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？