When does Core Data flush to disk?

Question

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:

• 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.

It does not help to call [NSManagedObjectContext save:]. The behavior is the same.

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)

Answer 1

Stopping the simulator with the stop button in Xcode is probably the same as smashing your iPhone with a hammer (but preserving the SSD). Basically, your app and everything else just stops. There's no reasonable way to ensure that data is saved when that happens.

Answer 2

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.

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.