Swift 組合錯誤：'Publisher' 上的方法 requires.Failure'（又名'WeatherError'）和'Never' 是等效的

Question

我現在正在學習 Swift Combine，找到了非常簡單的視頻教程，但是由於某種原因，當我嘗試在 PassthroughSubject<Int, WeatherError>() 中使用我的枚舉時出現錯誤

檢查此代碼：

import Combine 

enum WeatherError: Error {
   case thingsJustHappen
   
}
let weatherPublisher = PassthroughSubject<Int, WeatherError>()


let subscriber = weatherPublisher
   .filter {$0 > 10}
   .sink { value in
       print("\(value)")
   }

weatherPublisher.send(10)
weatherPublisher.send(30)

“.filter”被突出顯示，錯誤是：

Referencing instance method 'sink(receiveValue:)' on 'Publisher' 
requires the types 'Publishers.Filter<PassthroughSubject<Int, WeatherError>>.Failure' 
(aka 'WeatherError') and 'Never' be equivalent

令人驚訝的是，這段代碼在視頻教程中有效。 我怎樣才能使我的 WeatherError 和 Never 相等？？？

Answer 1

您需要同時提供處理程序、完成處理程序和值處理程序：

let subscriber = weatherPublisher
    .filter { $0 > 10 }
    .sink(receiveCompletion: { _ in }, receiveValue: { value in
       print("\(value)")
    })

這是必需的，因為單參數sink僅適用於永遠不會失敗的發布者：

extension Publisher where Self.Failure == Never {
    /// ... many lines of documentation omitted
    public func sink(receiveValue: @escaping ((Self.Output) -> Void)) -> AnyCancellable
}

Answer 2

如果您將類型更改為Never ，它將起作用：

let weatherPublisher = PassthroughSubject<Int, Never>()

或者創建一個新的 Published 變量：

@Published var weather = 0

let weatherPublisher = PassthroughSubject<Int, WeatherError>()

let weatherSubscriber = weather
   .filter { $0 > 10 }
   .sink { print($0) }

let subscriber = weatherPublisher
   .sink { [weak self] value in
       self?.weather = value
   }

Answer 3

在 Xcode 13 & iOS 15.4 中，此代碼需要括號才能編譯。

extension Publisher where Self.Failure ==  Never {
    // because the publisher can NEVER FAIL - by design!
    public  func sink(receiveValue: @escaping ((Self.Output) -> Void)) -> AnyCancellable {  }
}