iOS Core Data Fetch Request, how to use

Question

I am using Core Data to see whether messages in a table view have been seen before by the user. The way I do this is to save the message Id to Core Data the first time it is seen, and then I run a fetch request when I update the table view to see if there is an entry in the persistent memory with the same Id. Now what I want to know is how I should most effectively implement my fetch request, based on how time consuming it is. Should I either run a request that returns all saved message Ids as an array when the view is loaded, and then in cellForRowAtIndexPathcheck if that array contains that cell's message Id, or run the fetch request with a predicate in cellForRowAtIndexPath? The latter would be my preferred method, but If i have 100 or so cells I wondered if this would be poor etiquette. Any help would be very much appreciated.

This is my fetch Request :

func persistQuery(predicateValueString: String!) -> Bool! {

let fetchRequest = NSFetchRequest(entityName: "LogItem")
let predicate = NSPredicate(format: "itemText == %@", predicateValueString)

fetchRequest.predicate = predicate

var didFindResult = true

if let fetchResults = managedObjectContext!.executeFetchRequest(fetchRequest, error: nil) as? [LogItem] {

    if fetchResults.count == 0 {
        didFindResult=false
    }
}

return didFindResult

}

Answer 1

The best way is to use a NSFetchedResultsController. It will optimize the fetching and the memory footprint as well. It is specifically designed for table views.

To get started, take a look at the Xcode template (Master/Detail, check Core Data). It is really quite simple.

Make sure you also implement the delegate methods - they will automatically be called when your managed objects change, so there is only minimal code that is executed to update the UI (an only if the object is actually on screen).

Presumably each of your table view cells represent a LogItem (the NSManagedObject subclass) with a property to indicate the read status. Once you change that, the delegate method will try to update it based on the index path.

That's all there is to it. With the fetched results controller you get a lot of optimization for free, so I would strongly recommend using it whenever you populate a table view with Core Data entities.