Shows how to properly make singletons/sharedInstances in objective-C, using a dispatch_once_t token

singletons properly

// MyObject.h
@interface MyObject : NSObject

+ (instancetype)sharedInstance;

@end

// MyObject.m

@implementation MyObject

+ (instancetype)sharedInstance {
    static dispatch_once_t once;
    static MyObject *sharedInstance;
    dispatch_once(&once, ^{
        sharedInstance = [[self alloc] init];
    });
    return sharedInstance;
}

@end