实现iOS 9 Task Switcher动画

升级到iOS 9以后,发现新的task switcher的动画蛮有趣的,于是就动手实现了下,最终效果如下~

思路

  1. 首先我们需要一个横向的scroll view,可以用UICollectionView,也可以自己实现一个。scroll view里每一页都是一张card,一屏5张card:
|                        |
|card card card card card|
|                        |
  1. 其次,我们需要在scrollViewDidScroll中判断每张card距离中心的距离,根据这个值来调整它的alpha,scale以及x轴的translation。

alpha:右边的card alpha都是1,左边的越靠左alpha越小
scale: 从左往右依次变大
translation:除了中间的card,所有的card都会右偏,而为了让中间card大部分都露出来,右边的card偏移需要比左边大

开工

1. 横向滚动的scroll view

我们可以自己实现一个横向无限滚动的scroll view, 具体可以参考: http://tech.glowing.com/cn/practice-in-uiscrollview/

在scrollViewDidScroll中,我们提供一个delegate方法,告诉使用者每一页距离中心的位置,以便apply各种transform到这个view上,delegate方法如下:

@protocol InfiniteScrollViewDelegate <NSObject>
- (void)updateView:(UIView *)view withProgress:(CGFloat)progress scrollDirection:(ScrollDirection)direction;
@end

说明一下progress的含义,如果一屏有5个visible views的话,那么它的值会从-2变化到2:

|                   |
|-2...-1...0...1...2|
|                   |
2. 根据每一页的位置来设置它的transform

首先是alpha,中心右边的card alpha都是1,而左边的会越来越淡,所以我们可以这样写:

if (progress >= 0) {  
    view.alpha = 1;
} else {
    view.alpha = 1 - fabs(progress) * 0.2;
}

其次是scale,由左往右依次变大:

CGAffineTransform transform = CGAffineTransformIdentity;  
CGFloat scale = 1 + (progress) * 0.03;  
transform = CGAffineTransformScale(transform, scale, scale);  

最后是x轴的translation,除了中间的card,所有的card都会往右偏,而为了让中间card大部分都露出来,右边的card偏移需要比左边大

CGFloat translation = 0;  
if (progress > 0) {  
    translation = fabs(progress) * SCREEN_WIDTH / 2.2;
} else {
    translation = fabs(progress) * SCREEN_WIDTH / 15;
}
transform = CGAffineTransformTranslate(transform, translation, 0);  

完整的实现:

- (void)updateView:(UIView *)view withProgress:(CGFloat)progress scrollDirection:(ScrollDirection)direction
{
    // adjust z-index of each views
    NSMutableArray *views = [[self.scrollView allViews] mutableCopy];
    [views sortUsingComparator:^NSComparisonResult(UIView *view1, UIView *view2) {
        return view1.tag > view2.tag;
    }];
    for (UIView *view in views) {
        [view.superview bringSubviewToFront:view];
    }

    // alpha
    if (progress >= 0) {
        view.alpha = 1;
    } else {
        view.alpha = 1 - fabs(progress) * 0.2;
    }

    CGAffineTransform transform = CGAffineTransformIdentity;    
    // scale
    CGFloat scale = 1 + (progress) * 0.03;
    transform = CGAffineTransformScale(transform, scale, scale);
    // translation
    CGFloat translation = 0;
    if (progress > 0) {
         translation = fabs(progress) * SCREEN_WIDTH / 2.2;
    } else {
         translation = fabs(progress) * SCREEN_WIDTH / 15;
    }
    transform = CGAffineTransformTranslate(transform, translation, 0);
    view.transform = transform;
}

最后是完整的demo代码:https://github.com/Glow-Inc/TaskSwitcherDemo