OC单例写法

#import "SingleDemo.h"

static SingleDemo* _instance = nil;

@implementation SingleDemo

-(instancetype)init{

    if (self = [super init]) {

    }

    return self;

}

+(instancetype)shareInstance{

    static dispatch_once_t onceToken;

    dispatch_once(&onceToken, ^{

        _instance = [[SingleDemo alloc]init];

    });

    return _instance;

}

@end