#import "ViewController.h"     @interface ViewController ()  @property (strong, nonatomic)UILabel *label;  @end     @implementation ViewController     - (void)viewDidLoad {      [super viewDidLoad];      // Do any additional setup after loading the view, typically from a nib.      CGRect labelFrame = CGRectMake(100, 100,100, 50);      self.label = [[UILabel alloc] initWithFrame:labelFrame];      self.label.font = [UIFont fontWithName:@"Helvetica" size:36];      self.label.text = @"XxxX";      self.label.textAlignment = NSTextAlignmentCenter;      self.label.backgroundColor = [UIColor brownColor];      [self.view addSubview:self.label];      [self rotateLabelDown];  }     - (void)rotateLabelDown{      [UIView animateWithDuration:10 animations:^{          self.label.layer.anchorPoint = CGPointMake(0.5, 0.5);          //self.label.transform = CGAffineTransformMakeRotation(-90);  //逆时针旋转          self.label.transform = CGAffineTransformMakeRotation(M_PI);      } completion:^(BOOL finished) {          [self rotateLabelUp];      }];  }     - (void)rotateLabelUp{      [UIView animateWithDuration:10 animations:^{          self.label.layer.anchorPoint = CGPointMake(0.5, 0.5);          self.label.transform = CGAffineTransformMakeRotation(0);      } completion:^(BOOL finished) {          [self rotateLabelDown];      }];  }     - (void)didReceiveMemoryWarning {      [super didReceiveMemoryWarning];      // Dispose of any resources that can be recreated.  }     @end