track a UIScrollView's direction, based on http://stackoverflow.com/a/16953031

ScrollDirection.m

typedef NS_ENUM(NSInteger, UIScrollViewScrollDirection) {
    UIScrollViewScrollDirectionDown = -1,
    UIScrollViewScrollDirectionNone = 0,
    UIScrollViewScrollDirectionUp = 1,
};

@interface FRJViewController ()

@property (nonatomic) UIScrollViewScrollDirection scrollDirection;

@end

@implementation FRJViewController

- (id)init
{
    if ( self = [super init] )
    {
        UIScrollView *scrollView = [UIScrollView alloc] initWithFrame:CGRectZero];
        // configuration
        [self.view addSubview:scrollView];

        [scrollView addObserver:self forKeyPath:@"contentOffset" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];
    }
  
    return self;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    CGFloat newContentOffset = [[change objectForKey:@"new"] CGPointValue].y;
    CGFloat oldContentOffset = [[change objectForKey:@"old"] CGPointValue].y;
    CGFloat diff = newContentOffset - oldContentOffset;

    if ( diff < 0 ) 
    {
        //scrolling down
        self.scrollDirection = UIScrollViewScrollDirectionDown;
    }
    else if ( diff > 0 ) 
    {
        // scrolling up
        self.scrollDirection = UIScrollViewScrollDirectionUp;
    }
    else 
    {
        // not scrolling
        self.scrollDirection = UIScrollViewScrollDirectionNone;
    }
}

// ...

@end