A question about dispatch_sync

[leonerd42]

In the initializer of ImageCache:

dispatch_sync(ioQueue, { () -> Void in
    self.fileManager = NSFileManager()
})

Why not use self.fileManager = NSFileManager() directly?

[onevcat]

While NSFileManager.defaultManager() is thread-safe, the file manager created by NSFileManager() is not. So there is a possibility of creating the file manager in a thread, but using it in another. An ioQueue could ensure all file IOs are in the same thread.

See the "Threading Considerations" section in NSFileManager doc.

[leonerd42]

Do you mean that self.fileManager may be used when initializing an instance of ImageCache?

[onevcat]

Hi, in fact here should be no problem to just use fileManager = NSFileManager(), or even use a defaultManager. But if you want to add some delegate later, you will need a new created file manager and consider the thread problem.

Here I prefer a new file manager since Kingfisher is a framework and should be isolated in the io operations from the "host" app, which might be using the shared file manager.