Thread-safe Singletons in Modern Objective-C (The Short Way)

EVCCommon.h

// the block that's passed in needs to return an object
#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;

EVCSomeClass.m

#import "EVCCommon.h"

@implementation EVCSomeClass

+(instancetype)shared {
  DEFINE_SHARED_INSTANCE_USING_BLOCK(^{
    return [EVCSomeClass new];
  });
}

@end