使用转场动画写图片无限循环

图片的无限循环在iOS开发中非常常见，常见的实现方式有三种：使用UIScrollView，使用UICollectionView，和今天我要献丑的转场动画。
完成后的效果如图：

本程序用到了3张图片，分别命名为0，1，2.

** 第一步：声明一下图片的个数和显示图片的控件以及当前显示的图片下标。**

NSInteger const  IMAGE_COUNT = 3;

@interface TransitionAnimationController ()
{
     UIImageView *_imageView;
     NSInteger _currentIndex;
}

第二步：初始化UI，添加手势。

- (void)viewDidLoad {
    [super viewDidLoad];

    [self setupUI];
}

- (void)setupUI
{
    //定义图片控件
    _imageView = [[UIImageView alloc] initWithFrame:[UIScreen mainScreen].applicationFrame];
    _imageView.contentMode = UIViewContentModeScaleAspectFit;
    _imageView.image = [UIImage imageNamed:@"0.jpg"];
    [self.view addSubview:_imageView];

    //添加手势
    UISwipeGestureRecognizer *leftSwipeGesture = [[UISwipeGestureRecognizer alloc] initWithTarget:self action:@selector(leftSwipe:)];
    leftSwipeGesture.direction = UISwipeGestureRecognizerDirectionLeft;
    [self.view addGestureRecognizer:leftSwipeGesture];

    UISwipeGestureRecognizer *rightSwipeGesture =[[UISwipeGestureRecognizer alloc] initWithTarget:self action:@selector(rightSwipe:)];
    rightSwipeGesture.direction = UISwipeGestureRecognizerDirectionRight;
    [self.view addGestureRecognizer:rightSwipeGesture];
}

第三步：编写手势的响应方法。

#pragma mark - 左滑
- (void)leftSwipe:(UISwipeGestureRecognizer *)gesture
{
    [self transitionAnimation:YES];
}

#pragma mark - 右滑
- (void)rightSwipe:(UISwipeGestureRecognizer *)gesture
{
    [self transitionAnimation:NO];
}

第四步：编写转场动画。

#pragma mark - 转场动画
- (void)transitionAnimation:(BOOL)isNext
{
    //创建转场动画
    CATransition *transition = [[CATransition alloc] init];

    //设置动画类型
    transition.type = @"push";

    //设置子类型
    if (isNext) {
        transition.subtype = kCATransitionFromRight;
    }
    else
    {
        transition.subtype = kCATransitionFromLeft;
    }

    //设置动画时常
    transition.duration = .8f;

    //设置转场后的新视图
    _imageView.image = [self getImage:isNext];

    //添加转场动画
    [_imageView.layer addAnimation:transition forKey:nil];
}

第五步：取得当前图片。
#pragma mark - 取得当前图片
- (UIImage *)getImage:(BOOL)isNext
{
if (isNext) {
_currentIndex = (_currentIndex + 1) % IMAGE_COUNT;
}
else
{
_currentIndex = (_currentIndex - 1 + IMAGE_COUNT) % IMAGE_COUNT;
}

    NSString *imageName = [NSString stringWithFormat:@"%ld.jpg", (long)_currentIndex];
    return [UIImage imageNamed:imageName];
}

在取得当前图片中用到了一个求余算法，为了方便理解我这里用Python敲了一下：

到这里就算是结束，下篇见。