核心数据问题。 数据无法保存

Question

So I have a utility app and I am trying to save some text into a "To" and "Message: text field on the Flipside View Controller. However, my data won't save. I am new to objective C and I have been using multiple different tutorials to the point where I have totally confused myself. Hopefully you can help me out. Not sure what else to do at this point...

FlipsideViewController.m

#import "CCCFlipsideViewController.h"
#import "CCCAppDelegate.h"
#import "CCCMainViewController.h"
#import "MessageDetails.h"

@interface CCCFlipsideViewController ()
{
   // NSManagedObjectContext *context;
}
@end

@implementation CCCFlipsideViewController
@synthesize allMessageDetails;
@synthesize managedObjectContext;

- (void)awakeFromNib
{
    [super awakeFromNib];

    CCCAppDelegate *appDelegateController = [[CCCAppDelegate alloc]init];
    self.managedObjectContext = appDelegateController.managedObjectContext;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    // Do any additional setup after loading the view, typically from a nib.

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription
                                   entityForName:@"MessageDetails" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    NSError *error;

    self.allMessageDetails = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];

    /*
    NSManagedObject *managedObject; = [_fetchedResultsController valueForKey:@"to"];
    self.toTextField.text = managedObject to;

    messageDetails.to = [allMessageDetails firstObject];
    self.toTextField.text = messageDetails.to;

    messageDetails.message = [allMessageDetails valueForKey:@"message"];
    self.messageTextField.text = messageDetails.message;
    */
    NSLog(@"The 'to' is currently at %@ after viewdidload", self.toTextField.text);

    }

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (BOOL)textFieldShouldReturn:(UITextField *)textField
{
   return [textField resignFirstResponder]; //function says that if (bool) the text field is open and the keyboard hits return, text field is to resign first responder.
}

#pragma mark - Actions
- (IBAction)done:(id)sender
{
    [self.delegate flipsideViewControllerDidFinish:self];
}

- (IBAction)resignFirstResponder:(id)sender {

    [self.toTextField resignFirstResponder];
    [self.messageTextField resignFirstResponder];
    NSLog(@"Resigned First Responder");
}


- (IBAction)save:(id)sender {

    // Create a new instance of the entity managed by the fetched results controller.
   NSManagedObjectContext *context = [self.fetchedResultsController managedObjectContext];
    NSEntityDescription *entity = [[self.fetchedResultsController fetchRequest] entity];
    NSManagedObject *newManagedObject = [NSEntityDescription insertNewObjectForEntityForName:[entity name] inManagedObjectContext:context];

    // If appropriate, configure the new managed object.
    [newManagedObject setValue:self.toTextField.text forKey:@"to"];
    [newManagedObject setValue:self.messageTextField.text forKey:@"message"];

    // Save the context.
    NSError *error = nil;
    if (![context save:&error]) {
        /*
         Replace this implementation with code to handle the error appropriately.

         abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.
         */
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }


}


#pragma mark -
#pragma mark Fetched results controller

- (NSFetchedResultsController *)fetchedResultsController {

    if (_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }

    /*
     Set up the fetched results controller.
     */
    // Create the fetch request for the entity.
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    // Edit the entity name as appropriate.
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"MessageDetails" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    // Set the batch size to a suitable number.
    [fetchRequest setFetchBatchSize:20];

    // Edit the sort key as appropriate.
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"to" ascending:NO];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

    [fetchRequest setSortDescriptors:sortDescriptors];

    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;


    NSError *error = nil;
    if (![_fetchedResultsController performFetch:&error]) {
        /*
         Replace this implementation with code to handle the error appropriately.

         abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.
         */
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return _fetchedResultsController;
}


@end

Answer 1

I didn't look at all your code because there was a problem near the top that negates everything you do thereafter. Don't alloc/init your app delegate in awakeFromNib or anywhere else for that matter. The one and only instance of your app delegate already exists (I have no idea what happens when there is more than one app delegate).

CCCFlipsideViewController needs to gain access to the managed object context through another means. Perhaps CCCMainViewController (or another view controller) could set the CCCFlipsideViewController's managedObjectContext property. If CCCMainViewController does not have access to the managed object context, have the app delegate pass that context to it.

Example: App delegate sets a managedObjectContext property on the root view controller; the root view controller, in turn, sets the managedObjectContext property on a child view controller (like your flipside VC), etc.

Answer 2

You don't seem to ever actually set self.messageTextField.text or self.toTextField.text to anything -- you have commented out code in your viewDidLoad method which sets these fields. bilobatum is entirely correct about your AppDelegate issue as well -- you could also use something like

[((NSObject*)[UIApplication sharedApplication].delegate) valueForKey: @"managedObjectContext"];

to get the app delegate for your application if you want to fix that fast, though long term bilobatum's solution to this is better design.

Answer 3

Honestly, I think you've done quite a number on this code... ;)

OK, first off, in your save method, don't create another NSManagedObjectContext, use the instance variable you already declared, "managedObjectContext."

Secondly, I think you've made things way too complicated for yourself... Storing Core Data is actually shockingly simple once you've created the NSManagedObject subclasses and set up everything in the App Delegate...

It seems as if you wouldn't need any info from the "fetchedResultsController" at that point in your code, since you're saving, not fetching. Maybe try changing your save method to something like:

- (IBAction)save:(id)sender {

    NSEntityDescription *entity = [NSEntityDescription insertNewObjectForEntityForName:@"MessageDetails" inManagedObjectContext:self.managedObjectContext];

    // If appropriate, configure the new managed object.
    [entity setValue:self.toTextField.text forKey:@"to"];
    [entity setValue:self.messageTextField.text forKey:@"message"];

    // Save the context.
    NSError *error = nil;
    [self.managedObjectContext save:&error]

    if (error) {
        /*
         Replace this implementation with code to handle the error appropriately.

         abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.
     */
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

}

---

Edit: And to get the managed object context from the app delegate...

Right after your @synthesize's, create a variable for the App Delegate.

AppDelegate* appDelegateController;

And in viewDidLoad, initialize it:

appDelegateController = (AppDelegate*)[[UIApplication sharedApplication] delegate];

Right after viewDidLoad (or anywhere you want), you can stick in a method to declare the managed object context:

- (NSManagedObjectContext*)managedObjectContext {
    return appDelegateController.managedObjectContext;
}

Then back in viewDidLoad, call that method with:

self.managedObjectContext = [self managedObjectContext];