AnySubscriber，例如 RxCocoa 的按钮点击

Question

我使用以下基于UIViewController 和RxSwift/RxCocoa 的代码编写了一个非常简单的MVVM 模式来绑定UIButton 点击事件来触发一些Observable 工作并监听结果：

import UIKit
import RxSwift
import RxCocoa

class ViewController: UIViewController {

    @IBOutlet weak var someButton: UIButton!

    var viewModel: ViewModel!
    private var disposeBag = DisposeBag()

    override func viewDidLoad() {
        super.viewDidLoad()
        viewModel = ViewModel()
        setupBindings()
    }

    private func setupBindings() {
        someButton.rx.tap
        .bind(to: self.viewModel.input.trigger)
        .disposed(by: disposeBag)

        viewModel.output.result
            .subscribe(onNext: { element in
            print("element is \(element)")
            }).disposed(by: disposeBag)
    }
}

class ViewModel {

    struct Input {
        let trigger: AnyObserver<Void>
    }

    struct Output {
        let result: Observable<String>
    }

    let input: Input
    let output: Output

    private let triggerSubject = PublishSubject<Void>()

    init() {
        self.input = Input(trigger: triggerSubject.asObserver())
        let resultObservable = triggerSubject.flatMap { Observable.just("TEST") }
        self.output = Output(result: resultObservable)
    }
}

它编译并运行良好。但是，我需要用SwiftUI 来Combinify 这个模式，所以我将该代码转换为以下代码：

import SwiftUI
import Combine

struct ContentView: View {
    var viewModel: ViewModel
    var subscriptions = Set<AnyCancellable>()

    init(viewModel: ViewModel) {
        self.viewModel = viewModel
        setupBindings()
    }

    var body: some View {

        Button(action: {
            // <---- how to trigger viewModel's trigger from here
        }, label: {
            Text("Click Me")
        })
    }

    private func setupBindings() {
        self.viewModel.output.result.sink(receiveValue: { value in
            print("value is \(value)")
            })
            .store(in: &subscriptions) // <--- doesn't compile due to immutability of ContentView
    }
}

class ViewModel {

    struct Input {
        let trigger: AnySubscriber<Void, Never>
    }

    struct Output {
        let result: AnyPublisher<String, Never>
    }

    let input: Input
    let output: Output

    private let triggerSubject = PassthroughSubject<Void, Never>()

    init() {
        self.input = Input(trigger: AnySubscriber(triggerSubject))

        let resultPublisher = triggerSubject
            .flatMap { Just("TEST") }
            .eraseToAnyPublisher()

        self.output = Output(result: resultPublisher)
    }
}

由于两个错误（在代码中注释），此示例无法编译：

（1）问题一：如何像上面RxSwift的情况一样，从按钮的actionclosure中触发发布者的工作？

(2) 问题 2 与架构设计有关，而不是编译错误： 错误说：... Cannot pass immutable value as inout argument: 'self' is immutable ...，那是因为SwiftUI 视图是结构，它们被设计为只能通过各种绑定（@State、@ObservedObject 等...）进行更改，我有两个相关的子问题问题2：

[A]：在SwiftUI 视图中sink 发布者是否被认为是一种不好的做法？这可能需要一些解决方法来将cancellable 存储在View 的结构范围内？

[B]：就 MVVM 架构模式而言，哪个更适合 SwiftUI/Combine 项目：使用具有 [ Input[Subscribers]、Output[AnyPublishers] ] 模式的 ViewModel，或者 ObservableObject ViewModel 带有 [@Published 属性]？

Answer 1

我在理解最佳 mvvm 方法时遇到了同样的问题。 推荐也看看这个帖子Best data-binding practice in Combine + SwiftUI?

将发布我的工作示例。应该很容易转换成你想要的。

SwiftUI 视图：

struct ContentView: View {
    @State private var dataPublisher: String = "ggg"
    @State private var sliderValue: String = "0"
    @State private var buttonOutput: String = "Empty"


    let viewModel: SwiftUIViewModel
    let output: SwiftUIViewModel.Output

    init(viewModel: SwiftUIViewModel) {
        self.viewModel = viewModel
        self.output = viewModel.bind(())
    }

    var body: some View {
        VStack {
            Text(self.dataPublisher)
            Text(self.sliderValue)
            Slider(value: viewModel.$sliderBinding, in: 0...100, step: 1)
            Button(action: {
                self.viewModel.buttonBinding = ()
            }, label: {
                Text("Click Me")
            })
            Text(self.buttonOutput)
        }
        .onReceive(output.dataPublisher) { value in
            self.dataPublisher = value
        }
        .onReceive(output.slider) { (value) in
            self.sliderValue = "\(value)"
        }
        .onReceive(output.resultPublisher) { (value) in
            self.buttonOutput = value
        }
    }
}

AbstractViewModel：

protocol ViewModelProtocol {
    associatedtype Output
    associatedtype Input

    func bind(_ input: Input) -> Output
}

视图模型：

final class SwiftUIViewModel: ViewModelProtocol {
    struct Output {
        let dataPublisher: AnyPublisher<String, Never>
        let slider: AnyPublisher<Double, Never>
        let resultPublisher: AnyPublisher<String, Never>
    }

    typealias Input = Void

    @SubjectBinding var sliderBinding: Double = 0.0
    @SubjectBinding var buttonBinding: Void = ()

    func bind(_ input: Void) -> Output {
        let dataPublisher = URLSession.shared.dataTaskPublisher(for: URL(string: "https://www.google.it")!)
            .delay(for: 5.0, scheduler: DispatchQueue.main)
            .map{ "Just for testing - \($0)"}
            .replaceError(with: "An error occurred")
            .receive(on: DispatchQueue.main)
            .share()
            .eraseToAnyPublisher()

        let resultPublisher = _buttonBinding.anyPublisher()
            .dropFirst()
            .flatMap { Just("TEST") }
            .share()
            .eraseToAnyPublisher()

        return Output(dataPublisher: dataPublisher,
                      slider: _sliderBinding.anyPublisher(),
                      resultPublisher: resultPublisher)
    }
}

SubjectBinding 属性包装器：

@propertyWrapper
struct SubjectBinding<Value> {
    private let subject: CurrentValueSubject<Value, Never>

    init(wrappedValue: Value) {
        subject = CurrentValueSubject<Value, Never>(wrappedValue)
    }

    func anyPublisher() -> AnyPublisher<Value, Never> {
        return subject.eraseToAnyPublisher()
    }

    var wrappedValue: Value {
        get {
            return subject.value
        }
        set {
            subject.value = newValue
        }
    }

    var projectedValue: Binding<Value> {
        return Binding<Value>(get: { () -> Value in
            return self.subject.value
        }) { (value) in
            self.subject.value = value
        }
    }
}

Answer 2

所以我最近也想知道如何做到这一点，因为我们还没有开始在 SwiftUI 中写出视图。

我创建了一个辅助对象，它封装了从函数调用到发布者的转换。我称它为中继。

@available(iOS 13.0, *)
struct Relay<Element> {
    var call: (Element) -> Void { didCall.send }
    var publisher: AnyPublisher<Element, Never> { 
        didCall.eraseToAnyPublisher() 
    }

    // MARK: Private

    private let didCall = PassthroughSubject<Element, Never>()
}

具体来说，您可以声明一个私有中继并像这样使用它；

    Button(action: relay.call, 
    label: {
        Text("Click Me")
    })

然后你就可以随心所欲了。

relay.publisher