gistfile1.m

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    const CGFloat threshHold = -60.0f;
    const float minOpacity = 0.2;
    CGFloat currentOffset = [scrollView contentOffset].x;
    
    if (currentOffset < 0.0)
    {
        CGFloat currentProgress = currentOffset / threshHold;
        [scrollView setAlpha:MAX(1.0 - (1.0 * currentProgress), minOpacity)];

        if (currentOffset / threshHold >= 1.7)
            [self.navigationController popViewControllerAnimated:YES];
    }
}

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
    const CGFloat threshHold = -60.0f;
    CGFloat currentOffset = [scrollView contentOffset].x;

    if (currentOffset / threshHold >= 1.0)
        [self.navigationController popViewControllerAnimated:YES];

}