从TableViewController的CoreData数据库中获取数据

Question

I have a TableViewController using the CoreData database. I have another UIviewController from which I want to read the TableViewController's database. What I did was as follow.

//In UIviewController
-(NSArray *)fetchRecordedDatainsqldatabase
{
    // construct a fetch request

    NSError *error;
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"TrackerList" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];
    //[fetchRequest setFetchBatchSize:20];
    // Create the sort descriptors array.
    NSSortDescriptor *descriptor = [[NSSortDescriptor alloc] initWithKey:@"descript" ascending:YES];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:descriptor, nil];
    [fetchRequest setSortDescriptors:sortDescriptors];
    // return the result of executing the fetch request
    return [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];}

I have a property of

@property (strong, nonatomic) NSManagedObjectContext *managedObjectContext;

But managedObjectContext is always nil, at the line
NSEntityDescription *entity = [NSEntityDescription entityForName:@"TrackerList" 
inManagedObjectContext:self.managedObjectContext];

So the program is always crash when it reaches to that line. What could be the problem?

Answer 1

Generally you can use the managedObjectContext provided to you in the stub code in AppDelegate . If thats the case you can use:

AppDelegate *appD = [[UIApplication sharedApplication] delegate];

then instead of the line:

NSEntityDescription *entity = [NSEntityDescription entityForName:@"TrackerList" 
inManagedObjectContext:self.managedObjectContext];

use:

NSEntityDescription *entity = [NSEntityDescription entityForName:@"TrackerList" 
inManagedObjectContext:appD.managedObjectContext];

and you should replace the return statement to this:

return [appD.managedObjectContext executeFetchRequest:fetchRequest error:&error];

If you are creating your own NSManagedObjectContext object, then you should be setting its persistentStoreCoordinator (which in turns need the managed object model and set the persistent store type).

You can see how to do all this in AppDelegate.m , if you have checked 'use core-data' when creating the project.

Anyway in your case, you already are using a managedObjectContext successfully in your first view controller. So you just need to get the same object over in the second view controller.

For your method to work, you just have to add one line at the top of the code block you provided:

self.managedObjectContext = [[[UIApplication sharedApplication] delegate] managedObjectContext];

Answer 2

Managed Object Context needs to be initialized with a Persistent Store Coordinator and which needs a Managed Object Model . XCode used to provide a boiler plate code for all these implementation in AppDelegate.

As an alternative solution you can try using MagicalRecord

You can set up core data by

[MagicalRecord setupCoreDataStackWithStoreNamed:@"Database.sqlite"];

And you can fetch all the trackerlist values in a context by

NSManagedObjectContext *context = [NSManagedObjectContext defaultContext];
[TrackerList findAllSortedBy:@"descript" ascending:YES inContext:context];

Following link would guide you better How to make programming in Core Data pleasant