Singleton Category implementation

gistfile1.txt

Singleton.h
-------------

@protocol Singleton
@optional

+ (instancetype)sharedInstance;

@end


Singleton.m
-------------

#import "Singleton.h"
#import <objc/runtime.h>

@implementation NSObject (singleton)

+ (instancetype)sharedInstance
{
    if (![self conformsToProtocol:@protocol(Singleton)])
    {
        [self doesNotRecognizeSelector:_cmd];
    }

    @synchronized (self)
    {
        id instance = objc_getAssociatedObject(self, _cmd);
        if (!instance)
        {
            instance = [[self alloc] init];
            objc_setAssociatedObject(self, _cmd, instance, OBJC_ASSOCIATION_RETAIN);
        }
        return instance;
    }
}

@end


Usage
-------

#import "Singleton.h"

@interface MyObject : NSObject <Singleton>

@end

// you can now safely access shared instance of MyObject
// by using the +sharedInstance method