怎么判断scrollView往什么方向滑动

/**
* 可以监听ScrollView的上下滑动 ，实现ScrollListener接口，调用setScrollListener(ScrollListener l)方法。
* SCROLL_UP :ScrollView正在向上滑动
* SCROLL_DOWN :ScrollView正在向下滑动
* @author xm
*/
public class ObservableScrollView extends ScrollView {

private ScrollListener mListener;

public static interface ScrollListener {
public void scrollOritention(int oritention);
}

/**
* ScrollView正在向上滑动
*/
public static final int SCROLL_UP = 0x01;

/**
* ScrollView正在向下滑动
*/
public static final int SCROLL_DOWN = 0x10;

/**
* 最小的滑动距离
*/
private static final int SCROLLLIMIT = 40;

public ObservableScrollView(Context context) {
super(context, null);
}

public ObservableScrollView(Context context, AttributeSet attrs) {
super(context, attrs, 0);
}

public ObservableScrollView(Context context, AttributeSet attrs,
int defStyle) {
super(context, attrs, defStyle);
}

@Override
protected void onScrollChanged(int l, int t, int oldl, int oldt) {
super.onScrollChanged(l, t, oldl, oldt);

if (oldt > t && oldt - t > SCROLLLIMIT) {// 向下
if (mListener != null)
mListener.scrollOritention(SCROLL_DOWN);
} else if (oldt < t && t - oldt > SCROLLLIMIT) {// 向上
if (mListener != null)
mListener.scrollOritention(SCROLL_UP);
}
}

public void setScrollListener(ScrollListener l) {
this.mListener = l;
}
}