Adding Events

Get the Current Location

When you create a new Event object, you need to set its location. You get the location from the location manager. If it’s not able to provide a location, then don’t continue.

>> Add the following to RootViewController implementation file:

- (void)addEvent {

    CLLocation *location = [locationManager location];

    if (!location) {

        return;

    }

}

Create and Configure the Event object

You typically create a managed object using a convenience method—insertNewObjectForEntityForName:inManagedObjectContext:—of NSEntityDescription, which returns a properly initialized instance of the correct class for the entity you specify, inserted into the managed object context. (For more about the initialization process, see “Managed Objects” in Core Data Programming Guide). After you’ve created the object, you can set its property values.

You get the latitude and longitude from the location as scalar values, so you need to convert these to NSNumber objects for the Event object. You could get the time stamp from the location as well, but this is a constant value in Simulator. Instead, here you can use date method of NSDate to get a date object representing the current date and time.

>> Add the following code at the end of the current implementation of addEvent:

// Create and configure a new instance of the Event entity.

Event *event = (Event *)[NSEntityDescription insertNewObjectForEntityForName:@"Event" inManagedObjectContext:managedObjectContext];

CLLocationCoordinate2D coordinate = [location coordinate];

[event setLatitude:[NSNumber numberWithDouble:coordinate.latitude]];

[event setLongitude:[NSNumber numberWithDouble:coordinate.longitude]];

[event setCreationDate:[NSDate date]];

This illustrates the general technique you use to update a managed object, whether it’s a new managed object you created, or one you fetch from the store. You simply use accessor methods, just as you would any other Cocoa object. Importantly, though, changes are not pushed to the persistent store until you explicitly save the context.

Save the New Event

Remember that the managed object context acts like a scratch pad (see “Managed Objects and the Managed Object Context”). Whatever changes you make—whether editing property values or adding or deleting whole objects—aren’t actually committed to the persistent store (file) until you save the context. Typically, in an iOS application, you save changes as soon as the user has made them.

>> Add the following code at the end of the current implementation of addEvent:

NSError *error = nil;

if (![managedObjectContext save:&error]) {

    // Handle the error.

}

In common with several Core Data methods, the NSManagedObjectContext save: method takes an error parameter and returns a Boolean value to indicate success or failure. The situation is really no different from that in any other application; it’s just that the return value from the save: method and the error parameter tend to bring into sharper focus the possibility of a problem occurring.

Handling Errors

It’s up to you to decide how you handle a Core Data error.

In a scenario as simple as that described in “Save the New Event”—where the only change you expect is the addition of a single object—if the data can’t be saved it’s likely to be indicative of some sort of catastrophic failure from which recovery might be difficult or impossible. In this situation you might just present an alert sheet telling the user to restart the application.

In a more complex scenario, the user might have changed property values, or added or deleted managed objects in such a way that either an individual object is in an inconsistent state (validation fails) or the object graph as a whole is inconsistent. If you have more than one managed object context, it’s also possible that the persistent store was updated when changes made in a different context were committed and so the objects in the current context are inconsistent with the corresponding records in the store.

In general, you can interrogate the error object to find out what went wrong.

You should think carefully about what the user experience should be in the event of an error occurring. What information should you present to the user? What options might you give them for recovering from the problem? These are not questions that Core Data is able to answer.

Update the Events Array and the Table View

Finally, you need to add the new Event object to the events array, then update the table view. Since this is a new Event, and Events are displayed with most recent events at the top of the list, add the new object to the beginning of the events array, add a corresponding row to the top of the table view, then scroll the table view to show the new row.

>> Add the following code at the end of the current implementation of addEvent:

[eventsArray insertObject:event atIndex:0];

NSIndexPath *indexPath = [NSIndexPath indexPathForRow:0 inSection:0];

[self.tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:indexPath]

    withRowAnimation:UITableViewRowAnimationFade];

[self.tableView scrollToRowAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:0] atScrollPosition:UITableViewScrollPositionTop animated:YES];

The next task is to complete the implementation of the table view data-source methods to display the events.