ObservablePublisher.swift

import Combine
import Foundation

class ObservablePublisher<T, U: Error>: ObservableObject {
    var result: Result<T, U>?
    
    var value: T? {
        if let result = self.result, case .success(let value) = result {
            return value
        }
        else {
            return nil
        }
    }
    var error: U? {
        if let result = self.result, case .failure(let error) = result {
            return error
        }
        else {
            return nil
        }
    }
    
    var cancellable: AnyCancellable?
    
    init<P: Publisher>(publisher: P) where P.Output == T, P.Failure == U {
        cancellable = publisher
            .receive(on: DispatchQueue.main)
            .sink { (completion) in
                guard case .failure(let error) = completion else {
                    return
                }

                self.result = .failure(error)
                self.objectWillChange.send()
            } receiveValue: { (value) in
                self.result = .success(value)
                self.objectWillChange.send()
            }
    }
}

extension Publisher {
    var asObservableObject: ObservablePublisher<Output, Failure> {
        ObservablePublisher(publisher: self)
    }
}