Objective-c Singleton

Singleton

@implementation Singleton

static id _sharedManager;

+ (instancetype)sharedManager
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _sharedManager = [[self alloc] init];
    });
    return _sharedManager;
}

+ (id)alloc 
{
    @synchronized(self){
        NSAssert(_sharedManager == nil, @"Attempt to allocate a second instance of singleton %@", [self class]);
        _sharedManager = [super alloc];
        return _sharedManager;
    }
    return nil;
}

@end