Class to handle CloudKit server change tokens

ServerChangeTokenCache.swift

import CloudKit

final class ChangeTokenCache {

    // MARK: - Zone support

    /// Determines a unique UserDefaults key for storing zone change tokens
    /// like from CKFetchRecordZoneChangesOperation
    private static func userDefaultsKey(_ zoneID: CKRecordZone.ID) -> String {
        return "token_" + zoneID.ownerName + zoneID.zoneName
    }

    /// Persists a zone change token in standard UserDefaults
    static func setZoneToken(_ zoneID: CKRecordZone.ID, token: CKServerChangeToken?) {
        setToken(userDefaultsKey(zoneID), token: token)
    }

    /// Fetches a zone change token from standard UserDefaults
    static func getZoneToken(_ zoneID: CKRecordZone.ID) -> CKServerChangeToken? {
        return getToken(userDefaultsKey(zoneID))
    }
    
    /// Clears a zone change token from standard UserDefaults
    static func clearZoneToken(_ zoneID: CKRecordZone.ID) {
        UserDefaults.standard.removeObject(forKey: userDefaultsKey(zoneID))
    }


    // MARK: - Database support

    /// Determines a unique UserDefaults key for storing database change tokens
    /// like from CKFetchDatabaseChangesOperation
    private static func userDefaultsKey(_ database: CKDatabase) -> String {
        return "token_db\(database.databaseScope.rawValue)"
    }

    /// Persists a database change token in standard UserDefaults
    static func setDatabaseToken(_ database: CKDatabase, token: CKServerChangeToken?) {
        setToken(userDefaultsKey(database), token: token)
    }

    /// Fetches a database change token from standard UserDefaults
    static func getDatabaseToken(_ database: CKDatabase) -> CKServerChangeToken? {
        return getToken(userDefaultsKey(database))
    }

    // MARK: - Actual workers

    /// Private method that archives a token as data and persists that to UserDefaults
    private static func setToken(_ userDefaultsKey: String, token: CKServerChangeToken?) {
        if let token = token,
           let data = try? NSKeyedArchiver.archivedData(withRootObject: token, requiringSecureCoding: false) {
            UserDefaults.standard.set(data, forKey: userDefaultsKey)
        } else {
            UserDefaults.standard.removeObject(forKey: userDefaultsKey)
        }
    }

    /// Private method that gets data from UserDefaults and unarchives it as a token
    private static func getToken(_ userDefaultsKey: String) -> CKServerChangeToken? {
        guard let data = UserDefaults.standard.value(forKey: userDefaultsKey) as? Data else { return nil }
        var token: CKServerChangeToken?
        do {
            token = try NSKeyedUnarchiver.unarchivedObject(ofClass: CKServerChangeToken.self, from: data)
        } catch {
            token = nil
        }
        return token
    }
}