dispatch_once创建单列

static  DMHolidayManager *sharedInstance;
@implementation DMHolidayManager

+ (DMHolidayManager *)sharedManager{

static dispatch_once_t oneToken;
dispatch_once(&oneToken, ^{
if (sharedInstance == nil) {
sharedInstance = [[DMHolidayManager alloc] init];
}
});
return sharedInstance;
}