#import "ViewController.h"@interface ViewController ()@property (strong, nonatomic) IBOutlet UIImageView *imageViewFirst;@property (strong, nonatomic) IBOutlet UIImageView *imageViewSecond;@property(nonatomic,assign)__block BOOL rectToVisible;@end@implementation ViewController- (void)viewDidLoad{    [super viewDidLoad];    _rectToVisible=YES;    [self.view addSubview:_imageViewFirst];        }-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{    if (_rectToVisible) {                [UIView transitionFromView:self.imageViewFirst toView:self.imageViewSecond duration:1 options:UIViewAnimationOptionTransitionFlipFromLeft completion:^(BOOL finished) {                        NSLog(@"animatioin complete");            _rectToVisible=!_rectToVisible;                    }];    }    else    {        [UIView transitionFromView:self.imageViewSecond toView:self.imageViewFirst duration:1 options:UIViewAnimationOptionTransitionFlipFromRight completion:^(BOOL finished) {                        NSLog(@"animatioin complete");            _rectToVisible=!_rectToVisible;                    }];    } }@end