如何 "forward" @Published 值

Question

我对 SwiftUI 和 Combine 很陌生，尽管我对 Swift 有很多经验，对 ReactiveKit 也有一点经验，但我发现很难获得一些基本的东西。

例如，我试图在我的 ViewModel 上添加一个 isLoggedIn 属性，它应该只是 "forward" UserManager class' isLoggedIn 属性。对于 ReactiveKit，这是相当微不足道的，但是对于 SwiftUI/Combine 我无法正常工作。该值仅设置一次，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

以下应该有效。如果您不存储订阅者，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

另一种方法是用手动 objectWillChange 替换 ViewModel 中的 @Published var isLoggedIn。

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()
    }
  }

}

如何 "forward" @Published 值

How to "forward" a @Published value

swiftui

combine