Android: ScrollView监听滑动到顶端和底端

在项目中需要监听ScrollView滑动到顶端和底端的时候以实现自己的ScrollView，那么怎样去监听呢？今天查看了一下ScrollView的源码，找到了一种方法。先看一下源码中的overScrollBy()方法：

 1 protected boolean overScrollBy(int deltaX, int deltaY,

 2             int scrollX, int scrollY,

 3             int scrollRangeX, int scrollRangeY,

 4             int maxOverScrollX, int maxOverScrollY,

 5             boolean isTouchEvent) {

 6         final int overScrollMode = mOverScrollMode;

 7         final boolean canScrollHorizontal =

 8                 computeHorizontalScrollRange() > computeHorizontalScrollExtent();

 9         final boolean canScrollVertical =

10                 computeVerticalScrollRange() > computeVerticalScrollExtent();

11         final boolean overScrollHorizontal = overScrollMode == OVER_SCROLL_ALWAYS ||

12                 (overScrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && canScrollHorizontal);

13         final boolean overScrollVertical = overScrollMode == OVER_SCROLL_ALWAYS ||

14                 (overScrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && canScrollVertical);

15         int newScrollX = scrollX + deltaX;

16         if (!overScrollHorizontal) {

17             maxOverScrollX = 0;

18         }

19         int newScrollY = scrollY + deltaY;

20         if (!overScrollVertical) {

21             maxOverScrollY = 0;

22         }

23         // Clamp values if at the limits and record

24         final int left = -maxOverScrollX;

25         final int right = maxOverScrollX + scrollRangeX;

26         final int top = -maxOverScrollY;

27         final int bottom = maxOverScrollY + scrollRangeY;

28         boolean clampedX = false;

29         if (newScrollX > right) {

30             newScrollX = right;

31             clampedX = true;

32         } else if (newScrollX < left) {

33             newScrollX = left;

34             clampedX = true;

35         }

36         boolean clampedY = false;

37         if (newScrollY > bottom) {

38             newScrollY = bottom;

39             clampedY = true;

40         } else if (newScrollY < top) {

41             newScrollY = top;

42             clampedY = true;

43         }

44         onOverScrolled(newScrollX, newScrollY, clampedX, clampedY);

45         return clampedX || clampedY;

46     }

此方法有9个参数，参数太多有点让人模糊。这里我们不管他们是什么意思，直接看36-43行代码，很显然这是一个垂直方向的边界检查，而这也正是我们想要的。因此我们可以重写此方法，在此方法中实现监听，代码如下：

 1  @Override

 2     protected boolean overScrollBy(int deltaX, int deltaY, int scrollX, int scrollY, int scrollRangeX, int scrollRangeY, int maxOverScrollX, int maxOverScrollY, boolean isTouchEvent) {

 3 

 4         int newScrollY = scrollY + deltaY;

 5         final int bottom = maxOverScrollY + scrollRangeY;

 6         final int top = -maxOverScrollY;

 7         if (newScrollY > bottom) {

 8             System.out.println("滑动到底端");

 9         }

10         else if (newScrollY < top) {

11             System.out.println("滑动到顶端");

12         }

13         return super.overScrollBy(deltaX, deltaY, scrollX, scrollY, scrollRangeX, scrollRangeY, maxOverScrollX, maxOverScrollY, isTouchEvent);

14     }

是不是很简单呢？