Macro for creating your "shared instance" using GCD

ExampleClass.m

@implementation MySharedThing

+ (id)sharedInstance
{
  DEFINE_SHARED_INSTANCE_USING_BLOCK(^{
    return [[self alloc] init];
  });
}

@end

GCDSingleton.h

#define DEFINE_SHARED_INSTANCE_USING_BLOCK(block) \
  static dispatch_once_t pred = 0; \
  __strong static id _sharedObject = nil; \
  dispatch_once(&pred, ^{ \
    _sharedObject = block(); \
  }); \
  return _sharedObject; \

This is a convenient way of creating a shared instance of an object using GCD and dispatch_once. I didn't come up with this technique (I believe I saw it first in a 2010 WWDC video) but I did wrap it up in this macro for convenience.

This code is written to use ARC (hence the __strong).

If you can't use GCD, but still want a singleton, please follow this advice.

Just wanted to say thanks. I too created a macro on the same premise to be a bit more generic:

/*!
 * @function Singleton GCD Macro
 */
#ifndef SINGLETON_GCD
#define SINGLETON_GCD(classname)                        \
                                                        \
+ (classname *)shared##classname {                      \
                                                        \
    static dispatch_once_t pred;                        \
    __strong static classname * shared##classname = nil;\
    dispatch_once( &pred, ^{                            \
        shared##classname = [[self alloc] init]; });    \
    return shared##classname;                           \
}                                                           
#endif

This assumes the init is a standard init.

Then to implement in the .h

@interface MyClass : NSObject
+ (MyClass *) sharedMyClass;
@end

and in the .m

#import "MyClass.h"

@implementation MyClass

SINGLETON_GCD(MyClass);

- (id) init {
    if ( (self = [super init]) ) {
        // Initialization code here.
    }   
    return self;
}
@end

I assume that __strong is not necessary if this is a Mac OS X app?
Also, allocWithZone can be ignored when using Garbage Collection?

Thanks again...

What if the sharedInstance is destroyed anywhere? Can it be created again?

No, it acts like a traditional singleton in that respect - only one can ever be created.

This code is error in Xcode 4.6

GCDSingleton.h:7:25: note: expanded from macro 'DEFINE_SHARED_INSTANCE_USING_BLOCK'
  return _sharedObject; \

Last \ is not need.

Thank you for sharing such a great technique! Here is a slightly modified version:

#define SHARED_INSTANCE(...) ({\
    static dispatch_once_t pred;\
    static id sharedObject;\
    dispatch_once(&pred, ^{\
        sharedObject = (__VA_ARGS__);\
    });\
    sharedObject;\
})

It allows two forms of shared object initialization: one-line

+ (instancetype) sharedInstance {
    return SHARED_INSTANCE( [[self alloc] init] );
}

// after pre-processing:

+ (instancetype) sharedInstance {
    return ({
        static dispatch_once_t pred;
        static id sharedObject;
        dispatch_once(&pred, ^{
            sharedObject = ( [[self alloc] init] );
        });
        sharedObject;
    });
}

and multi-line (notice curly braces around the block of code):

+ (instancetype) sharedInstance {
    return SHARED_INSTANCE({
        NSLog(@"creating shared instance");
        CGFloat someValue = 84 / 2.0f;
        [[self alloc] initWithSomeValue:someValue]; // no return statement
    });
}

// after pre-processing:

+ (instancetype) sharedInstance {
    return ({
        static dispatch_once_t pred;
        static id sharedObject;
        dispatch_once(&pred, ^{
            sharedObject = ({
                NSLog(@"creating shared instance");
                CGFloat someValue = 84 / 2.0f;
                [[self alloc] initWithSomeValue:someValue];
            });
        });
        sharedObject;
    });
}

It can be used in the right part of an assignment as well:

- (void) someMethod {
    MethodPrivateHelper *helper = SHARED_INSTANCE( [[MethodPrivateHelper alloc] init] );
    // do smth with the helper
}

This modification utilizes two language features: GCC compound expressions extension, which is also supported by Clang, and C99 variadic macros support.

I am afraid that in this way it can't prevent creating multiple instances by 'alloc'.