12102020-iOS-SimpleMath-solution-3.swift

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)
		}
	}
}