Core Data what is created with UIManagedDocument

3
votes

I am unsure about the structure of coreData and how the objects are saved in a directory. So what I know is that you create an instance of UIManagedDocument and make a URL for it and where it will save files. Then you call "SaveToURL", what exactly is created when you call this? is it the core data stack? Then when you save information into the entities you declared is a separate file created within the stack for each set of information in an entity? Finally, what exactly is a context of a UIManagedDocument.

These are the three main questions

  1. What is created when you call "SaveToURL", is it a document or a file or a stack?
  2. When you save information in entities, are separate files created within this file/stack?
  3. what is a UIManagedDocument context?
1
objective-cxcodecore-datansurluimanageddocument

1 Answers

10
votes

I strongly suggest you read Core Data Programming Guide and start with Core Data Basics Chapter.

UIManagedDocument is a special kind of document, an UIDocument subclass, that stores its data using Core Data Framework. So it combines the power of document architecture and core data capabilities.

You can read more about document based architecture from Document Based App Programming Guide for iOS and I recommend WWDC2011 Storing Documents in iCloud using iOS5 session video. I also recommend Stanford CS193P: iPad and iPhone App Development (Fall 2011) Lecture 13.

  1. What is created when you call saveToURL:forSaveOperation:completionHandler: is an implementation detail of UIManagedDocument and UIDocument and you should not really worry or depend on it. However in current implementation a folder containing an sqlite database file is being created.

  2. No. All entities will be contained in a single database file also more generally called: a persistent store. It is possible to use more than one persistent store, but those are more advanced use cases and UIManagedDocument currently uses one.

  3. UIManagedDocument's context refers to a NSManagedObjectContext from underlying Core Data Framework. UIManagedDocument actually operates two of those in parallel to spin off IO operations to a background thread. When it comes to the nature of a context itself here's a quote from Core Data Programming Guide:

    You can think of a managed object context as an intelligent scratch pad. When you fetch objects from a persistent store, you bring temporary copies onto the scratch pad where they form an object graph (or a collection of object graphs). You can then modify those objects however you like. Unless you actually save those changes, however, the persistent store remains unaltered.

But it really is a good idea to take a look at the lectures and other material I posted above to get a general picture of the technologies used and their potential value to you as a developer in different situations.