Data loading using protocols

DataLoading.swift

import UIKit
import PlaygroundSupport

enum CoreError: Error, LocalizedError {
    case unknown

    var errorDescription: String? {
        switch self {
        case .unknown:
            return "Unknown error occured"
        }
    }
}

enum ViewState<Content> {
    case loading
    case loaded(data: Content)
    case failure(error: Error)
}

class LoadingView: UIView {}

class ErrorView: UIView {
    func show(error: Error) {
        // show the error
        print("Got error: \(error.localizedDescription)")
    }
}

protocol DataLoading {
    associatedtype Data

    var state: ViewState<Data> { get set }
    var loadingView: LoadingView { get }
    var errorView: ErrorView { get }

    func update()
}

extension DataLoading where Self: UIView {
    func update() {
        switch state {
        case .loading:
            loadingView.isHidden = false
            errorView.isHidden = true
        case .failure(let error):
            loadingView.isHidden = true
            errorView.isHidden = false
            errorView.show(error: error)
        case .loaded(_):
            loadingView.isHidden = true
            errorView.isHidden = true
        }
    }
}

class SimpleView: UIView, DataLoading {
    let loadingView: LoadingView = LoadingView()
    let errorView: ErrorView = ErrorView()

    var state: ViewState<String> = .loading {
        didSet {
            update()
            guard case .loaded(let data) = state else { return }
            print(data)
        }
    }

    override init(frame: CGRect) {
        super.init(frame: frame)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

let view = SimpleView(frame: .zero)
PlaygroundPage.current.needsIndefiniteExecution = true
PlaygroundPage.current.liveView = view

view.state = .loaded(data: "Data loaded!")
view.state = .failure(error: CoreError.unknown)
PlaygroundPage.current.finishExecution()