单例： GCD

OC

#import 
@interface Student : NSObject
//获取单例对象的第二种方式
+ (instancetype)shareInstance;
@end

#import "Student.h"
@implementation Student
//使用GCD的方式实现单例
//Apple推荐的方式
+ (instancetype)shareInstance {
        static Student *instance = nil;
        static dispatch_once_t onceToken;    
    dispatch_once(&onceToken, ^{        
        instance = [[Student alloc] init];
    });    
    return instance;
}
@end

Swift

import UIKit

//自己使用GCD实现一个单例
class Singleton: NSObject {
    
    class var sharedInstance: Singleton {
        
        struct Static {
            
            static var onceToken: dispatch_once_t = 0
            static var myInstance: Singleton?
        }
        
        dispatch_once(&Static.onceToken) { 
            Static.myInstance = Singleton()
        }
        
        return Static.myInstance!
    }
    

}