Singleton by Objective-C

SingletonDemo.h

#import <UIKit/UIKit.h>

@interface SingletonDemo : NSObject

+ (instancetype)shared;

@end

SingletonDemo.m

#import "SingletonDemo.h"

@implementation SingletonDemo

+ (instancetype)shared {
    static SingletonDemo *instance = nil;
    static dispatch_once_t once_token;
    dispatch_once(&once_token, ^{
        instance = [[SingletonDemo alloc] init];
    });
    return instance;
}

@end