Swift Combine：使用其他 Publisher 的后续 Publisher（使用 CombineLatest）不会 "fire"

Question

我正在尝试使用 SwiftUI（从 22:50 开始复制 WWDC 2019 session "Combine in Practice" https://developer.apple.com/videos/play/wwdc2019/721/ 中给出的 "Wizard School Signup"-示例（与 session).

期间使用的 UIKit 相反

我已经根据示例创建了所有发布者：validatedEMail、validatedPassword 和 validatedCredentials。虽然 validatedEMail 和 validatedPassword 工作正常，但使用 CombineLatest 消耗两个发布者的 validatedCredentials 永远不会触发

//
//  RegistrationView.swift
//
//  Created by Lars Sonchocky-Helldorf on 04.07.19.
//  Copyright © 2019 Lars Sonchocky-Helldorf. All rights reserved.
//

import SwiftUI
import Combine

struct RegistrationView : View {
    @ObjectBinding var registrationModel = RegistrationModel()

    @State private var showAlert = false
    @State private var alertTitle: String = ""
    @State private var alertMessage: String = ""

    @State private var registrationButtonDisabled = true

    @State private var validatedEMail: String = ""
    @State private var validatedPassword: String = ""

    var body: some View {
        Form {
            Section {
                TextField("Enter your EMail", text: $registrationModel.eMail)
                SecureField("Enter a Password", text: $registrationModel.password)
                SecureField("Enter the Password again", text: $registrationModel.passwordRepeat)
                Button(action: registrationButtonAction) {
                    Text("Create Account")
                }
                .disabled($registrationButtonDisabled.value)
                    .presentation($showAlert) {
                        Alert(title: Text("\(alertTitle)"), message: Text("\(alertMessage)"))
                }
                .onReceive(self.registrationModel.validatedCredentials) { newValidatedCredentials in
                    self.registrationButtonDisabled = (newValidatedCredentials == nil)
                }
            }

            Section {
                Text("Validated EMail: \(validatedEMail)")
                    .onReceive(self.registrationModel.validatedEMail) { newValidatedEMail in
                        self.validatedEMail = newValidatedEMail != nil ? newValidatedEMail! : "EMail invalid"
                }
                Text("Validated Password: \(validatedPassword)")
                    .onReceive(self.registrationModel.validatedPassword) { newValidatedPassword in
                        self.validatedPassword = newValidatedPassword != nil ? newValidatedPassword! : "Passwords to short or don't matchst"
                }
            }
        }
        .navigationBarTitle(Text("Sign Up"))
    }

    func registrationButtonAction() {
        let trimmedEMail: String = self.registrationModel.eMail.trimmingCharacters(in: .whitespaces)

        if (trimmedEMail != "" && self.registrationModel.password != "") {
            NetworkManager.sharedInstance.registerUser(NetworkManager.RegisterRequest(uid: trimmedEMail, password: self.registrationModel.password)) { (status) in
                if status == 200 {
                    self.showAlert = true
                    self.alertTitle = NSLocalizedString("Registration successful", comment: "")
                    self.alertMessage = NSLocalizedString("please verify your email and login", comment: "")
                } else if status == 400 {
                    self.showAlert = true
                    self.alertTitle = NSLocalizedString("Registration Error", comment: "")
                    self.alertMessage = NSLocalizedString("already registered", comment: "")
                } else {
                    self.showAlert = true
                    self.alertTitle = NSLocalizedString("Registration Error", comment: "")
                    self.alertMessage = NSLocalizedString("network or app error", comment: "")
                }
            }
        } else {
            self.showAlert = true
            self.alertTitle = NSLocalizedString("Registration Error", comment: "")
            self.alertMessage = NSLocalizedString("username / password empty", comment: "")
        }
    }
}

class RegistrationModel : BindableObject {
    @Published var eMail: String = ""
    @Published var password: String = ""
    @Published var passwordRepeat: String = ""

    public var didChange = PassthroughSubject<Void, Never>()

    var validatedEMail: AnyPublisher<String?, Never> {
        return $eMail
            .debounce(for: 0.5, scheduler: RunLoop.main)
            .removeDuplicates()
            .flatMap { username in
                return Future { promise in
                    self.usernameAvailable(username) { available in
                        promise(.success(available ? username : nil))
                    }
                }
        }
        .eraseToAnyPublisher()
    }

    var validatedPassword: AnyPublisher<String?, Never> {
        return Publishers.CombineLatest($password, $passwordRepeat)
            .debounce(for: 0.5, scheduler: RunLoop.main)
            .map { password, passwordRepeat in
                guard password == passwordRepeat, password.count > 5 else { return nil }
                return password
        }
        .eraseToAnyPublisher()
    }

    var validatedCredentials: AnyPublisher<(String, String)?, Never> {
        return Publishers.CombineLatest(validatedEMail, validatedPassword)
            .map { validatedEMail, validatedPassword in
                guard let eMail = validatedEMail, let password = validatedPassword else { return nil }
                return (eMail, password)
        }
        .eraseToAnyPublisher()
    }


    func usernameAvailable(_ username: String, completion: (Bool) -> Void) {
        let isValidEMailAddress: Bool = NSPredicate(format:"SELF MATCHES %@", "[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,64}").evaluate(with: username)

        completion(isValidEMailAddress)
    }
}

#if DEBUG
struct RegistrationView_Previews : PreviewProvider {
    static var previews: some View {
        RegistrationView()
    }
}
#endif

我希望在提供有效用户名（有效 E-Mail-address）和两个长度正确的匹配密码时启用表单按钮。负责这两项任务的两个发布者工作，我可以在我为调试目的添加的两个文本中的用户界面中看到 validatedEMail 和 validatedPassword。

只有第三个 Publisher（也与上面 32:20 视频中显示的代码进行比较）永远不会触发。我确实在这些发布者中设置了断点，在行的 validatedPassword 发布者中：

guard password == passwordRepeat, password.count > 5 else { return nil }

在那里停得很好，但在 validatedCredentials Publisher 中有一个类似的断点：

guard let eMail = validatedEMail, let password = validatedPassword else { return nil }

从未达到。

我做错了什么？

编辑：

为了使运行在Xcode-beta11.0 beta 4 didChange下的上述代码需要替换为willChange

Answer 1

您可能需要将其中一些发布者的验证分组到一个消费者中。有一个很酷的 playground 概述了 combine 框架，这就是他们如何做类似的 use case。在示例中，他们在同一订阅者中验证用户名和密码。在向用户名和密码发布者发布某些内容之前，订阅者不会执行。

如果您想将它们分开，那么您需要添加更多的发布者，这些发布者基本上概述了密码是否有效和用户名是否有效的状态。然后让订阅者在用户名和密码发布者都有效时监听。

Answer 2

只需更换

.debounce(for: 0.5, scheduler: RunLoop.main)

和

.throttle(for: 0.5, scheduler: RunLoop.main, latest: true)

由于发布者订阅中没有昂贵的代码，因此基本上不需要延迟处理。使用 latest: true 节流关键事件将以几乎相同的方式完成工作。

我不是 React 编程专家，无法判断背后的原因，我假设是设计选择。

Answer 3

我在这里回答了这个问题：https://forums.swift.org/t/crash-in-swiftui-app-using-combine-was-using-published-in-conjunction-with-state-in-swiftui/26628/9 非常友好和乐于助人的 Nanu Jogi，他不在 Whosebug 上。

比较直接：

添加这一行：

        .receive(on: RunLoop.main) // run on main thread

在 validatedCredentials 中，因此它看起来像这样：

var validatedCredentials: AnyPublisher<(String, String)?, Never> {
    return Publishers.CombineLatest(validatedEMail, validatedPassword)

        .receive(on: RunLoop.main) // <<—— run on main thread

        .map { validatedEMail, validatedPassword in
            print("validatedEMail: \(validatedEMail ?? "not set"), validatedPassword: \(validatedPassword ?? "not set")")

            guard let eMail = validatedEMail, let password = validatedPassword else { return nil }

            return (eMail, password)

    }
    .eraseToAnyPublisher()

这就是所有需要的。

这里再放一遍完整的代码供参考（针对 Xcode 11.0 beta 5 (11M382q) 更新）：

//
//  RegistrationView.swift
//  Combine-Beta-Feedback
//
//  Created by Lars Sonchocky-Helldorf on 09.07.19.
//  Copyright © 2019 Lars Sonchocky-Helldorf. All rights reserved.
//

import SwiftUI
import Combine

struct RegistrationView : View {
    @ObservedObject var registrationModel = RegistrationModel()

    @State private var registrationButtonDisabled = true

    @State private var validatedEMail: String = ""
    @State private var validatedPassword: String = ""

    var body: some View {
        Form {
            Section {
                TextField("Enter your EMail", text: $registrationModel.eMail)
                SecureField("Enter a Password", text: $registrationModel.password)
                SecureField("Enter the Password again", text: $registrationModel.passwordRepeat)
                Button(action: registrationButtonAction) {
                    Text("Create Account")
                }
                .disabled($registrationButtonDisabled.wrappedValue)
                    .onReceive(self.registrationModel.validatedCredentials) { newValidatedCredentials in
                        self.registrationButtonDisabled = (newValidatedCredentials == nil)
                }
            }

            Section {
                Text("Validated EMail: \(validatedEMail)")
                    .onReceive(self.registrationModel.validatedEMail) { newValidatedEMail in
                        self.validatedEMail = newValidatedEMail != nil ? newValidatedEMail! : "EMail invalid"
                }
                Text("Validated Password: \(validatedPassword)")
                    .onReceive(self.registrationModel.validatedPassword) { newValidatedPassword in
                        self.validatedPassword = newValidatedPassword != nil ? newValidatedPassword! : "Passwords to short or don't match"
                }
            }
        }
        .navigationBarTitle(Text("Sign Up"))
    }

    func registrationButtonAction() {

    }
}

class RegistrationModel : ObservableObject {

    @Published var eMail: String = ""
    @Published var password: String = ""
    @Published var passwordRepeat: String = ""

    var validatedEMail: AnyPublisher<String?, Never> {
        return $eMail
            .debounce(for: 0.5, scheduler: RunLoop.main)
            .removeDuplicates()
            .map { username in
                return Future { promise in
                    print("username: \(username)")
                    self.usernameAvailable(username) { available in
                        promise(.success(available ? username : nil))
                    }
                }
        }
        .switchToLatest()
            .eraseToAnyPublisher()
    }

    var validatedPassword: AnyPublisher<String?, Never> {
        return Publishers.CombineLatest($password, $passwordRepeat)
            .debounce(for: 0.5, scheduler: RunLoop.main)
            .map { password, passwordRepeat in
                print("password: \(password), passwordRepeat: \(passwordRepeat)")
                guard password == passwordRepeat, password.count > 5 else { return nil }
                return password
        }
        .eraseToAnyPublisher()
    }

    var validatedCredentials: AnyPublisher<(String, String)?, Never> {
        return Publishers.CombineLatest(validatedEMail, validatedPassword)
            .receive(on: RunLoop.main)
            .map { validatedEMail, validatedPassword in
                print("validatedEMail: \(validatedEMail ?? "not set"), validatedPassword: \(validatedPassword ?? "not set")")
                guard let eMail = validatedEMail, let password = validatedPassword else { return nil }
                return (eMail, password)
        }
        .eraseToAnyPublisher()
    }


    func usernameAvailable(_ username: String, completion: (Bool) -> Void) {
        let isValidEMailAddress: Bool = NSPredicate(format:"SELF MATCHES %@", "[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,64}").evaluate(with: username)

        completion(isValidEMailAddress)
    }
}

#if DEBUG
struct RegistrationView_Previews : PreviewProvider {
    static var previews: some View {
        RegistrationView()
    }
}
#endif

Swift Combine：使用其他 Publisher 的后续 Publisher（使用 CombineLatest）不会 "fire"

Swift Combine: subsequent Publisher that consumes other Publishers (using CombineLatest) doesn't "fire"

swift

swiftui

ios13

combine