Using a singleton to create an array accessible by multiple views

Question

It's a classic problem.

I would like to access an array of objects from anywhere within my app. I would also like to do this using a singleton. My questions are:

1. Where do I instantiate my singleton object?
2. Where do I instantiate my NSMutable array of objects?
3. How do I refer to this array from anywhere within my project?

All code and examples are greatly appreciated!

EDIT 1

This is what I have so far. I can't figure out though how to access this array of bananas correctly and consistently:

#import <Foundation/Foundation.h>

@interface Singleton : NSObject {
    NSMutableArray *bananas;
}

@property (nonatomic, retain) NSMutableArray *bananas;

@end


#import "Singleton.h"

static Singleton *mySingleton;

@implementation Singleton

@synthesize bananas;

#pragma mark SingletonDescption stuff

+ (Singleton *)mySingleton
{
    if(!mySingleton){
        mySingleton = [[Singleton alloc]init];
    }

    return mySingleton;
}

+ (id)allocWithZone:(NSZone *)zone
{
    if (!mySingleton) {
        mySingleton = [super allocWithZone:zone];
        return mySingleton;
    } else {
        return nil;
    }
}

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

- (void)release
{
    // NO OP
}

@end

EDIT 2

This is how I'm trying to use my singleton object to have an array of objects placed in a table cell. Nothing is happening and the table cell comes up blank :(

- (id)init
{
    [super initWithStyle:UITableViewStylePlain];

    // bananas = [[NSMutableArray alloc] init];

    Singleton *mySingleton = [[Singleton alloc]init];
    mySingleton.bananas = [[NSMutableArray alloc]init];

    UIImage *imageA = [UIImage imageNamed:@"A.png"];
    UIImage *imageB = [UIImage imageNamed:@"B.png"];
    UIImage *imageC = [UIImage imageNamed:@"C.png"];

    Banana *yellowBanana = [[Banana alloc] initWithName:@"Yellow" description:@"Beautiful" weight:22.0 icon:imageA];
    Banana *greenBanana =  [[Banana alloc] initWithName:@"Green" description:@"Gorgeous" weight:12.0 icon:imageB];
    Banana *rottenBanana = [[Banana alloc] initWithName:@"Rotten" description:@"Ugly" weight:8.0 icon:imageC];

    [mySingleton.bananas addObject:yellowBanana];
    [mySingleton.bananas addObject:greenBanana];
    [mySingleton.bananas addObject:rottenBanana];
}

Answer 1

Do your singleton like this:

@interface Singleton : NSObject
@property (nonatomic, retain) NSMutableArray *bananas;
+(Singleton*)singleton;
@end
@implementation Singleton
@synthesize bananas;
+(Singleton *)singleton {
    static dispatch_once_t pred;
    static Singleton *shared = nil;
    dispatch_once(&pred, ^{
        shared = [[Singleton alloc] init];
        shared.bananas = [[NSMutableArray alloc]init];
    });
    return shared;
}
@end

The singleton is initialized the first time you use it. You can call it from anywhere at any time:

NSLog(@"%@",[Singleton singleton].bananas);

Answer 2

1. You use lazy instantiation, that is, a class method that returns your singleton object. The first time this method is called, it creates the instance, all other times thereafter, it just returns the already available instance (retained in a class variable).

2. I thought the point of your singleton was to hold this array? You could either create it in the singleton's initializer, or create it lazily when it is needed the first time.

3. In your AppName-pefix.pch file, you #import its class. This global import will be available in your whole application.