Question

I come from more of an Android development background so apologies if this is a stupid question, but it’s just wracking my brain and I can’t see what is wrong. I have a Singleton Class implementation as follows:

Header file:

@interface SingletonClass : NSObject
{
}

@property(nonatomic, retain) NSMutableArray *categoryArray;

+ (SingletonClass *)sharedInstance;
- (id) init;
- (void)setCategory: (NSMutableArray *) x;
- (NSMutableArray *)getCategory;

@end 

Class Implementation:

#import "SingletonClass.h"

@implementation SingletonClass
@synthesize categoryArray;


static SingletonClass *sharedInstance = nil;

+ (SingletonClass *)sharedInstance {
    if (sharedInstance == nil) {
        sharedInstance = [[super allocWithZone:NULL] init];
    }
    return sharedInstance;
}

- (id)init
{
    self = [super init];

    if (self) {
        categoryArray = [[NSMutableArray alloc] init];
    }

    return self;
}

+ (id)allocWithZone:(NSZone*)zone {
    return [self sharedInstance];
}

- (id)copyWithZone:(NSZone *)zone {
    return self;
}

- (void)setCategory: (NSMutableArray *)category_array{
    categoryArray = category_array;
}

- (NSMutableArray *)getCategory{
    return categoryArray;
}

@end

I have 2 Tabs each of which I try to access the Singleton Object which holds the Arrays I need using:

SingletonClass* myapp = [SingletonClass sharedInstance];
categories = [myapp getCategory];

When switching tabs it works when the Singleton Object is not called, but as soon as I use it I get the SIGABRT error. (Think it’s a memory warning). Are Singleton Instances not sharable across Tabs?

Answer 1

Yes, singletons may certainly be shared across tabs.

a) If you’re not using ARC, then your reference counting is off.

b) This sounds like a reference counting offset (can happen with ARC, MRC or GC). There is a diagnostic mode “Enable Zombies”. Basically, it means that objects are not actually deallocated and ‘zombified’ when their reference count reaches zero. A zombified object will then note an error whenever it is messaged. You can use this and record reference counts for objects using the Zombies instrument in Instruments.