How to pass database from AppDelegate to ViewController - ios

Question

I don't know why this code error.
Please help.
I read some articles and I think the problem is about context.
What Should I do?
This program is about shows data in coredata to label in viewcontroller.
AppDelegate.h
#import <UIKit/UIKit.h>
#import <CoreData/CoreData.h>
#interface AppDelegate : UIResponder <UIApplicationDelegate>
#property (strong, nonatomic) UIWindow *window;
#property (readonly, strong, nonatomic) NSManagedObjectContext *managedObjectContext;
#property (readonly, strong, nonatomic) NSManagedObjectModel *managedObjectModel;
#property (readonly, strong, nonatomic) NSPersistentStoreCoordinator *persistentStoreCoordinator;
- (void)saveContext;
- (NSURL *)applicationDocumentsDirectory;
#end
AppDelegate.m
#import "AppDelegate.h"
#import "Test.h"
#import "ViewController.h"
#interface AppDelegate ()
#end
#implementation AppDelegate
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
// Override point for customization after application launch.
NSManagedObjectContext *context = [self managedObjectContext];
Test *t = [NSEntityDescription insertNewObjectForEntityForName:#"Test"
inManagedObjectContext:context];
t.name = #"please";
return YES;}
ViewController.h
#import <UIKit/UIKit.h>
#interface ViewController : UIViewController
#property (weak, nonatomic) IBOutlet UILabel *label;
#property (nonatomic,strong) NSArray *temp;
#property (nonatomic,strong) NSManagedObjectContext* managedObjectContext;
#end
ViewController.m
#import "ViewController.h"
#import "Test.h"
#interface ViewController ()
#end
#implementation ViewController
#synthesize managedObjectContext;
#synthesize temp;
- (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:#"Test"
inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];
NSError *error;
self.temp = [managedObjectContext executeFetchRequest:fetchRequest error:&error];
for(Test *info in temp){
_label.text = info.name;
}
}
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
#end
Test.h
#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>
#interface Test : NSManagedObject
#property (nonatomic, retain) NSString * name;
#end
Test.m
#import "Test.h"
#implementation Test
#dynamic name;
#end
I don't know why this code error.
Please help.
I read some articles and I think the problem is about context.
What Should I do?

Answer 1

In your view controller, replace the line:
#synthesize managedObjectContext;
With this:
- (NSManagedObjectContext *) managedObjectContext {
return ((AppDelegate *)[[UIApplication sharedApplication] delegate]).managedObjectContext;
}
Instead of storing another, different object context in your view controller, this property will return the object context that you set up in the app delegate.
There are other ways to do this, such as creating a Core Data helper class following the Singleton pattern (as #NewYork167 suggests), but this should at least solve your current problem.

Answer 2

For any future reference, you can also try subclassing the NSManagedObjectContext like this:
#interface MyManagedObjectContext : NSManagedObjectContext
+ (MyManagedObjectContext *)mainThreadContext;
#end
#implementation MyManagedObjectContext
+ (MyManagedObjectContext *)mainThreadContext;
{
static MyManagedObjectContext *moc;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
moc = [[self alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
// Setup persistent store coordinator here
});
return moc;
}
#end
Reference: Best practices for passing NSManagedObjectContext around to UITabBarController child view controllers?