Why is my managedObjectContext coming back nill? - iphone

Question

I'm trying to use CLLocation Manager to get someone's location whenever they move and store the lat, lng, and timestamp into core data, then display that in a table view tab. However, the output in the console always indicates that managedObjectContext is nill by throwing this log coredataproject[12478:11903] After managedObjectContext: <NSManagedObjectContext: 0x7248230>
Here's the relevant code in my AppDelgate implementation file
#import "AppDelegate.h"
#import "RootViewController.h"
#import "FirstViewController.h"
#implementation AppDelegate
#synthesize window;
#synthesize navigationController;
#pragma mark -
#pragma mark Application lifecycle
- (void)applicationDidFinishLaunching:(UIApplication *)application {
// Configure and show the window.
RootViewController *rootViewController = [[RootViewController alloc] initWithStyle:UITableViewStylePlain];
NSManagedObjectContext *context = [self managedObjectContext];
if (!context) {
NSLog(#"Could not create context for self");
}
rootViewController.managedObjectContext = context;
UINavigationController *aNavigationController = [[UINavigationController alloc] initWithRootViewController:rootViewController];
self.navigationController = aNavigationController;
[window addSubview:[navigationController view]];
[window makeKeyAndVisible];
}
/**
applicationWillTerminate: saves changes in the application's managed object context before the application terminates.
*/
- (void)applicationWillTerminate:(UIApplication *)application {
NSError *error;
if (managedObjectContext != nil) {
if ([managedObjectContext hasChanges] && ![managedObjectContext save:&error]) {
// Handle the error.
}
}
}
Here is the FirstViewController.M code, where I'm getting the location and storing it in the core data 'Event' entity
- (void)viewDidLoad
{
locationManager =[[CLLocationManager alloc] init];
locationManager.delegate = self;
locationManager.desiredAccuracy = kCLLocationAccuracyBest;
locationManager.distanceFilter = kCLDistanceFilterNone;
[locationManager startUpdatingLocation];
-(void) locationmanager: (CLLocationManager *) manager
didUpdateToLocation: (CLLocation *) newLocation
fromLocation: (CLLocation *) oldLocation
{
CLLocation *location = [locationManager location];
if (!location) {
return;
}
/*
Create a new instance of the Event entity.
*/
RootViewController *rootviewcontroller = [RootViewController alloc];
Event *event = (Event *)[NSEntityDescription insertNewObjectForEntityForName:#"Event" inManagedObjectContext:rootviewcontroller.managedObjectContext];
// Configure the new event with information from the location.
CLLocationCoordinate2D coordinate = [location coordinate];
[event setLatitude:[NSNumber numberWithDouble:coordinate.latitude]];
[event setLongitude:[NSNumber numberWithDouble:coordinate.longitude]];
// Should be the location's timestamp, but this will be constant for simulator.
// [event setCreationDate:[location timestamp]];
[event setTimeStamp:[NSDate date]];
// Commit the change.
NSError *error;
if (![rootviewcontroller.managedObjectContext save:&error]) {
NSLog(#"Save Error");
}
//RootViewController *rootviewcontroller = [RootViewController alloc];
[rootviewcontroller.eventsArray insertObject:event atIndex:0];
NSIndexPath *indexPath = [NSIndexPath indexPathForRow:0 inSection:0];
[rootviewcontroller.tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
[rootviewcontroller.tableView scrollToRowAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:0] atScrollPosition:UITableViewScrollPositionTop animated:YES];
}
And finally here is the RootViewController file where I'm trying to fetch and display what's in the core data. It's when I click this tab that the console tells me that managedObjectConsole is nill
- (void)viewDidLoad {
[super viewDidLoad];
if (managedObjectContext == nil)
{
managedObjectContext = [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext];
NSLog(#"After managedObjectContext: %#", managedObjectContext);
}
// Set the title.
self.title = #"Locations";
/*
Fetch existing events.
Create a fetch request; find the Event entity and assign it to the request; add a sort descriptor; then execute the fetch.
*/
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Event" inManagedObjectContext:managedObjectContext];
[request setEntity:entity];
// Order the events by time stamp, most recent first.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"timeStamp" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
// Execute the fetch -- create a mutable copy of the result.
NSError *error = nil;
NSMutableArray *mutableFetchResults = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];
if (mutableFetchResults == nil) {
NSLog(#"Mutable Fetch Results equals nill");
}
// Set self's events array to the mutable array, then clean up.
[self setEventsArray:mutableFetchResults];
}
I'm also doing some more stuff with organizing the table data once it's there, but I don't think that's where the problem is.
I'm unsure as to why nothing would be in managedObjectContext, as it should have the location data from the location manager. I'm not too familiar with Core data, so I'm probably just doing something simple wrong, but any insight would be appreciated!

Answer 1

Your mistake is in the didUpdateToLocation method. There you create a new instance of RootViewController. You just need to save the newLocation to CoreData and need the MOC for that (not the RootViewController). So you need to find a way to pass the MOC to the FirstViewController. You can do this similar like you did in the AppDelegate or like this:
managedObjectContext = [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext];
Why do you reset the MOC of RootViewController in viewDidLoad? You already passed it in applicationDidFinishLaunching!
I'd recommend to use a NSFetchedResultsController for the table view. It automatically detects changes of the data and reloads the table if needed, just implement the delegate correctly. Here is a helpful tutorial on this: http://www.raywenderlich.com/999/core-data-tutorial-how-to-use-nsfetchedresultscontroller