创建单例

static TRDataManager *_dataManagerByUnsafe = nil;+ (instancetype)sharedDataManagerByUnsafe {    if (!_dataManagerByUnsafe) {        _dataManagerByUnsafe = [[TRDataManager alloc] init];    }    return _dataManagerByUnsafe;}////////////////////////////////////static TRDataManager *_dataManagerBySafe = nil;+ (instancetype)sharedDataManagerBySafe {    //1.创建一个静态的一次性任务对象    static dispatch_once_t onceToken;    //2.调用一次性任务方法    dispatch_once(&onceToken, ^{        //创建单例对象        _dataManagerBySafe = [[TRDataManager alloc] init];    });    //3.返回单例对象    return _dataManagerBySafe;}//重写alloc方法; 或者重写allocWithZone+ (instancetype)allocWithZone:(struct _NSZone *)zone {    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        _dataManagerBySafe = [super allocWithZone:zone];    });    return _dataManagerBySafe;}//当copy当前类型对象时候，返回唯一的单例对象- (id)copyWithZone:(NSZone *)zone {    return _dataManagerBySafe;}////////////////////////////////创建单例的方式三static TRDataManager *_dataManagerByInit = nil;+ (void)initialize {    if (self == [TRDataManager class]) {        //初始化操作;一定能保证只调用一次+线程安全的        _dataManagerByInit = [[TRDataManager alloc] init];    }}+ (instancetype)sharedDataManagerByInit {    return _dataManagerByInit;}