NSFileCoordinator does not work reliably across applications and share extensions

Number:rdar://18341292 Date Originated:9/15/2015
Status:Open Resolved:
Product:iOS SDK Product Version:iOS 8, last beta prior to GM
Classification: Reproducible:Yes
WWDC Session 217 indicates that NSFileCoordinator can be used for synchronizing file access. In practice, we've found it to be very unreliable and inconsistent.

Steps to Reproduce:
With extension and application processes running at the same time, modify a file in the extension via NSFileCoordinator's `
coordinateWritingItemAtURL:options:error:byAccessor:` method.

Expected Results:
Expect NSFilePresenter in the application to be alerted when writing is about to begin, and when has completed, via `relinquishPresentedItemToWriter:` and `presentedItemDidChange` respectively.

Actual Results:
These methods are either:

* Not called at all
* Only called when switching between applications
* Called, but only after a method that would cause the app to overwrite the data that the extension had just written (either `savePresentedItemChangesWithCompletionHandler:` or `relinquishPresentedItemToReader:` was called first

iOS 8 SDK, last beta prior to GM

iPhone 5S, last iOS 8 beta prior to GM


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