两种单例模式的写法

单例是类只允许创建单件对象,即这些类在当前进程中的唯一实例。

不使用GCD

#import "ServiceManager.h"
static ServiceManager *defaultManager;

@implementation ServiceManager

+(ServiceManager *)defaultManager{
    if(!defaultManager)
        defaultManager=[[self allocWithZone:NULL] init];
    return  defaultManager;
}

@end

@interface Teacher:NSObeject

+ (Teacher *)getInstace;
@end

static Teacher *teacher = nil;

@implementation Teacher
+ (Teacher *)getInstance 
{
    if (teacher == nil) {
    teacher = [[Teacher alloc]init];
}
    return teacher;
}

//单例：应用里面的唯一用户，游戏里面的主角，特点：唯一，全局，方便获取
//单例是非常好用的传值和存值的方法

//单例的获取
+ (Person *)heightPerson;


+ (Person *)heightPerson
{   //保证使用时初始化一次，并保证唯一
    //建立静态对象,置为nil

    static Person *person = nil;

    if (person == nil) {
        //保证不释放
        person = [[Person alloc]init];
        //
            }

    return person;
}

使用GCD

#import "ServiceManager.h"
@implementation ServiceManager

+(ServiceManager *)sharedManager{

  static dispatch_once_t predicate;
  static ServiceManager * sharedManager;

  dispatch_once(&predicate, ^{
    sharedManager=[[ServiceManager alloc] init];
  });
  return sharedManager;
}
@end

@implementation ShatedInstance {
    static dispatch_onece_t predicate;
    static SharedInstace *sharedInstance;

    dispatch_once(&predicate, ^{
    sharedInstance = [SharedManager alloc]init];
});
    return sharedInstance;
}
@end

+ (UserNameSington *)shareSaveName;

static UserNameSington * _userNameInfor = nil;
@implementation UserNameSington

+ (UserNameSington *)shareSaveName
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{

        _userNameInfor = [[UserNameSington alloc]init];

        _userNameInfor.dataString = [[NSMutableDictionary alloc]init];
    });

    return _userNameInfor;
}
@end

dispatch_once这个函数， 它可以保证整个应用程序生命周期中某段代码只被执行一次！