swift - 结合:不能将 `.assign` 与结构一起使用 - 为什么？

Question

在尝试使用Combine 分配值时，我看到了一些我不太理解的结构与类行为。

代码:

import Foundation
import Combine

struct Passengers {
  var women = 0
  var men = 0
}

class Controller {
  @Published var passengers = Passengers()
  var cancellables = Set<AnyCancellable>()
  let minusButtonTapPublisher: AnyPublisher<Void, Never>

  init() {
    // Of course the real code has a real publisher for button taps :)
    minusButtonTapPublisher = Empty<Void, Never>().eraseToAnyPublisher()

    // Works fine:
    minusButtonTapPublisher
      .map { self.passengers.women - 1 }
      .sink { [weak self] value in
        self?.passengers.women = value
      }.store(in: &cancellables)

    // Doesn't work:
    minusButtonTapPublisher
      .map { self.passengers.women - 1 }
      .assign(to: \.women, on: passengers)
      .store(in: &cancellables)
  }
}

我得到的错误是 Key path value type 'ReferenceWritableKeyPath<Passengers, Int>' cannot be converted to contextual type 'WritableKeyPath<Passengers, Int>' .

使用 sink 的版本而不是 assign工作正常，当我转动时 Passengers进类后，assign版本也可以正常工作。我的问题是:为什么它只适用于一个类？这两个版本(sink 和assign)最后真的做同样的事情，对吧？他们都更新了 women属性(property)在 passengers .

(当我将 Passengers 更改为一个类时，sink 版本不再有效。)

Answer 1

实际上它是明确记录的 - 将发布者中的每个元素分配给对象上的属性。这是 Assign 的功能、设计订阅者 - 仅适用于引用类型。

extension Publisher where Self.Failure == Never {

    /// Assigns each element from a Publisher to a property on an object.
    ///
    /// - Parameters:
    ///   - keyPath: The key path of the property to assign.
    ///   - object: The object on which to assign the value.
    /// - Returns: A cancellable instance; used when you end assignment of the received value. Deallocation of the result will tear down the subscription stream.
    public func assign<Root>(to keyPath: ReferenceWritableKeyPath<Root, Self.Output>, on object: Root) -> AnyCancellable
}