CoreData DetailTableView BAD_ACCESS 错误

Question

Maybe I'm not going about showing a detail for a selected row using CoreData, but I can't figure out why I'm getting a "BAD_ACCESS" error. I've googled around and can't find what I'm looking for.

Basically I use CoreData to populate the data for a Table View. It retrieves all of the title attributes for all of the entities. When the user clicks on a row, I have a Detail View that needs to show the description for that entity. I think I need to make a new NSManagedObjectContext and a new NSEntityDescription for a new NSFetchRequest in my DetailViewController and then use a NSPredicate to say "where title = [user selected title]". I get an error when I select a row. See code:

- (void)viewDidLoad
{
    // Do any additional setup after loading the view from its nib.

    // Get the objects from Core Data database
    Caregiver_Activity_GuideAppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *context = [appDelegate managedObjectContext];
    NSEntityDescription *entityDescription = [NSEntityDescription
                                              entityForName:@"Definition"
                                              inManagedObjectContext:context];
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    [request setEntity:entityDescription];

    NSPredicate *pred = [NSPredicate predicateWithFormat:@"(title = %@)", self.title];
    [request setPredicate:pred];

    NSError *error;
    NSArray *objects = [context executeFetchRequest:request error:&error];
    if (objects == nil) {
        NSLog(@"There was an error!");
        // Do whatever error handling is appropriate
    }

    for (NSManagedObject *oneObject in objects) {
        [definitionDescriptionTextView setText:[oneObject valueForKey:@"desc"]];
    }

    [objects release];
    [request release];

    [super viewDidLoad];
}

I comment out that code and everything works. But when I try to debug with breakpoints, nothing catches. So I'm more confused.

I know CoreData is probably overkill for what I'm doing but this is a learning app for me.

EDIT: I didn't include that I'm using a sqlite database that is pre-populated with the entities.

You can also download my project on my github page .

Answer 1

Normally, with a Core Data backed Master-Detail interface, you don't fetch for the Detail view.

When you select a row in the Master tableview, you are selecting a particular managed object instance. You then pass that managed object instance to the detail view. There is no need to refetch the object that you selected in the tableview.

A good example of this would be the Contacts app. The Master tableview would be a list of Contact objects (displaying the name.) When you select a row, the Master tableview controller takes the specific Contact object associated with the selected row and then passes it to the Detail view controller which then populates the Detail view using data taking from the properties of the passed Contact object.

So, that entire code block where the error occurs is unnecessary.

However, the immediate error in this code is that you are releasing an object you didn't create. In this line:

 NSArray *objects = [context executeFetchRequest:request error:&error];

... you are not creating a NSArray instance with a init , new or create method. Instead, you are merely receiving an autoreleased NSArray instance created and returned by the context NSManagedObjectContext instance. When you release an object you did not create here:

 [objects release];

... you cause the crash.

Conversely, you do create a NSFetchRequest here:

 NSFetchRequest *request = [[NSFetchRequest alloc] init];

... because you used init so you do have to balance that with:

[request relwase];

BTW, this type of code should not be put in viewDidLoad as the method is only called when the view is read in the first time from the nib file on disk. That is only guaranteed to happen once as the view may remain in memory when the user switches to another view. Instead, put code that needs to run each time the view appears in viewWillAppear .