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~
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。
其次,我们需要在scrollViewDidScroll中判断每一页距离中心的距离,根据这个值来调整它的alpha,scale以及x轴的translation。
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| | |
首先是alpha,可以发现中心右边的page 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page,所有的page都会往右偏,而为了让中间page大部分都露出来,右边的page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; }
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