Question

I’m fairly new to programming in Objective-C. While I have been able to find my way, there now is an issue I cannot solve, which is either caused by a mistake I made or because I have a fundamental misunderstanding about classes.

Essentially, I want one class to change a variable (or object) in another class. Here is the code I have:

// LocationManager.h
#import <Foundation/Foundation.h>
#import <CoreLocation/CoreLocation.h>

@interface LocationManager : NSObject <CLLocationManagerDelegate> {
    CLLocationManager *locationManager;
    CLLocation *locationByCustomLocation;
}

@property (nonatomic, retain) CLLocation *locationByCustomLocation;

@end

Of course, there’s a corresponding implementation file: LocationManager.m. It synthesizes the locationByCustomLocation variable.

The point is that from another class, I’d like to manipulate the locationByCustomLocation variable.

// viewCustomLocation.h
#import <UIKit/UIKit.h>

@interface viewCustomLocation : UIViewController <UITableViewDataSource, UITableViewDelegate> {
    UITableView *tblLocation;
    UITableViewCell *cell;
}

—

//viewCustomLocation.m
#import "viewCustomLocation.h"
#import "LocationManager.h"

@class LocationManager;

@implementation viewCustomLocation

@synthesize tblLocation;
@synthesize cell;

// some view related selectors here, but it boils down to this one:

- (void)dismissView:(id)sender
{
    [self dismissModalViewControllerAnimated:YES];

    LocationManager *locationManager = [[LocationManager alloc] init];

    // I made sure with NSLog that the customLoc variable contains the expected data
    CLLocation *customLoc = [[CLLocation alloc] initWithLatitude:place.coordinate.latitude longitude:place.coordinate.longitude];

   [locationManager setLocationByCustomLocation:customLoc];
}

Now, if I use NSLog in LocationManager.m to see what’s in the LocationByCustomLocation variable, I would expect the same data as in customLoc. Instead, the variable still seems empty.

I think the problem is that I created a copy of the LocationManager class, thus filling the LocationByCustomLocation variable in the copied class, rather than the original one, which is what I want. I can’t figure out how to talk to the original LocationManager class.

I know of a few ways to work around this issue, but I would like to know how to achieve it this way to improve my fundamental understanding of the language.

Thanks for reading!

Answer 1

That’s because you are allocating a new instance of LocationManager. You can either connect the two controllers between them, like declaring properties and setting them accordingly.

For example, if you instantiate controller B from controller A, you should implement a property for controller B, like firstController, so :

B *controller = [[B alloc] init];
controller.firstController = A;

and then from inside B controller, you control what happens in controller A

An alternate way is to instantiate and control everything from the ApplicationDelegate. It’s a more powerful pattern.