Fetching Core Data in the background

Question

Hello All, I have a Navigation View with a Table View, when a row is clicked, the row indexPath is passed to the next view.

in the Details view viewDidLoad, i am fetching data from Core Data. i use the fetching from the application delegate

[appDelegate loadItem:i];

As you can see i am passing one integer only which carries the row number.

the question is: How can i make this call process in another thread.(in the background) I need this because sometimes the result of the fetch is too big, so the processing takes 3 seconds delaying pushing the Details View.

I need to display an Activity Indicator for the three seconds, so i need to put the Fetch in another thread to be able to use the UI for the indicator while the fetch is being processed.

I want the simplest way as i am a newbie. Posting some codes will be great. or links :)

Answer 1

Mixing multithreading and Core Data is not a simple task. The "Multi-Threading with Core Data" section of the Core Data Programming Guide describes how to interact with Core Data on multiple threads, including all the things that you need to be careful of.

Basically, you will need to create a separate managed object context for each thread. These contexts can share access to one managed object model and persistent store. For your case, they suggest the following:

You use two managed object contexts associated with a single persistent store coordinator. You fetch in one managed object context on a background thread, and pass the object IDs of the fetched objects to another thread. In the second thread (typically the application's main thread, so that you can then display the results), you use the second context to fault in objects with those object IDs (you use objectWithID: to instantiate the object).

It sounds like the BackgroundFetching sample application shows how to do this, but I don't have it on my system.

However, before you get too far into multithreading your fetch request, I'd take a hard look into why it's taking so long to load. I'd first recommend using -setFetchBatchSize: on your NSFetchRequest to limit the number of objects loaded into memory via your fetch (which will save you a lot of memory, too). Next, I'd use -setPropertiesToFetch: to limit the properties fetched to only those you'll be using immediately.