对核心数据执行提取时出现间歇性崩溃

Question

I have a published app that is occasionally crashing when performing a core data fetch. I know where the crashes are occurring, however I can't figure out why they are happening.

The app uses a tab view controller, and the crash occurs in a tab that has a tableView as the initial view. Because I need this tableView to update whenever data is changed elsewhere in the app, I'm performing a fetch in the viewWillAppear method.

Here is the relevant code:

lazy var fetchedResultsController: NSFetchedResultsController<Day> = {
    let request: NSFetchRequest<Day> = Day.fetchRequest()
    request.sortDescriptors = [NSSortDescriptor(key: "date", ascending: false)]
    request.predicate = NSPredicate(format: "calories > %@", "0")

    let frc = NSFetchedResultsController(fetchRequest: request, managedObjectContext: managedObjectContext, sectionNameKeyPath: nil, cacheName: nil)

    return frc
}()

override func viewWillAppear(_ animated: Bool) {
    do {
        try fetchedResultsController.performFetch()
    } catch {
        print("Could not fetch results")
    }
    tableView.reloadData()
}

And here is an image of the call stack.

I haven't been able to recreate the crash on my device or on the simulator, so I really don't know how to go about fixing this bug. I'd appreciate any advice on solving this.

Here's a screenshot of the calories attribute in the core data model.

Here's the class method for creating a Day entity.

    class func dayWithInfo(date: Date, inManagedContext context: NSManagedObjectContext) -> Day {
    let defaults = UserDefaults.standard

    let formatter = DateFormatter()
    formatter.dateStyle = .full
    let dateString = formatter.string(from: date)

    let request: NSFetchRequest<Day> = Day.fetchRequest()
    request.predicate = NSPredicate(format: "dateString = %@", dateString)

    if let day = (try? context.fetch(request))?.first {
        if let yesterday = Calendar.current.date(byAdding: .day, value: -1, to: Date()) {
            // Update the macro goals if the date is the current or future date
            if date > yesterday {
                day.proteinGoal = defaults.double(forKey: Constants.UserDefaultKeys.proteinValueKey)
                day.carbohydrateGoal = defaults.double(forKey: Constants.UserDefaultKeys.carbohydratesValueKey)
                day.lipidGoal = defaults.double(forKey: Constants.UserDefaultKeys.lipidsValueKey)
                day.fiberGoal = defaults.double(forKey: Constants.UserDefaultKeys.fiberValueKey)
                day.calorieGoal = defaults.double(forKey: Constants.UserDefaultKeys.caloriesValueKey)
            }
        }

        return day
    } else {
        let day = Day(context: context)

        // Set the date as a string representation of a day
        day.dateString = dateString
        day.date = date

        // Set the calorie and macronutrient goals from user defaults
        day.proteinGoal = defaults.double(forKey: Constants.UserDefaultKeys.proteinValueKey)
        day.carbohydrateGoal = defaults.double(forKey: Constants.UserDefaultKeys.carbohydratesValueKey)
        day.lipidGoal = defaults.double(forKey: Constants.UserDefaultKeys.lipidsValueKey)
        day.fiberGoal = defaults.double(forKey: Constants.UserDefaultKeys.fiberValueKey)
        day.calorieGoal = defaults.double(forKey: Constants.UserDefaultKeys.caloriesValueKey)

        // Creat meals and add their ralationship to the day
        if let meals = defaults.array(forKey: Constants.UserDefaultKeys.meals) as? [String] {
            for (index, name) in meals.enumerated() {
                _ = Meal.mealWithInfo(name: name, day: day, slot: index, inManagedContext: context)
            }
        }
        return day
    }

}

Answer 1

Allow me to posit a theory about what I think is happening. I might be making incorrect assumptions about your project, so please correct anything I get wrong. You likely understand all the below, but I'm trying to be thorough, so please excuse anything obvious.

You have a calories attribute of your model class that is implemented as an optional attribute with no default at the model level . Something like the below.

You also do not implement func validateCalories(calories: AutoreleasingUnsafeMutablePointer<AnyObject?>, error: NSErrorPointer) -> Bool in your managed object subclass, so it's possible to save an instance with a nil (remember that Core Data is still Objective-C, so your Double Swift attribute is an NSNumber in Core Data, so nil is perfectly valid for an optional attribute).

You can specify that an attribute is optional — that is, it is not required to have a value. In general, however, you are discouraged from doing so — especially for numeric values (typically you can get better results using a mandatory attribute with a default value — in the model — of 0). The reason for this is that SQL has special comparison behavior for NULL that is unlike Objective-C's nil. NULL in a database is not the same as 0, and searches for 0 will not match columns with NULL.

At this point, it would be easy to confirm the hypothesis. Simply edit the SQLite database used for Core Data by your app in the simulator and set the calories attribute of a single record to null and see if you crash in the same way.

If so, do a Core Data migration on your next release and remove the Optional designation on the model attribute and provide a Default value of zero.