实现循环中倒序播放图片

如题，用了下面代码：

UIImageView* campFireView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 175, 175)];
 // load all the frames of our animation
 campFireView.animationImages = [NSArray arrayWithObjects:
                                 [UIImage imageNamed:@"a1.png"],
                                 [UIImage imageNamed:@"a2.png"],
                                 [UIImage imageNamed:@"a3.png"],
                                 [UIImage imageNamed:@"a4.png"],
                                 [UIImage imageNamed:@"a5.png"],
                                 [UIImage imageNamed:@"a6.png"],
                                 [UIImage imageNamed:@"a7.png"],
                                 [UIImage imageNamed:@"a8.png"],
                                 [UIImage imageNamed:@"a9.png"],
                                 [UIImage imageNamed:@"a10.png"],
                                 nil];

 // all frames will execute in 1.75 seconds
 campFireView.animationDuration = 0.75;
 // repeat the annimation forever
 campFireView.animationRepeatCount = 0;
 // start animating
 [campFireView startAnimating];
 // add the animation view to the main window 
 [self.view addSubview:campFireView];   //  [campFireView release];

在a1到a10都运行正常，怎么以相反的顺序继续？应该是a1到a10再到a1?

定义一个变量，记录运行到第几个。写一个循环，判断，如果运行到末尾，再从头开始。