//scroll delegate
 -(void)scrollViewDidEndDragging:(UIScrollView *)scrollView         
willDecelerate:(BOOL)decelerate
{
    if (!decelerate) {
        [self scrollToPageWith:scrollView];
    }
}
- (void)scrollViewDidEndDecelerating:(UIScrollView 
*)scrollView
{
    [self scrollToPageWith:scrollView];
}
-(void)scrollToPageWith:(UIScrollView *)scrollView
{
    CGPoint pInView = [self.mainCollectionView.superview 
convertPoint:self.mainCollectionView.center 
toView:self.mainCollectionView];
    // 獲取中間cell的indexPath
    NSIndexPath *indexPath = [self.mainCollectionView 
indexPathForItemAtPoint:pInView];

    CGFloat newX=140+205*(indexPath.row-1);
    if (indexPath.row==0) {
        newX=0;
    }

    CGFloat leftX=cellWidth-(kMainScreen_Width-cellWidth-
20-15)+20;
    if (indexPath.row==(_arrayList.count-1)) {
        newX=newX-leftX;
    }
    [scrollView setContentOffset:CGPointMake(newX, 0) 
animated:YES];
}