Global variable NSMuteableArray using Singleton Class

Question

I'm having trouble creating a nice way of passing a collection around to different view controllers. For example, I created a custom class called Message with a bunch of attributes. I want to have a global NSMutableArray of those stored in a global variable of sorts called messages that I can add to or get from anywhere. Everyone on Stackoverflow says not to use your delagate class to store global variables so I created a singleton class called Shared. In there I created a property for the NSMutableArray called messages like this:

@interface Shared : NSObject {

}

@property (nonatomic, retain) NSMutableArray *messages;

+(Shared *) sharedInstance;

@end

And my .h file is (the important part):

#import "Shared.h"
static Shared* sharedInstance;

@implementation Shared

@synthesize messages;

static Shared *sharedInstance = nil;

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

    }
    return self;
}

-(void) initializeSharedInstance {

}

+ (Shared *) sharedInstance{
    @synchronized(self) {
        if (sharedInstance == nil){
            sharedInstance = [[self alloc] init];
            [sharedInstance initializeSharedInstance];

        }
        return (sharedInstance);
    }
}

In my other view controller, I first import "Shared.h", then try this:

[[Shared sharedInstance].messages addObject:m];

NSLog([NSString stringWithFormat:@"Shared messages = %@", [Shared sharedInstance].messages]);

It keeps printing null instead of the the collection of m objects. Any thoughts?

Answer 1

You need to have a static variable.

In .h:

@interface Shared : NSObject
{
    NSMutableArray *messages;
}

@property (nonatomic, retain) NSMutableArray *messages;

+ (Shared*)sharedInstance;

@end

in .m:

static Shared* sharedInstance;

@implementation Shared

@synthesize messages;


+ (Shared*)sharedInstance
{
    if ( !sharedInstance)
    {
        sharedInstance = [[Shared alloc] init];

    }
    return sharedInstance;
}

- (id)init
{
    self = [super init];
    if ( self )
    {
        messages = [[NSMutableArray alloc] init];
    }
    return self;
}

Answer 2

A thought:

@synthesize generates setter and getter methods, it doesn't init your variable. Where do you do that? I can't see it in the excerpts you posted.

Answer 3

The following is not an answer to your issue, but instead a suggestion to an alternative approach that (in my opinion) is 'cleaner' in use.

An alternative to using a Singleton to store app-wide could be to define a class with class methods that retrieves values from the NSUserDefaults. This class could be imported into the prefix header (*.pch) so you can access it from every other class in the project.

Methods inside this class could look like this:

inside Settings.h:

// for this example I'll use the prefix for a fictional company called SimpleSoft (SS)
extern NSString *kSSUserLoginNameKey;

+ (NSString *)userLoginName;
+ (void)setUserLoginName:(NSString *)userLoginName; 

inside Settings.m:

kSSUserLoginNameKey = @"SSUserLoginNameKey";

+ (NSString *)userLoginName 
{
    return [[NSUserDefaults standardUserDefaults] valueForKey:kSSUserLoginNameKey];
}

+ (void)setUserLoginName:(NSString *)userLoginName 
{
    [[NSUserDefaults standardUserDefaults] setValue:userLoginName forKey:kSSUserLoginNameKey];
    [[NSUserDefaults standardUserDefaults] synthesize];
}

Of course in a setup like this NSUserDefaults is the singleton that is being accessed through a convenience class. This class acts as a wrapper around the NSUserDefaults singleton. Values can be accessed like this:

NSString userLoginName = [Settings userLoginName];
[Settings setUserLoginName:@"Bob"];

Other objects -like Arrays- could be accessed in much the same way. One thing to be careful with (much the same as with your current approach) is to be careful not to access a class like this from every other class. Components that are intended to be reusable should pass values, so the components of the application don't become too tightly coupled to the settings class.