OC单例：

+(instancetype)sharedInstance{
    static demo *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[self alloc] init];
        //初始化
    });
    return sharedInstance;
}

//使用
[demo sharedInstance]

Swift 3.0

static var shareInstance:Demo{
        struct MyStatic{
            static var instance :Demo = Demo()
        }
        
        //初始化
        return MyStatic.instance;
    }
  
//使用  
Demo.shareInstance