iOS IPC via NSFileCoordinator and NSFilePresenter

Source：http://mjtsai.com/blog/2014/11/21/ios-ipc-via-nsfilecoordinator-and-nsfilepresenter/

为什么每次我想到的东西，别人都已经研究了。⊙﹏⊙b汗

Tom Harrington:

There’s still no full IPC mechanism on iOS. NSDistributedNotification hasn’t made the jump from OS X to iOS and probably never will. But file coordination and presentation can serve the same purpose, as long as the apps use the same app group.

When I was adding file coordination and presentation to my demo app, I realized that they could also be used for notifications between an app and its extensions. If one of them does a coordinated write while the other is using a file presenter for the file, the call topresentedItemDidChange happens almost instantly. Notification is the whole purpose of that method, so it makes sense it would work this way. I want to be notified if a specific file changes, and that’s how I get the notification.

But you don’t need to care about the file contents to be interested in notifications. If you just want a notification, choose a file name and use it as the notification mechanism. Any time one process needs to notify the other, make a change to the file. The other will get a file presenter call, and the notification is complete. It feels sort of like a hack but really this is exactly how the API is designed to work.

Update (2014-11-22): Apple TN2408 (via JanApotheker):

Using file coordination in an app extension to access a container shared with its containing app may result in a deadlock. This is usually the case if a process is suspended mid coordinated I/O. This can be more prevalent on iOS where most apps will be suspended after a short period of time after being moved to the background. Extensions should use alternatives to file coordination.

[…]

Regardless of this issue, the containing app (and all applications) should properly use background task assertions around file operations they require completed in a shared container (with or without extensions). This includes all writes or deletions. Such a process might still be killed by jetsam but at a much lower frequency.