iOS中無限循環(huán)滾動簡單處理實現(xiàn)原理分析

+ (instancetype)loopScrollViewWithFrame:(CGRect)frame;

 //默認(rèn)滾動到要顯示的第一張圖片
 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個對象，因此index取offset.x/width后的整數(shù)
 NSInteger index = scrollView.contentOffset.x/width;
 //這個比值很重要
 CGFloat ratio = scrollView.contentOffset.x/width;
 //從顯示的最后一張往后滾，自動跳轉(zhuǎn)到顯示的第一張
 if (index == self.loopImageList.count-1) {
  self.currentIndex = 1;
  NSIndexPath *indexPath = [NSIndexPath indexPathForItem:self.currentIndex inSection:0];
  [self scrollToIndexPath:indexPath animated:NO];
  return;
 }
 //從顯示的第一張往前滾，自動跳轉(zhuǎn)到顯示的最后一張
 //這里判斷條件為contentOffset.x和寬的比值，在往前滾快要結(jié)束的時候，能達(dá)到無縫切換到顯示的最后一張的效果
 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);
}