ReactiveViewModel.swift

//
//  ReactiveViewModel.swift
//  MVRxVM
//
//  Created by Cristhian Leon on 2022.
//

import Foundation
import Combine

protocol ReactiveViewModel {
    /// Makes reference to any view state the view model uses.
    associatedtype ViewState
    
    /// Since Swift still doens't support wrappingProperties in protocols,
    /// this property is required to implement. This won't affect the behavior,
    /// rather just satisfies the compiler.
    var viewState: Published<ViewState>.Publisher { get }
    
    /// Allows view models be able to map the state into properties,
    /// remove duplicates, and receive events on the main queue.
    /// This is to force view to run on sink on UI queues.
    func stateOf<T: Equatable>(_ transform: @escaping (ViewState) -> T) -> AnyPublisher<T, Never>
}

extension ReactiveViewModel {
    func stateOf<T: Equatable>(_ transform: @escaping (ViewState) -> T) -> AnyPublisher<T, Never> {
        viewState
        // convert the whole state into a property of it.
            .map(transform)
        // Avoids the view to react multiple times when the value hasn't changed.
            .removeDuplicates()
        // since this is called ONLY from the UI, receveing on main queue is required.
            .receive(on: DispatchQueue.main)
        // convert it to any.
            .eraseToAnyPublisher()
    }
}