计时器的写法

@interface MyPlanarCodeViewController ()
//定义计时器
@property (nonatomic, strong) NSTimer *timer;
@end
  
@implementation MyPlanarCodeViewController
-(ApiGetCodeExcutor *)apiGetCodeExcutor{
    if (_apiGetCodeExcutor == nil) {
        _apiGetCodeExcutor = [ApiGetCodeExcutor excutor];
    }
    return _apiGetCodeExcutor;
}
- (void)viewDidLoad {
    [super viewDidLoad];
    //启动计时器
    [self startTimer];
}
- (void)startTimer {
    //60秒调用一次timerRepeat:方法
    _timer = [NSTimer scheduledTimerWithTimeInterval:60 target:self selector:@selector(timerRepeat:) userInfo:nil repeats:YES];
    [[NSRunLoop currentRunLoop] addTimer:_timer forMode:NSDefaultRunLoopMode];
    [_timer fire];
}
//重复进行的操作放到这里
-(void)timerRepeat:(NSTimer *)timer {
    ApiRequestStateHandler * apiRequest = [ApiRequestStateHandler apiRequestStateHandlerOnSuccess:^(ApiExcutor *apiExcutor, BOOL *cache) {
        if(self.apiGetCodeExcutor.status == 1){
            [self.codeImageView sd_setImageWithURL:[NSURL URLWithString:self.apiGetCodeExcutor.imageURL] placeholderImage:nil completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
                [self hideHud];
                [[SDImageCache sharedImageCache] removeImageForKey:self.apiGetCodeExcutor.imageURL];
            }];
            self.codeImageView.userInteractionEnabled = NO;
        }else{
            [self hideHud];
            [self showAlertViewWithTitle:@"请等待老师开启签到后点击二维码区域重新获取"];
        }
    } onFail:^(ApiExcutor *apiExcutor, NSError *error) {
        //
        [self hideHud];
        [self showHint:@"网络通讯异常"];
    }];
    [self.apiGetCodeExcutor getQRCodeWithUserid:[CUserManager sharedManager].user.userid withCourseId:self.course.sjddid type:self.type withDeviceIdentifier:device withApiRequestStateHandler:apiRequest];
}
//销毁计时器。避免计时器一直在运行状态
- (void)viewDidDisappear:(BOOL)animated {
    [_timer invalidate];
    _timer = nil ;
}
//- (void)dealloc {
//    [_timer invalidate];
//     _timer = nil;
//}
@end