问UICollectionView不会在设置contentInset后滚动

EN

- (void)viewDidLayoutSubviews {
    [super viewDidLayoutSubviews];

    CGFloat itemWidth = self.collectionView.bounds.size.width/3.0;
    NSInteger count = [self collectionView:self.collectionView numberOfItemsInSection:0];
    self.collectionView.contentSize = (CGSize){ .width=itemWidth*count, .height=self.collectionView.bounds.size.height };
    // uncomment this line, and the scroll code in the swipes below fails to work
    //self.collectionView.contentInset = UIEdgeInsetsMake(0, itemWidth, 0, itemWidth);
    self.collectionView.contentOffset = (CGPoint){ .x=self.collectionView.contentSize.width/2.0, .y=0 };
}

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
    CGFloat width = self.view.bounds.size.width/3.0;
    return (CGSize){ .width=width, .height=collectionView.bounds.size.height };
}

- (NSIndexPath *)centerIndexPath {
    CGRect visibleRect = (CGRect){.origin = self.collectionView.contentOffset, .size = self.collectionView.bounds.size};
    CGPoint visiblePoint = CGPointMake(CGRectGetMidX(visibleRect), CGRectGetMidY(visibleRect));
    return [self.collectionView indexPathForItemAtPoint:visiblePoint];
}

- (void)swipeLeft:(UISwipeGestureRecognizer *)gr {
    NSIndexPath *centerIndexPath = [self centerIndexPath];
    NSLog(@"at %@", centerIndexPath);

    if (centerIndexPath.row < [self collectionView:self.collectionView numberOfItemsInSection:0]-1) {
        [self.collectionView scrollToItemAtIndexPath:centerIndexPath atScrollPosition:UICollectionViewScrollPositionLeft animated:YES];
    }
}

- (void)swipeRight:(UISwipeGestureRecognizer *)gr {
    NSIndexPath *centerIndexPath = [self centerIndexPath];
    NSLog(@"at %@", centerIndexPath);

    if (centerIndexPath.row > 0) {
        [self.collectionView scrollToItemAtIndexPath:centerIndexPath atScrollPosition:UICollectionViewScrollPositionRight animated:YES];
    }
}