NSFilePresenter protocol should be implemented by objects that allow the user to view or edit the content of files or directories. You use file presenters in conjunction with an
NSFileCoordinator object to coordinate access to a file or directory among the objects of your application and between your application and other processes. When changes to an item occur, the system notifies objects that adopt this protocol and gives them a chance to respond appropriately.
- iOS 5.0+
- macOS 10.7+
- tvOS 9.0+
- watchOS 2.0+
You use the methods of this protocol to respond to actions about to be taken on the presented file or directory. When another object or process uses a file coordinator to begin reading or writing a file or directory, the file coordinator notifies all presented objects interested in the item first. It notifies the presenter objects by invoking one of the methods defined by this protocol on that object. The actual invocation of that method occurs on the operation queue in the
presentedItemOperationQueue property. Your file presenter must provide this queue. If your queue supports the concurrent execution of operations, the methods of your presenter object must be thread-safe and able to run in multiple queues simultaneously.
You can use file presenters to coordinate access to a file or directory among your application’s objects. If another process uses a file coordinator for the same file or directory, your presenter objects are similarly notified whenever the other process makes its changes. Your presenter objects are not notified about changes made directly using low-level read and write calls to the file. Only changes that go through a file coordinator result in notifications.
For information about how to use file presenters with a file coordinator object, see
File Presenters and iOS
If your app enters the background with an active file presenter, any other processes that perform a coordinated read or write on the presented file can deadlock. To prevent this situation, call
presentedItemURL to remove the file presenter in the
applicationDidEnterBackground(_:) method or in response to a
UIApplicationDidEnterBackground notification. Call
presentedItemOperationQueue to add the file presenter again in the
applicationWillEnterForeground(_:) method or in response to a