KVO监听scrollView的滚动方向

1. 设置监听

[self.mainScrollView addObserver:self forKeyPath:@"contentOffset" options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:nil];

2. 监听回调处理

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    if ([keyPath isEqualToString:@"contentOffset"]) {
        NSValue *oldvalue = change[NSKeyValueChangeOldKey];
        NSValue *newvalue = change[NSKeyValueChangeNewKey];
        CGFloat oldoffset_y = oldvalue.UIOffsetValue.vertical;
        CGFloat newoffset_y = newvalue.UIOffsetValue.vertical;
        NSLog(@"Old:%f\nNew:%f",oldoffset_y,newoffset_y);
        if (newoffset_y > oldoffset_y) {
            NSLog(@"向下滑动");
        }  else if(newoffset_y < oldoffset_y) {
            NSLog(@"向上滑动");
        }
    } else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

3. 移除监听

- (void)dealloc
{
    [self.mainScrollView removeObserver:self forKeyPath:@"contentOffset"];
}

利用KVO来监听scrollView类contentOffset的变化