1.iOS8 provided asynchronousFetchRequest，and we can also create a 'private context' to fetch results, so what is the difference between asynchronousFetchRequest and 'create a private context'?
2.The type of NSFetchedResultsController's context must be MainQueueConcurrencyType?(block the UI?) Is there any solution to resolve this?
AsynchronousFetchRequest will create another context and perform the fetch on that context. Creating a private context yourself means that you can work on managedObjects on the background context without having to block the main thread while that work is being performed. If you have your own context though your going to have to transfer your managedObjects onto the main thread yourself though, while async fetch request is already doing that. FetchedResutlsControllers do not necessarily need a context that is MainQueueConcurrency, but do remember that if its PrivateQueueConcurrency, than the cache won't work and you will need to use performBlock: method in order to work with the objects. Your UI can get blocked while fetching objects for an FRC, but it shouldn't take a long time. IF you need speed from core data index your entities first. If you want to make sure you have data before a fetch, you can use an AsynchronousFetchRequest with a countForFetchRequest to just have a number returned and act accordingly.
