swift - RxSwift withLatestFrom怪异行为

今天，我将代码从ReactiveSwift迁移到RxSwift，并来到了这个奇怪的场景。

我在Observable类中有一个由withLatestFrom运算符组成的ViewModel，它仅在构成它时在ViewModel类的初始值设定项内进行的测试订阅上发出，而在我在ViewController中进行的订阅。

此Observable中的withLatestFrom运算符正在接受另一个Observable，该withLatestFrom也已由[!]运算符组成为其参数。

// emit phrases when viewDidLoad emits
let thePhrases = self.viewDidLoadSubject.withLatestFrom(self.configureWithPhrasesSubject)

// This is the problematic Observable
let printThePhrases = self.buttonTappedSubject.withLatestFrom(thePhrases)

这是我用来展示这种怪异行为的代码，您可以在XCode中运行它，并将调试器输出过滤器设置为ViewController，以忽略Simulator的垃圾输出：

import UIKit
import RxSwift

public final class RxTestViewModel {
    public init() {
        // emit configuredWithPhrases when viewDidLoad emits
        let configPhrases = self.viewDidLoadSubject
            .withLatestFrom(self.configureWithPhrasesSubject)
            .filter { $0 != nil }
            .map { $0! }

        // Show phrases to be printed on viewDidLoad
        self.toBePrinted = configPhrases.asObservable()

        _ = self.toBePrinted.subscribe(onNext: {
            print("[!][\(Thread.current)] -- ViewModel.toBePrinted.onNext -> \($0)")
        })

        // Print first phrase whenever buttonTapped() is called
        self.printSomething = self.buttonTappedSubject
            .withLatestFrom(self.configureWithPhrasesSubject
            .filter { $0 != nil }
            .map { $0! })

        _ = self.printSomething.subscribe(onNext: {
            print("[!][\(Thread.current)] -- ViewModel.printSomething.onNext -> \($0)")
        })
    }

    // MARK: Inputs
    private let configureWithPhrasesSubject = BehaviorSubject<[String]?>(value: nil)
    public func configureWith(phrases: [String]) {
        print("[!][\(Thread.current)] -- ViewModel.configureWith")
        self.configureWithPhrasesSubject.on(.next(phrases))
    }

    private let viewDidLoadSubject = PublishSubject<Void>()
    public func viewDidLoad() {
        print("[!][\(Thread.current)] -- ViewModel.viewDidLoad")
        self.viewDidLoadSubject.on(.next( () ))
    }

    private let buttonTappedSubject = PublishSubject<Void>()
    public func buttonTapped() {
        print("[!][\(Thread.current)] -- ViewModel.buttonTapped")
        self.buttonTappedSubject.on(.next( () ))
    }

    // MARK: Outputs
    public let printSomething: Observable<[String]>
    public let toBePrinted: Observable<[String]>
}

public final class RxTestViewController: UIViewController {

    private let button: UIButton = UIButton()
    private let viewModel: RxTestViewModel = RxTestViewModel()

    public static func instantiate() -> RxTestViewController {
        let vc = RxTestViewController()
            vc.viewModel.configureWith(phrases: ["First phrase", "Second phrase", "Third phrase"])

        return vc
    }
}

extension RxTestViewController {
    public override func viewDidLoad() {
        super.viewDidLoad()

        self.setupButton()
        self.setupViewModel()

        self.viewModel.viewDidLoad()
    }
}

extension RxTestViewController {
    private func setupViewModel() {
        _ = self.viewModel.toBePrinted
            .subscribeOn(ConcurrentMainScheduler.instance)
            .subscribe(onNext: {
                print("[!][\(Thread.current)] -- RxTestViewController.toBePrinted.onNext -> \($0)")
                self.viewModel.buttonTapped()
            })

        _ = self.viewModel.printSomething
            .subscribeOn(ConcurrentMainScheduler.instance)
            .subscribe(onNext: {
                print("[!][\(Thread.current)] -- RxTestViewController.printSomething.onNext -> \($0)")
        })
    }
}

extension RxTestViewController {
    private func setupButton() {
        // Add to view
        self.view.addSubview(self.button)

        // Button config
        self.button.setTitle("CLICK ME", for: .normal)
        self.button.addTarget(self, action: #selector(buttonTapped), for: .touchUpInside)

        // Auto-layout
        self.button.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            self.button.centerXAnchor.constraint(equalTo: self.view.centerXAnchor),
            self.button.centerYAnchor.constraint(equalTo: self.view.centerYAnchor)])
    }

    @objc
    private func buttonTapped() {
        self.viewModel.buttonTapped()
    }
}

预期结果应为：

[!][<NSThread: 0x600001fee280>{number = 1, name = main}] -- ViewModel.configureWith
[!][<NSThread: 0x600001fee280>{number = 1, name = main}] -- ViewModel.viewDidLoad
[!][<NSThread: 0x600001fee280>{number = 1, name = main}] -- ViewModel.toBePrinted.onNext -> ["First phrase", "Second phrase", "Third phrase"]
[!][<NSThread: 0x600001fee280>{number = 1, name = main}] -- RxTestViewController.toBePrinted.onNext -> ["First phrase", "Second phrase", "Third phrase"]
[!][<NSThread: 0x600001fee280>{number = 1, name = main}] -- ViewModel.buttonTapped
[!][<NSThread: 0x600001fee280>{number = 1, name = main}] -- ViewModel.printSomething.onNext -> ["First phrase", "Second phrase", "Third phrase"]
[!][<NSThread: 0x600001fee280>{number = 1, name = main}] -- RxTestViewController.printSomething.onNext -> ["First phrase", "Second phrase", "Third phrase"]

但是我得到了：

[!][<NSThread: 0x600001fee280>{number = 1, name = main}] -- ViewModel.configureWith
[!][<NSThread: 0x600001fee280>{number = 1, name = main}] -- ViewModel.viewDidLoad
[!][<NSThread: 0x600001fee280>{number = 1, name = main}] -- ViewModel.toBePrinted.onNext -> ["First phrase", "Second phrase", "Third phrase"]
[!][<NSThread: 0x600001fee280>{number = 1, name = main}] -- RxTestViewController.toBePrinted.onNext -> ["First phrase", "Second phrase", "Third phrase"]
[!][<NSThread: 0x600001fee280>{number = 1, name = main}] -- ViewModel.buttonTapped
[!][<NSThread: 0x600001fee280>{number = 1, name = main}] -- ViewModel.printSomething.onNext -> ["First phrase", "Second phrase", "Third phrase"]

如您所见，观察者订阅不是在ViewModel调用的，而是在latestFrom调用的。

但是有趣的是，如果我再次调用Observable的buttonTapped()触发函数（withLatestFrom），则将按预期方式调用ViewModel订阅和ViewController订阅。

另外，如果我从configPhrases可观察链中删除toBePrinted运算符，并且仅将其添加到withLatestFrom可观察链中，则所有工作均按预期进行。

这使我认为将withLatestFrom应用于已经应用运算符的Observable会出错。

最佳答案

我没有看到您正在谈论的行为，可能是因为我提供了视图控制器而不是在测试工具中使用它？

无论如何，请记住，当您执行a.withLatestFrom(b)时，如果a在b发出任何值之前发出一个值，则运算符将过滤掉该发出。那可能是你的问题吗？