12102020-iOS-SimpleMath-Solution.swift

//  SimpleMath
//  Copyright (c) Filip Lazov 2020
//  MIT license - see LICENSE file for more info

import Foundation

final class UserDefaultsStorage: Storage {
	
  let modelVersion: String
  let key: String

  init(withKey: String, modelVersion: String) {
    key = withKey
    self.modelVersion = modelVersion
  }

	// Saving Settings bundle store, from cache.
  func store(settingsBundle: SettingsBundle) {
    var data = storedMigrationData
    data.settingsBundle = settingsBundle
    save(storedData: data)
  }

	// Loading of the settings bundle
  func loadSettingsBundle() -> SettingsBundle {
    storedMigrationData.settingsBundle
  }

	// Save onboarding bundle
  func store(onboardingBundle: OnboardingBundle) {
    var data = storedMigrationData
    data.onboardingBundle = onboardingBundle
    save(storedData: data)
  }

	// Loading onboarding bundle
  func loadOnboardingBundle() -> OnboardingBundle {
    storedMigrationData.onboardingBundle
  }
	
	private var storedDataCache: MigrationData?
}

// the model version is stored if migrations are needed in the future
#warning("Can we inline this into UserDefaultsStorage or get rid of it entirely?")
private struct MigrationData: Codable {
	var modelVersion: String
	var settingsBundle: SettingsBundle
	var onboardingBundle: OnboardingBundle
}

extension UserDefaultsStorage {
	
	/// Previously stored migration data.
	///
	/// - From the cache, if available. The cache has the same lifetime as the session.
	/// - If the cache is not available, try from UserDefaults, save to the cache.
	/// - If UserDefaults doesn't have any data, then return `defaultStoreData` and save to UserDefaults.
	///
	/// - Returns: a struct describing model version, settings bundle and onboarding bundle.
	private var storedMigrationData: MigrationData {
		
		// Do we have a cache?
		guard let cache = storedDataCache else {
			
			let defaultMigrationData = MigrationData(modelVersion: modelVersion,
																							 settingsBundle: .default,
																							 onboardingBundle: .default)
			
			// We do not. Do we have the previous data in UserDefaults?
			guard let savedJSONData = UserDefaults.standard.value(forKey: key) as? Data else {
				// We don't have previous data.
				print("No data found for key \(key), generating default values")
				storedDataCache = defaultMigrationData
				save(storedData: storedDataCache!)
				return storedDataCache!
			}
			
			// We do have the previous data in UserDefaults.
			// Transform it into a JSON.
			//
			// `storedData` should be a variable because we're using it as fallback.
			var storedData = defaultMigrationData
			do {
				storedData = try JSONDecoder().decode(MigrationData.self, from: savedJSONData)
			} catch {
				print("error reading stored data \(error), generating default values")
			}
			
			storedDataCache = storedData
			return storedData
		}
		
		return cache
	}
	
	#warning("Can we make this function throw?")
	private func save(storedData: MigrationData) {
		storedDataCache = storedData
		do {
			try UserDefaults.standard.set(JSONEncoder().encode(storedData), forKey: key)
		} catch {
			print("failed saving data :", error)
		}
	}
}