iOS中无限循环滚动简单处理实现原理分析

+ (instancetype)loopScrollViewWithFrame:(CGRect)frame;

 //默认滚动到要显示的第一张图片
 if (self.imageCollectionView.contentOffset.x == 0) {
  NSIndexPath *indexPath = [NSIndexPath indexPathForItem:1 inSection:0];
  [self scrollToIndexPath:indexPath animated:NO];
  self.currentIndex = 1;
}

- (void)setImageList:(NSMutableArray *)imageList {
 _imageList = imageList;
 self.loopImageList = [NSMutableArray arrayWithArray:imageList];
 if (imageList.count>0) {
  [self.loopImageList insertObject:[imageList lastObject] atIndex:0];
  [self.loopImageList addObject:[imageList objectAtIndex:0]];
 }
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
 CGFloat width = self.bounds.size.width;
 //在loopImageList中，有n+2个对象，因此index取offset.x/width后的整数
 NSInteger index = scrollView.contentOffset.x/width;
 //这个比值很重要
 CGFloat ratio = scrollView.contentOffset.x/width;
 //从显示的最后一张往后滚，自动跳转到显示的第一张
 if (index == self.loopImageList.count-1) {
  self.currentIndex = 1;
  NSIndexPath *indexPath = [NSIndexPath indexPathForItem:self.currentIndex inSection:0];
  [self scrollToIndexPath:indexPath animated:NO];
  return;
 }
 //从显示的第一张往前滚，自动跳转到显示的最后一张
 //这里判断条件为contentOffset.x和宽的比值，在往前滚快要结束的时候，能达到无缝切换到显示的最后一张的效果
 if (ratio <= 0.01) {
  self.currentIndex = self.imageList.count;
  NSIndexPath *indexPath = [NSIndexPath indexPathForItem:self.currentIndex inSection:0];
  [self scrollToIndexPath:indexPath animated:NO];
  return;
 }
 if (self.currentIndex != index) {
  self.currentIndex = index;
 }
 NSLog(@"currentIndex = %ld",self.currentIndex);
}