Correct way to create/use a Singleton NSMutableArray for Xcode 4

Question

I've reviewed (and tried) a bunch of the threads here regarding Singletons and NSMutableArrays. I'm new to Objective-C so please bear with me.

I simply want to create a few arrays that can be accessed from any view/.m file.

What is the best (or most concise) coding for a Singleton?

Below is what I have now and I get 1 warning at .m '@implementation' - "Incomplete implementation" 1 error at usage in a view .m file - "initializer element is not a compile-time constant"

This is the code I have now - my GlobalData.h file:

#import <Foundation/Foundation.h>

@interface GlobalData : NSObject {    
    NSMutableArray *listOfHeadings;
    NSMutableArray *listOfItems1;
    NSMutableArray *listOfItems2;
}    
@property(nonatomic,retain)NSMutableArray *listOfHeadings;
@property(nonatomic,retain)NSMutableArray *listOfItems1; 
@property(nonatomic,retain)NSMutableArray *listOfItems2; 
+(GlobalData*)getInstance;  
@end

My GlobalData.m file:

#import "GlobalData.h"

@implementation GlobalData
@synthesize listOfHeadings;
@synthesize listOfItems1;
@synthesize listOfItems2;
static GlobalData *instance=nil; 

+(GlobalData *)getInstance    
{    
    @synchronized(self)    
    {    
        if(instance==nil)    
        {    
            instance= [GlobalData new];    
        }    
    }    
    return instance;    
}    
@end

And in a view .m file (simplified):

#import GlobalData.h

GlobalData *globDat=[GlobalData getInstance]; //error occurs here

Can someone point out the trouble and if there's better coding, please enlighten me - thanks!

EDIT

Here's a few links I've tried to use:

Can i have a single NSMutableArray in my multiple views application?

iPhone help with singleton class

Answer 1

In this case, you might be doing more than you have to. Granted this certainly isn't always the best solution - but you can put your NSMutableArray as a property in your App Delegate and then easily refer to it from any view. By doing it this way - you aren't locking it in as a 'singleton' but there is a 'singleton instance' of it (this helps a great deal for testability).

I have simplified this process here:

YourAppDelegate.h

@property (nonatomic,retain) NSMutableArray *myArray;

YourAppDelegate.m

@synthesize myArray;

YourViewController.m

YourAppDelegate *appDelegate = (YourAppDelegate *)[[UIApplication sharedApplication] delegate];
NSMutableArray *myArrayFromAppDelegate = appDelegate.myArray;

From this point - you can do any manipulation on this value.

Answer 2

Here's the "modern" version of a single method to turn any class into a Singleton (in this case formatted as a code snippet). It works in iOS4.x or higher:

+(<#SingletonClassName#> *) sharedInstance 
{
    static <#SingletonClassName#> *_sharedClient = nil;

    static dispatch_once_t oncePredicate;
    dispatch_once(&oncePredicate, ^{
        _sharedClient = [[self alloc] init];
    });

    return _sharedClient;
}

But, do you really need a singleton of a single NSMutableArray? You could use the built-on singleton - your application delegate, which is got to by calling:

MyAppDelegate * appDelegate = (MyAppDelegate *)[[UIApplication sharedApplication] delegate];
[appDelegate.myMutableArray addObject:...];

Answer 3

The error initializer element is not a compile-time constant is not related to how you create your singleton. The error is how you are accessing your singleton. You are doing this outside of a function:

GlobalData *globDat=[GlobalData getInstance];

This means that you are trying to initialize a global variable (globDat) as the value of the expression [GlobalData getInstance]. You can only initialize global variables to expressions that are "compile-time constants". That means things like 0 or "fred" or 8/2. The value of [GlobalData getInstance] cannot be computed at compile-time, so it cannot be used to initialize the global variable.

Instead, you need to just use [GlobalData getInstance] inside your function bodies wherever you are currently trying to use the globDat variable.

As for the warning, Incomplete implementation, I don't see what's missing. Perhaps you didn't post all of the code from GlobalData.h. Anyway, you should be able to click the warning (where it appears on the right side of the editor window) and have Xcode show you what's missing.

Answer 4

This is the way I create my Singleton:

Singleton.h

#import <Foundation/Foundation.h>

@interface Singleton : NSObject {
    NSMutableArray *firstMutableArray;
    NSMutableArray *secondMutableArray;
}

@property (nonatomic, retain) NSMutableArray *firstMutableArray;
@property (nonatomic, retain) NSMutableArray *secondMutableArray;

+ (id)sharedSingleton;

@end

Sigleton.m

#import "Singleton.h"

static Singleton *sharedMySingleton = nil;

@implementation Singleton

@synthesize firstMutableArray;
@synthesize secondMutableArray;

#pragma mark Singleton Methods

+ (id)sharedSingleton {
    @synchronized(self) {
    if (sharedMySingleton == nil) {
        sharedMySingleton = [[super allocWithZone:NULL] init];
    }
    return sharedMySingleton;
}

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

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

- (id)retain {
    return self;
}

- (unsigned)retainCount {
    return UINT_MAX;
}

- (oneway void)release {
    // Never release
}

- (id)autorelease {
    return self;
}

- (id)init {
    if (self = [super init]) {
        firstMutableArray = [[NSMutableArray alloc] initWithObjects:nil];
        secondMutableArray = [[NSMutableArray alloc] initWithObjects:nil];
    }
    return self;
}

- (void)dealloc {
    [firstMutableArray release];
    [secondMutableArray release];
    [super dealloc];
}

@end

Then, when you want to call your Singleton:

#import "Singleton.h"

Singleton *singleton = [Singleton sharedSingleton];
singleton.firstMutableArray = ...
singleton.secondMutableArray = ...