基于iCloud同步UserDefaults的最佳实践及冲突解决

我开发的应用将各类设置与状态存储在UserDefaults中，希望实现多设备间的自动同步。但目前尝试的所有方案都会出现最新设置丢失的问题：比如用户离线一段时间后，或装有旧版UserDefaults数据的设备启动并上传旧数据，导致持有新数据的设备下载过时信息。

我已尝试使用用于同步UserDefaults与iCloud的Swift包Zephyr、GitHub上的同类Swift文件DefaultsCloudSync，还基于DefaultsCloudSync编写了自定义类，但均出现相同问题。

希望了解其他应用的解决方式，或是否存在完美解决方案，欢迎提供建议与见解。

感谢您的帮助！

以下是我的自定义iCloud服务类（代码有待优化）：

class iCloudBackupService: NSObject {

class func manualUpload() -> Result<Void, Error> {
    do {
        let defaultsDictionary = UserDefaults.standard.dictionaryRepresentation()
        let cloudStore = NSUbiquitousKeyValueStore.default
        for (key, obj) in defaultsDictionary {
            cloudStore.set(obj, forKey: key as String)
        }
        
        // Get the keys that need to be removed
        let keysToRemove = cloudStore.dictionaryRepresentation.keys.filter { key in
            !defaultsDictionary.keys.contains(key)
        }

        // Remove the keys from the dictionary
        for key in keysToRemove {
            cloudStore.removeObject(forKey: key)
        }
        
        cloudStore.synchronize()
        let excludedUserDefaults = UserDefaults()
        excludedUserDefaults.set(Date(), forKey: "cloudBackupLastUpload")
        print("Manually Updated iCloud")
        NotificationCenter.default.removeObserver(self, name: UserDefaults.didChangeNotification, object: nil);
        UserDefaults.standard.set(Date(), forKey: "icloudLastBackupDate")
        UserDefaults.standard.set("manual", forKey: "icloudLastBackupTrigger")
        NotificationCenter.default.addObserver(self, selector: #selector(self.updateiCloudFromUserDefaults(notification:)), name: UserDefaults.didChangeNotification, object: nil)
        
        return .success(())
    } catch let error {
        return .failure(error)
    }
}

class func manualRestore() -> Result<Void, Error> {
    do {
        // Your code to execute if localDate is older than or equal to cloudDate.
        stop()
        let iCloudDictionary = NSUbiquitousKeyValueStore.default.dictionaryRepresentation
        let userDefaults = UserDefaults.standard
        for (key, obj) in iCloudDictionary {
            userDefaults.set(obj, forKey: key as String)
        }
        userDefaults.synchronize()
        let excludedUserDefaults = UserDefaults()
        excludedUserDefaults.set(Date(), forKey: "cloudBackupLastLocalUpdate")
        print("Manually Updated local UserDefaults")
        if userDefaults.bool(forKey: "iCloudBackup") == true {
            start()
        }
        return .success(())
    } catch let error {
        return .failure(error)
    }
}

class func start() {
    // Note: NSUbiquitousKeyValueStoreDidChangeExternallyNotification is sent only upon a change received from iCloud, not when your app (i.e., the same instance) sets a value.
    NotificationCenter.default.addObserver(self, selector: #selector(self.updateUserDefaultsFromiCloud(notification:)), name: NSUbiquitousKeyValueStore.didChangeExternallyNotification, object: nil)
    
    NotificationCenter.default.addObserver(self, selector: #selector(self.updateiCloudFromUserDefaults(notification:)), name: UserDefaults.didChangeNotification, object: nil)
    
    print("CloudBackup service started")
    print("Enabled automatic synchronization of NSUserDefaults and iCloud.")
}

@objc class func updateUserDefaultsFromiCloud(notification: NSNotification?) {
    // Prevent loop of notifications by removing our observer before updating UserDefaults
    NotificationCenter.default.removeObserver(self, name: UserDefaults.didChangeNotification, object: nil)

    let iCloudDictionary = NSUbiquitousKeyValueStore.default.dictionaryRepresentation
    let userDefaults = UserDefaults.standard

    // Update UserDefaults with values from iCloud
    for (key, obj) in iCloudDictionary {
        userDefaults.set(obj, forKey: key as String)
    }
    userDefaults.synchronize()

    // Increment iCloud session automatic download times
    iCloudSessionAutomaticDownloadTimes += 1

    print("iCloudBackupService: Automatic Download. Times this Session: \(iCloudSessionAutomaticDownloadTimes)")

    // Re-enable UserDefaultsDidChangeNotification notifications
    NotificationCenter.default.addObserver(self, selector: #selector(self.updateUserDefaultsFromiCloud(notification:)), name: UserDefaults.didChangeNotification, object: nil)
}


@objc class func updateiCloudFromUserDefaults(notification: NSNotification?) {
    // Remove the observer before adding it again
    
    NotificationCenter.default.removeObserver(self, name: UserDefaults.didChangeNotification, object: nil)
    
    let defaultsDictionary = UserDefaults.standard.dictionaryRepresentation()
    let cloudStore = NSUbiquitousKeyValueStore.default
    var printKey = ""
    var keyIsRemovable = false
    
    
    for (key, obj) in defaultsDictionary {
        cloudStore.set(obj, forKey: key)
        printKey = key
        keyIsRemovable = false
    }

    // Get the keys that need to be removed
    let keysToRemove = cloudStore.dictionaryRepresentation.keys.filter { key in
        !defaultsDictionary.keys.contains(key)
    }

    // Remove the keys from the dictionary
    for key in keysToRemove {
        cloudStore.removeObject(forKey: key)
        printKey = key
        keyIsRemovable = true
    }

    // let iCloud know that new or updated keys, values are ready to be uploaded
    cloudStore.synchronize()

    // Increment iCloud session automatic upload times
    iCloudSessionAutomaticUploadTimes += 1

    print("iCloudBackupService: Automatic Upload. Times this Session: \(iCloudSessionAutomaticUploadTimes) Current Object: \(printKey). KeyIsRemovable: \(keyIsRemovable)")


    // Add the observer again
    NotificationCenter.default.addObserver(self, selector: #selector(self.updateiCloudFromUserDefaults(notification:)), name: UserDefaults.didChangeNotification, object: nil)
}

class func stop() {
    NotificationCenter.default.removeObserver(self, name: UserDefaults.didChangeNotification, object: nil);
    NotificationCenter.default.removeObserver(self, name: NSUbiquitousKeyValueStore.didChangeExternallyNotification, object: nil);
    print("CloudBackup service stopped")
}

deinit {
    NotificationCenter.default.removeObserver(self, name: UserDefaults.didChangeNotification, object: nil);
    NotificationCenter.default.removeObserver(self, name: NSUbiquitousKeyValueStore.didChangeExternallyNotification, object: nil);
}
}

解决方案建议 #

1. 为每个设置项添加时间戳 #

核心是避免盲目的全量覆盖，给每个UserDefaults键值对附加最后修改时间戳：

• 本地更新设置时，同时更新对应键的[key]_timestamp值为当前时间
• 从iCloud拉取数据时，对每个键对比本地与云端时间戳，仅保留时间戳更新的版本
• 上传本地数据到iCloud时，先对比云端时间戳，只有本地更新时才上传

2. 区分首次同步与增量同步 #

• 首次启动应用时，优先拉取云端数据（若存在），避免旧本地数据覆盖云端
• 日常同步采用增量方式，仅同步有变化的键，而非全量替换整个UserDefaults

3. 修复当前全量覆盖的逻辑问题 #

你的自定义类中updateUserDefaultsFromiCloud方法直接用云端所有数据覆盖本地，这是旧数据覆盖新数据的主要原因。修改方向：

• 遍历云端每个键，先获取本地对应的时间戳，仅当云端时间戳更新时才覆盖本地值
• 避免直接删除云端不存在的本地键，除非是用户主动删除的设置

4. 利用NSUbiquitousKeyValueStore的变更通知细节 #

处理NSUbiquitousKeyValueStore.didChangeExternallyNotification时，通过通知userInfo获取变更的具体键，只同步发生变化的键：

if let changedKeys = notification.userInfo?[NSUbiquitousKeyValueStoreChangedKeysKey] as? [String] {
    for key in changedKeys {
        // 对比时间戳后决定是否更新本地
    }
}

5. 增加冲突处理机制 #

对于无法通过时间戳判断的冲突（如两台设备同时修改同一设置）：

• 保留双方版本，让用户手动选择
• 针对不同设置类型制定规则（比如数值类型取最大值，文本类型保留最新修改的）

内容的提问来源于stack exchange，提问作者Victor Lobe