Core Data 什么时候刷新到磁盘? [英] When does Core Data flush to disk?
问题描述
我在我的应用程序中使用 Core Data，并发现了模拟器的一些奇怪行为:当我在我的应用程序中向 Core Data 添加一些数据并使用 XCode 中的停止"按钮退出模拟器时(或者只需按 Cmd-Q) 立即，下次我启动模拟器时，数据不会存储在数据库中.经过一些测试，我发现数据存储到磁盘:
I use Core Data in my application, and discovered some odd behavior of the simulator: When I add some data to Core Data in my app and quits the simulator using the "stop" button in XCode (or just press Cmd-Q) immediately, the data is not stored in the database the next time I start the simulator. After some testing, I found out, that the data is stored to disk:
- 当我按下主页按钮时.(几乎 - 如果退出速度非常快，可以在存储数据之前退出应用程序.)
- 如果我在添加数据后让应用程序运行一段时间(至少大约 20 秒).
- immediately if I press the home button. (Almost - it is possible to quit the application before data is stored if quitting very fast.)
- If I let the application run for some time (at least about 20 seconds) after adding data.
调用 [NSManagedObjectContext save:] 没有帮助.行为是一样的.
It does not help to call [NSManagedObjectContext save:]. The behavior is the same.
所以我的问题是:这种行为的原因是什么?有没有办法在退出之前强制刷新到磁盘?在设备上运行时，我是否有可能以这种方式丢失数据(就我个人而言，我没有看到任何风险，除了强制关闭，但奇怪的行为让我有点困扰)
So my question is: What is the reason for this kind of behavior? Are there any way to force flushing to disk before quitting? And are there any risk that I can loose data this way when running on a device (personally, I do not see any, except a force close, but the odd behavior bothers me a bit)
推荐答案
您可能正在使用嵌套上下文.如果你的上下文有一个父上下文，调用 -save: 只会保存到那个上下文，而不是文件系统.
You're probably using nested context's. If your context has a parent context, calling -save: will just save to that context, not to the file system.
嵌套上下文(通常)用于安全地使用后台队列上的上下文，并允许保存前台上下文而不必产生保存到文件系统的开销.保存到文件系统将在后台队列中进行，不会阻塞主线程.
Nested context are (usually) used to safely use a context on a background queue, and allowing a foreground context to save without having to incur the overhead of saving to the file system. The save to the file system will happen on a background queue and not block the main thread.
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