如何“转发”@Published值

Question

我对SwiftUI和Combine非常陌生，尽管我在使用Swift和ReactiveKit方面有很多经验，但我发现很难让一些基本的东西正常工作。

例如，我正试图在我的ViewModel上添加一个isLoggedIn属性，它应该只是“转发”UserManager类的isLoggedIn属性。在ReactiveKit中，这是相当琐碎的，但在SwiftUI/Combine中，我无法获得工作。该值只设置一次，然后再也不会更新。

class UserManager: ObservableObject {
  @Published private(set) var isLoggedIn = false

  // This class has all the actual logic for logging in, 
  // keeping track of the logged in user and the auth status, etc.
}

class ViewModel: ObservableObject {
  @Published var isLoggedIn = false

  private let userManager: UserManager

  init(userManager: UserManager) {
    self.userManager = userManager
    isLoggedIn = userManager.isLoggedIn // <- this doesn't work
    userManager.$isLoggedIn.assign(to: \.isLoggedIn, on: self) // <- neither does this
  }

  func logout() {
    userManager.logout()
  }
}

struct ContentView: View  {
  @ObservedObject var viewModel: ViewModel

  var body: some View {
    // this will use viewModel.isLoggedIn at some point
  }
}

Answer 1

下面的方法应该是可行的。如果你不存储订阅者，它会自动取消。

  private var subscribers = [AnyCancellable]()
  init(userManager: UserManager) {
    self.userManager = userManager
    userManager.$isLoggedIn
       .assign(to: \.isLoggedIn, on: self)
       .store(in: &subscribers) // << subscriber must be kept
  }

Answer 2

另一种方法是将ViewModel中的@Published var isLoggedIn替换为手动objectWillChange。

class ViewModel: ObservableObject {
  var isLoggedIn: Bool {userManager.isLoggedIn}

  private let userManager: UserManager
  private var anyCancellable: AnyCancellable?

  init(userManager: UserManager) {
    self.userManager = userManager

    anyCancellable = userManager.objectWillChange.sink {[weak self] _ in
      self?.objectWillChange.send()
    }
  }

}