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：

 protected boolean overScrollBy(int deltaX, int deltaY,
             int scrollX, int scrollY,
             int scrollRangeX, int scrollRangeY,
             int maxOverScrollX, int maxOverScrollY,
             boolean isTouchEvent) {
         final int overScrollMode = mOverScrollMode;
         final boolean canScrollHorizontal =
                 computeHorizontalScrollRange() > computeHorizontalScrollExtent();
         final boolean canScrollVertical =
                 computeVerticalScrollRange() > computeVerticalScrollExtent();
         final boolean overScrollHorizontal = overScrollMode == OVER_SCROLL_ALWAYS ||
                 (overScrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && canScrollHorizontal);
         final boolean overScrollVertical = overScrollMode == OVER_SCROLL_ALWAYS ||
                 (overScrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && canScrollVertical);
         int newScrollX = scrollX + deltaX;
         if (!overScrollHorizontal) {
             maxOverScrollX = 0;
         }
         int newScrollY = scrollY + deltaY;
         if (!overScrollVertical) {
             maxOverScrollY = 0;
         }
         // Clamp values if at the limits and record
         final int left = -maxOverScrollX;
         final int right = maxOverScrollX + scrollRangeX;
         final int top = -maxOverScrollY;
         final int bottom = maxOverScrollY + scrollRangeY;
         boolean clampedX = false;
         if (newScrollX > right) {
             newScrollX = right;
             clampedX = true;
         } else if (newScrollX < left) {
             newScrollX = left;
             clampedX = true;
         }
         boolean clampedY = false;
         if (newScrollY > bottom) {
             newScrollY = bottom;
             clampedY = true;
         } else if (newScrollY < top) {
             newScrollY = top;
             clampedY = true;
         }
         onOverScrolled(newScrollX, newScrollY, clampedX, clampedY);
         return clampedX || clampedY;
     }

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：

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

         int newScrollY = scrollY + deltaY;
         final int bottom = maxOverScrollY + scrollRangeY;
         final int top = -maxOverScrollY;
         if (newScrollY > bottom) {
             System.out.println("滑动到底端");
         }
         else if (newScrollY < top) {
             System.out.println("滑动到顶端");
         }
         return super.overScrollBy(deltaX, deltaY, scrollX, scrollY, scrollRangeX, scrollRangeY, maxOverScrollX, maxOverScrollY, isTouchEvent);
     }

是不是很简单呢？